S4D400
Introduction to ABAP Programming
for SAP S/4HANA
.
.
PARTICIPANT HANDBOOK
INSTRUCTOR-LED TRAINING
.
Course Version: 20
Course Duration: 5 Day(s)
e-book Duration: 8 Hours 15 Minutes
Material Number: 50151411
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iii
Typographic Conventions
American English is the standard used in this handbook.
The following typographic conventions are also used.
This information is displayed in the instructor’s presentation
Demonstration
Procedure
Warning or Caution
Hint
Related or Additional Information
Facilitated Discussion
User interface control
Example text
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iv
Contents
vii
Course Overview
1
Unit 1:
2
9
Lesson: The Development Model in SAP S/4HANA
Unit 2:
10
21
39
Unit 3:
Unit 4:
Unit 5:
Unit 6:
Unit 7:
Using Internal Tables
Lesson: Internal Table Types
Lesson: Working with Internal Tables
Unit 8:
131
139
141
149
157
Using Structures
Lesson: Using Structures
107
113
130
Calling Reusable Procedures
Lesson: Understanding Reuse
Lesson: Using Classes
Lesson: Using Function Modules
96
106
Debugging in ABAP
Lesson: Debugging in ABAP
78
82
86
95
Working with Variables in ABAP
Lesson: Working with Variables
70
77
The ABAP Development Environment
Lesson: ABAP Development Tools for Eclipse
Lesson: Working with Development Objects
40
69
The Development Model in SAP S/4HANA
Open SQL
Lesson: Understanding Open SQL
Lesson: Creating Database Tables
Lesson: Reading Data from the Database
Lesson: Preview: Core Data Services in SAP S/4HANA
Unit 9:
158
161
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Object-Orientation in ABAP
Lesson: A Short History of ABAP
Lesson: Modeling a Class
v
171
Unit 10:
172
179
183
189
192
198
Lesson: Creating Classes
Lesson: Creating Objects
Lesson: Calling Methods
Lesson: Using Constructors
Lesson: Using Factory Methods
Unit 11:
199
216
221
Creating and Using Classes
Using Inheritance
Lesson: Inheritance in ABAP
Lesson: Special Features of Inheritance
Unit 12:
222
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Using Interfaces
Lesson: Using Interfaces
vi
Course Overview
TARGET AUDIENCE
This course is intended for the following audiences:
●
Development Consultant
●
Technology Consultant
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Developer IT Adminstrator IT Support
●
Developer
●
Development Consultant
●
Technology Consultant
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vii
© Copyright. All rights reserved.
viii
UNIT 1
The Development Model
in SAP S/4HANA
Lesson 1
The Development Model in SAP S/4HANA
2
UNIT OBJECTIVES
●
Understand the Development Model in SAP S/4HANA
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1
Unit 1
Lesson 1
The Development Model in SAP S/4HANA
LESSON OBJECTIVES
After completing this lesson, you will be able to:
●
Understand the Development Model in SAP S/4HANA
The Programming Model
Figure 1: Evolution of ERP
SAP S/4HANA is SAP's new suite of business software, in which the letter 'S' stands for both
Suite and Simple. Throughout the history of SAP software, systems and processes became
more and more complicated, and the way in which users interacted with the software was
always oriented more towards highly-specialized power users than to occasional users.
SAP therefore recently decided to present a new product, simplified both in its
implementation and in its user experience. At the same time, SAP S/4HANA has retained the
programming language – ABAP – that it used in R/2 and R/3.
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Lesson: The Development Model in SAP S/4HANA
Figure 2: SAP S/4HANA Simplicity
From an IT value perspective, SAP S/4HANA creates unique opportunities to dramatically
simplify the landscape and reduce TCO with SAP HANA as the great simplifier. Business users
can leverage a simple and role-based user experience based on modern design principles,
minimizing training efforts while increasing productivity. Guided configurations ease creating
settings in the areas of system administration up to application customizing. Enterprises can
now significantly reduce their data footprint and work with larger data sets in one system to
save hardware costs, operational costs, and time.
Here are key facts on SAP S/4HANA:
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Faster analytics and reporting
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Less process steps
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ERP, CRM, SRM,SCM, PLM co-deployed
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All data: social, text, geo, graph, processing
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No locking, parallelism
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Actual data (25%) and historical (75%)
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Smaller total data footprint
●
SAP Fiori UX for any device
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3
Unit 1: The Development Model in SAP S/4HANA
Figure 3: From SAP HANA to SAP S/4HANA
SAP S/4HANA is the next-generation business suite from SAP powered by the SAP HANA
platform. This in-memory database, combined with platform features like real-time analytics
and the ability to process geospatial data provides faster, more powerful data processing and
has been the catalyst that has enabled much of the simplification that you will find in SAP S/
4HANA.
Figure 4: Systems as a Black Box
When an end user interacts with a software system, we say that, to them, it is a "black box".
This means that they make input into the system, and results come out, but they have no
access to or knowledge of the internal workings of the system itself. The vast majority of
desktop and mobile applications are black boxes – we can use them, but we cannot configure,
manipulate, or program them.
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4
Lesson: The Development Model in SAP S/4HANA
Figure 5: Systems as a White Box
To a programmer, a system is a white box (or sometimes called a glass box). They are
responsible for all of the programming activities from start to finish, which typically means
implementing a user interface layer, an application logic layer, and a database layer. The
advantage of this is, of course, that the developer has full control over the behavior of the
system. However, it is inefficient inasmuch as there are many repetitive tasks that he or she
must perform. The user interface layer always involves visualizing data from the application
layer, often in very similar ways. After all, the greater the similarity between different screens
in the system, the greater the familiarity will be to the end user, and familiarity in turn
promotes productivity.
Database programming is also repetitive, as programmers spend a lot of time on four key
operations, namely reading data from the database, creating new records, and updating or
deleting existing ones.
Figure 6: Programming with Frameworks and APIs
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5
Unit 1: The Development Model in SAP S/4HANA
A modern and complex software system, such as SAP S/4HANA, has a programming model
that is somewhere in between the black box and the white box approach. Clearly,
programmers need to be able to implement their own applications and extensions to existing
features, but at the same time, they should not have to spend too much time on all of the
repetitive tasks. The way to approach this is for a system to provide frameworks. A framework
is a subsystem within the overall software system that takes on a particular task by
generating and processing code for a particular function. However, a framework can only do
this in a very generic way, as it cannot guess what particular details a specific application will
need.
Consequently, while a framework is doing its job, it will very often provide a phase of the
operation in which you can write your own code to complement the generated code and
complete the task according to your own specifications. In order to access the information
that the framework is processing, there is almost always an application programming
interface (API) which is a set of functions that you can use to find out the current state of the
application and to influence what the framework does next.
The advantage of frameworks and APIs is that the system can provide a generic solution for
the parts of a process that are always the same, which leaves programmers free to
concentrate on the best way to solve their own individual problems.
Figure 7: Tasks for an ABAP Developer
In SAP S/4HANA, SAP has adopted the model of framework-supported development. Unlike
the classic SAP Business Suite, in which ABAP programmers had to deal with every aspect of
an application, many parts of an application can now be implemented without explicitly using
ABAP code.
The user interface layer of SAP S/4HANA is primarily implemented using SAPUI5. Data is
exchanged using SAP Gateway and the OData protocol. SAPUI5 is a framework that can be
manipulated and extended using JavaScript. Therefore, ABAP does not play a role in this
particular area. However, the user interface of an application in SAP S/4HANA may also be
implemented using conventional screen or Web Dynpro technology in the case of applications
that have been adopted from the SAP Business Suite and not reimplemented using SAPUI5.
These applications could be SAP standard applications, but could also be your own custom
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6
Lesson: The Development Model in SAP S/4HANA
developments from a previously used Business Suite system. Web Dynpro and conventional
screens both use ABAP in their implementations.
The database layer of a new SAP S/4HANA application will use Core Data Services to define
the data model and the Business Object Processing Framework (BOPF) to process changes
to data in the database. This is the approach that you should take when designing new
applications, but you should also be aware that existing applications that have not been
redesigned and reimplemented will use an all-ABAP approach to the database layer.
The application layer in SAP S/4HANA is used to perform calculations and validations on data
that has either been read from the database or input into the system. Here ABAP still plays a
strong role, and the relevant frameworks will normally provide a point at which you can
implement your own code.
Figure 8: ABAP Programming in This Class
The emphasis of this class is entirely on ABAP programming. ABAP programming in the new
SAP S/4HANA model has two major drawbacks for beginners; firstly, the ABAP code that you
write is encapsulated in routines that are called automatically by different frameworks.
Consequently, it is not possible to write meaningful code without understanding how these
frameworks work. Secondly, the user interface layer of the programming model does not use
ABAP techniques at all.
To help you to focus on developing your ABAP skills, you will be writing programs using the
ABAP Development Tools for Eclipse that run on the application server. Instead of using the
Fiori Launchpad, you will start them directly from the development environment. User input
and output is handled by routines that we have provided – all you need to do is to follow the
instructions to call them from your programs. Once you have mastered enough ABAP, you
can go on to learn the user interface side of things. Equally, it is possible that other teams or
team members will be responsible for front-end development while you and your colleagues
concentrate on developing back-end business logic.
Much of what you learn will involve processing data locally within a program. However, you will
also learn about Open SQL, ABAP's set of commands for communicating with the database,
to allow you to read the contents of database tables.
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7
Unit 1: The Development Model in SAP S/4HANA
LESSON SUMMARY
You should now be able to:
●
Understand the Development Model in SAP S/4HANA
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8
UNIT 2
The ABAP Development
Environment
Lesson 1
ABAP Development Tools for Eclipse
10
Lesson 2
Working with Development Objects
21
UNIT OBJECTIVES
●
Use the ABAP Development Tools in Eclipse
●
Understand software logistics in ABAP
●
Create a simple ABAP program
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9
Unit 2
Lesson 1
ABAP Development Tools for Eclipse
LESSON OBJECTIVES
After completing this lesson, you will be able to:
●
Use the ABAP Development Tools in Eclipse
ABAP Development Tools In Eclipse
Figure 9: ABAP Development Tools in Eclipse (ADT)
In the past, SAP had its own SAPGUI-based development environment, the ABAP Workbench.
As well as providing integrated access to all ABAP development objects, it was particularly
valued by developers for features such as versioning and the where-used list, which shows
exactly where a particular object has been reused by other objects.
As time moved on, SAP needed to provide development environments for products and tools
that were not SAPGUI-based, such as the SAP NetWeaver Developer Studio for Java
development, the MaxDB administration console, or the SAP HANA Studio. These all use the
popular cross-platform environment Eclipse.
Eclipse is a development environment, originally conceived for Java, that is extensible and can
support development for any platform. Eclipse itself is a framework for which vendors including SAP - can provide plug-ins that support development in a given language.
Despite the fact that ABAP already had the SAPGUI-based ABAP Workbench, SAP decided to
produce the ABAP Development Tools for Eclipse in recognition of the fact that an SAP
development project today may reach beyond pure ABAP into the HANA or SAP UI5 space.
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10
Lesson: ABAP Development Tools for Eclipse
Developers then need to access artefacts from the SAP HANA, SAP UI5 and ABAP spaces,
preferably from within the same tool. Eclipse provides this common ground, and ABAP
developers have the choice between working with the ABAP Development Tools for Eclipse
(ADT) and the classic SAPGUI-based ABAP Workbench.
In this course, you will be working with ADT. Note that, although the functions and features of
ADT and the ABAP Workbench are similar, they are not identical. In particular, there are some
kinds of ABAP development objects that can only be edited in ADT. In the S/4HANA
environment, this applies particularly to Core Data Services views.
Figure 10: Perspectives and Views
The Eclipse application is a framework within which you can perform many very different
tasks. Software vendors extend the framework by providing plug-ins for different
programming languages. Consequently, you can program in Java, C, C++, PHP, JavaScript and, of course, ABAP - all from the same application.
However, because different languages and platforms have different requirements, you will
use different toolsets within the same Eclipse window. Each of these toolsets is called a
perspective and a complete programming environment usually comprises several
perspectives - one for normal development, one for debugging, and probably further
perspectives for special tasks.
A perspective fills the entire Eclipse window and enables you to perform a particular task,
such as ABAP development. The different component tools that make up the perspective are
called views. The main view is normally a source code editor, but there will also be further
views showing the structural outline of a program, errors from the program check, and so on.
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11
Unit 2: The ABAP Development Environment
Figure 11: Creating an ABAP Project
In order to work in your system, you must first create an ABAP Project in ADT. The project
corresponds to logging onto the backend system. You can only work with objects when you
are actually logged on.
When you create an ABAP Project, you normally choose an entry from SAPLogon. However,
you could also specify the connection details for a system individually.
When you create the project, you log on using your user and password for the backend
system. You must also specify a logon language. This is not the language of the ADT user
interface, which is always English, but the original language that your development objects will
have in the backend system. Increasingly, companies are using English as their original
development language, although this is by no means mandatory.
Closing Eclipse simultaneously logs you off from the system. When you re-open Eclipse, you
must log back on. The simplest way to do this is to double-click the project in the Project
Explorer.
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12
Lesson: ABAP Development Tools for Eclipse
Figure 12: Eclipse and SAPGUI Editors
ABAP requires a lot of different editors for all kinds of different objects. Some have a state-ofthe-art implementation in Eclipse. This is particularly the case for heavily-used source-code
based tool such as the ABAP Editor for ABAP programs and the Class Builder for creating
classes.
Other tools, which may be heavily form-based instead of source-code based or that are
considered legacy tools do not have an Eclipse implementation. Instead, when you open an
object that requires one of these tools, the classic SAPGUI user interfaces appears. However,
the object still appears in-place in the Eclipse perspective and is fully-integrated into the
navigation of your session.
As time goes on, you will find that more and more SAPGUI-based editors - although not all of
them - are being replaced with Eclipse-based ones.
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13
Unit 2: The ABAP Development Environment
Figure 13: Opening Objects Using the Project Explorer
There are different ways to open objects in the ABAP Development Tools workbench. One
way is to access them through their package. Every development object in the SAP system
belongs to a package, which is a container for objects that logically belong together. In an
ABAP project, you can add packages that you use frequently to your favorites list, and you
can then browse them in the project explorer. Double-clicking an object opens it in the editor
area. Each new object that you open is displayed in its own new tab.
Figure 14: Object Search
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14
Lesson: ABAP Development Tools for Eclipse
If you do not know the package of an object, you can use the search function. Press Ctrl + H in
ADT to open the search dialog. You can then look for objects that fit a particular pattern. You
can restrict the search area as follows:
●
Project: The system searches the current project, which is the current system.
●
Workspace: The system searches all projects in the workspace. This means that the
search area covers multiple systems if you have projects in your workspace that log onto
different systems.
You can also restrict the search to just your favorite packages.
Figure 15: Open Object Dialog
If you know the name of an object, or a part of its name, you can also use the Open Object
dialog. To do this, press Ctrl + Shift + A and then enter a part of the object name in the filter
field. Here, you can also restrict the search to particular object types.
To restrict the object type, enter type: after your search string, followed by the kind of object
that you are looking for. Common types are:
prog: Program
devc: Package
tabl: Database table
clas: Class
For a full list of object types, you can enter type: in the filter field and then press Ctrl + Space.
This opens a dropdown list of all types.
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Unit 2: The ABAP Development Environment
Figure 16: First-Aid for ADT
When you begin to work with ADT, there are a few things that can seem unusual or go slightly
wrong. The first is the number of tabs that opens. Every time you navigate to a new
development object, ADT displays it in a new tab. The consequence of this is that, very soon,
you can lose track of where you are. In the figure, there are seven visible tabs, with another
eight open, but not visible in the toolbar.
To close all open tabs in one go, use the keyboard combination Ctrl + Shift + W. If there is
unsaved content in any of the tabs, ADT prompts you to save your work.
Another problem that can occur in ADT is that you might close an important view by mistake.
For example, in the figure above, the Project Explorer has disappeared. To fix this problem,
you can reset any perspective in Eclipse to a predefined arrangement of views. To do this,
Choose Window → Perspective → Reset Perspective from the menu and confirm the security
prompt.
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16
Lesson: ABAP Development Tools for Eclipse
Using SAP GUI Transactions in ADT
Figure 17: Using SAP GUI Transactions in Eclipse
There are certain tools that you will need to use during a development project but that do not
have an equivalent implementation in Eclipse. This is often the case when you need to use a
transaction from the area of system administration to check what is happening behind the
scenes when you run a program.
In order to avoid having to log onto the SAP system using SAP GUI as well as ADT, you can
use the keyboard shortcut Ctrl + 6 to open a new SAP GUI window in-place in ADT. When you
do this, you must specify the ABAP project to which the window should apply. ADT then
opens a SAP GUI window with the system, client, user, and logon language of the ABAP
project.
In this window, you work normally as if you had logged onto the system using SAP GUI. You
can enter transaction codes in the command field. Note that to access the menu of the SAP
GUI window, you must click the Menu button to the left of the command field.
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17
Unit 2: The ABAP Development Environment
Figure 18: The Repository Information System
One of the SAP GUI transactions that you will probably use when you are working with ADT is
the Repository Information System. This is a tool that allows you to search for different kinds
of development objects according to various criteria. Since there are over 4 million different
development objects in an SAP S/4HANA system, this is a very useful tool!
To start the Repository Information System, open a SAP GUI window in ADT (Ctrl+6) and
enter transaction code SE84. In the tree on the left-hand side of the screen, double-click on
the type of object for which you want to search. In this example, we are looking for a function
module. Function modules are useful in ABAP development as they provide a particular
function (such as a calculation or reading data from the database) in re-usable form. This
means that every other program in the system can use the function module instead of having
to reproduce the corresponding code themselves.
This example assumes that we are writing a program in which we need to know on which day
of the week a particular date falls. To do this, we have to guess what the function module
might be called. Using wildcards (the * characters) we can search for function modules whose
names contain the words DATE and DAY (bear in mind that the names of function modules
are generally English).
You will find out more about function modules later in the course.
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18
Lesson: ABAP Development Tools for Eclipse
Figure 19: Restricting The Search Area
As there are so many objects in the system, you will often want to restrict your search to a
particular area. SAP systems are divided into application components, each of which covers a
broad area of the system such as Controlling, Financial Accounting, Sales and Distribution,
and so on. Since each of these areas is itself still very large, they are divided into
subcomponents. In the example above, you can see CO for controlling, CO-PA underneath it
as the subcomponent for profitability analysis, and two further levels of subdivision below
that. Each development object in the system is ultimately assigned to one of these
components. If you want to search a particular application component, enter its name in the
Application Component field.
If you want to search a larger area – perhaps the entire area CO-PA, enter CO-PA* in the
Application Component field. This will search every application component whose name
begins with CO-PA.
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19
Unit 2: The ABAP Development Environment
Figure 20: The Hit List
When you execute the search in the Repository Information System, the system produces a
hit list of objects that meet the selection criteria. This list is often extensive, but still a greatly
reduced set of objects compared with the total number in the system (here there are 28
function modules in the hit list from a total of over 400,000 in the system as a whole).
To examine the definition of one of the objects from the list, double-click it in the list. Note
that if the object type has an Eclipse-based editor, the system will use it even though you are
working in a SAP GUI window. This would be the case here with function modules.
LESSON SUMMARY
You should now be able to:
●
Use the ABAP Development Tools in Eclipse
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20
Unit 2
Lesson 2
Working with Development Objects
LESSON OBJECTIVES
After completing this lesson, you will be able to:
●
Understand software logistics in ABAP
●
Create a simple ABAP program
Working with Packages and Change Requests
Figure 21: Working with Objects – The System Landscape
Your SAP system landscape consists of more than one system. Your business operations are
conducted in the production system. However, this system must only ever be used to process
live business data, which means that testing or development of new programs are strictly
forbidden. For this reason, you will have at least one more system for development, but
probably more than that.
The classic SAP system landscape consists of a development system, where development
can take place without disturbing (or being disturbed by) other activities. Your finished
programs will, of course, be required in the production system, but it is customary and good
practice to test them in a dedicated test system first. Following a successful test, they can
then be transferred into the production system and put into active use.
In theory, it is possible to do without a test system and perform tests in the development
system. However, this does not provide a particularly stable testing environment. Equally, it is
possible to have a system landscape that contains a stable and regulated validation or
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21
Unit 2: The ABAP Development Environment
preproduction system after the test stage. This is often found in highly regulated industries
such as the manufacture of medicines or medical equipment.
In order to move objects between systems in an orderly fashion, SAP systems are connected
using the Change and Transport System. Configuring this system is a job for the system
administrators, but as an ABAP developer, you will come into daily contact with it whenever
you create or modify a program or other development objects. In order to transport objects
between systems, you must assign each object that you create to a package and a change
request.
Figure 22: Packages in the ABAP Project
Packages in an SAP system have two major functions. The first, in common with packages in
other programming environments or folders in operating systems, is to group together
objects that are logically related. For example, all of the sample programs for this class are
grouped together in a package DEVS4D400.
The second function of a package is to determine how the objects that belong to it will be
transported into other systems. This is determined by the transport layer. The transport layer
is like a route map that describes which systems will receive the objects once they are ready.
Transport layers are configured by the system administrators. When you create a new
package, you merely select one of the predefined layers.
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22
Lesson: Working with Development Objects
Figure 23: Adding an Existing Package to Your Favorites
If you need to look regularly at the objects in a particular package, you can add it to your
favorites list in ADT. To do this, right-click the
Favorite Packages node in the Project Explorer
and choose Add a Package…
Enter the beginning of the name of the package that you want to find. ADT filters the results
as you type. Once you can see the package in the list, double-click it or select it and choose
OK. ADT adds the package to your favorites list.
If you now expand the package name, you can see all of the development objects that belong
to it. To open an object, double-click it.
Figure 24: Working with Objects – Packages
Within a system, you have access to all of the objects delivered by SAP. This means that you
can reuse them in your own applications or just look to see how a particular program works.
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23
Unit 2: The ABAP Development Environment
You should not, however, make changes to SAP programs, as these changes have to be
reconciled with new versions of these objects that might be shipped by SAP later.
This principle also applies to packages – if you want to create a new program of your own, you
must assign it to a package of your own, and all customer-specific objects must reside in the
customer namespace. To create an object in the customer namespace, you simply use Z or Y
as the first letter of the object's name. SAP developers are not allowed to ship objects whose
names begin with these letters, and so we have a simple but effective way of avoiding naming
collisions.
Figure 25: Working with Objects – Change Requests
When you create a new ABAP program, it exists in the development system but not in your
test or production system. In order to ensure that the program reaches these systems, you
must assign it to a change request. This is a compulsory step in creating the object.
Change requests are subdivided into tasks. A task gives one developer the right to assign his
or her objects to the change request. If you are working as a team on a particular project, you
should create one change request with a separate task for each developer. This ensures that
everyone's objects are transported together at the appropriate time.
You can assign any number of objects to a change request. In the development phase of a
project, you may end up with hundreds of objects in a single request. Later on, in the
maintenance phase, you may need to make a quick correction to a single object. You would
then have a much smaller request containing just that object.
Once you have completed and tested the program, you release the change request. This is a
signal to the Change and Transport System that the request can be imported into the test
system. This action creates the new programs in that system. Once you have tested the
objects in the test system, you can again release the request, which makes the objects
available for import into the production system. Now your end users can use your program.
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24
Lesson: Working with Development Objects
Figure 26: Changing Objects
Now let us look at what happens to an ABAP program that has already been released and
transported. Let us suppose that you need to add a new feature to the program. You do this in
the development system. Now you have a new version of the program in the development
system, but the old version in the test and production systems.
Luckily, the Change and Transport System will not allow this to happen: As soon as you make
a change to an existing program, you must assign the object to a change request. This
ensures that your changes to the program will be logged and transported into the test and
production systems.
Figure 27: Lifecycle of an Object
The lifecycle of an object therefore looks like this. When you create a new object, you must
assign it to a package. Although it is technically possible to reassign it to a different package
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Unit 2: The ABAP Development Environment
later, you will not often do this in practice. Once you have saved the new object, there will
never be a moment in its lifetime in which it is not assigned to a package.
You must also assign the object to a change request to ensure that your changes are
transported into the correct systems. Once you have completed and tested your objects, you
release the change request to trigger the transport into the next system. When you next
change the object, you must assign it to a new request. Thus an object may belong to many
change requests during its lifetime, and there will also be periods during which it is not
assigned to a request.
Creating an ABAP Program
Figure 28: Creating an ABAP Program
Once you have created a package in the customer namespace – one whose name begins with
Z or Y – you can go on to create your first program. The easiest way to do this is to right-click
the package name in the Package Explorer and choose
New → ABAP Program . You may also
use the keyboard shortcut Ctrl + Shift + N, which opens the Create ABAP Repository Object
dialog box. If you do this, enter
Program in the field marked Type Filter Text . You can then
select ABAP Program from the resulting list.
In the dialog box that follows, check that the suggested project and package are correct, and
change them if they are not.
The name of the program must begin with Z or Y. After that, it may contain the letters A-Z, the
digits 0-9, and the underscore characters. Accents, umlauts, and any other special characters
are not allowed.
ABAP does not distinguish between upper and lower case in program names. Regardless of
how you enter the name, the system converts all letters to upper case.
Once you have entered the name and description of the program, choose
Continue . You must
now choose a change request for your program. The dialog box shows a list of all change
requests in which you are already involved. Normally you would choose one of these requests.
However, if there is not a suitable request in the list, you can create one at this point.
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Lesson: Working with Development Objects
Figure 29: The Keyboard Shortcut Ctrl + Shift + N
The keyboard shortcut Ctrl+Shift+N is very useful, as it opens a dialog box from which you
can create any ABAP development object. When the dialog box appears, you must first check
that the object will be created in the correct project (check the Project field and change it if
necessary using the Browse button). You can then expand the tree to find the kind of object
that you want to create and double-click it to continue. However, you can also start typing the
name of the kind of object you want to create into the filter field as shown above; once you
have typed the letters
prog into the field, the filter only shows the object type ABAP Program.
Once you are familiar with the different object types that you need regularly, this is a very
quick means of navigation.
Figure 30: The ABAP Editor
Once you have created your program, it will appear in the ABAP Editor. Each ABAP
development object that you open is displayed in its own tab in the Editor area.
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Unit 2: The ABAP Development Environment
By default, the ABAP Editor should appear as displayed in the figure, The ABAP Editor.
However, since users can customize the display (see the figure, Customizing the Editor
Experience), it may look slightly different.
A new ABAP program contains a generated statement REPORT <program_name>. This is a
marker that indicates the beginning of the program. You may not place any other ABAP
statements before it.
The text in the illustration that does appear before REPORT is a block of comment lines.
Comments are notes for humans inside program code – the system ignores them, but we can
use them to remind ourselves what the program does at a particular point.
There are two ways to write a comment in ABAP: The first is to use the
* character at the very
beginning of the line. Then, the entire line is marked as a comment. The second way is to use
the ″ character anywhere in a line. The system then marks the rest of the line from this point
onwards as a comment.
Figure 31: Customizing the Editor Appearance
Eclipse allows developers a lot of flexibility to customize the appearance of its various
perspectives and views and in this respect, ADT is no exception. As with other tools, you can
find many settings by choosing
Window → Preferences . This opens a dialog box with a lot of
different settings; you will find the relevant ones under General and ABAP Development.
Remember that you can use the Type Filter Text input field to help you to locate a particular
setting.
Some settings – particularly the settings for automatic code formatting and advanced static
code checks – are project-specific. This means that you must configure them for each project
separately in the Project Properties. You can open this dialog box by right-clicking a project
name and choosing Properties . Alternatively, place the cursor on the project name and press
Alt + Enter.
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Lesson: Working with Development Objects
Writing Simple ABAP Code
Figure 32: The Simplest ABAP Method Call
The simplest thing you can do in ABAP is to call an existing piece of code that someone else
has written. There are various ways of doing this, depending on how the code was written in
the first place, but here we show you how to call a method.
A method is a function, and it always belongs to a class. To call the method, you must specify
both the name of the class and the name of the method. This is because there may be more
than one class with a method with that particular name. You join the name of the class and the
name of the method together using the component selector, which is an arrow made up of the
equals sign followed by the greater than sign.
A method call always concludes with parentheses. The opening parenthesis joins onto the
method name with no space. Inside the parentheses, there must always be at least one space.
Normally you would specify the parameters of the method in the parentheses. This means
passing values to the method that it needs to perform its function and receiving results back
from it. The method in the example has no parameters, so the parentheses remain empty.
This may be uncommon, but it is also not unusual.
At the end of every ABAP statement (the name for a single instruction in ABAP), there is
always a period.
All of the methods of the class cl_s4d_output are static methods. Later on, you will learn
about instance methods, how they differ from static methods, and how to use them.
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Unit 2: The ABAP Development Environment
Figure 33: Useful Editor Functions for Method Calls
To find out more about the method that you want to call, place the cursor anywhere in the
method name. You can then choose one of the following functions:
Press F2 to open the method documentation in a popup window in the Editor. This is useful if
you just want a quick look at the method definition. As soon as you click outside the popup
window, it disappears again.
Press Alt + F2 to open the method documentation in a separate tab below the Editor. The
same documentation appears, but does not disappear again until you close the tab yourself. If
you position the cursor on a different method name, the display changes to the
documentation of that method.
Press F3 to open the source code of the method. This opens the corresponding class in a new
tab in the Editor area so that you can examine how it is implemented. Note that you can do
this not only for your own code, but also for all SAP source code as well. To return to your own
program, click the corresponding tab or use the Alt + Left Arrow key combination to go
backwards along your navigation path.
Whenever you navigate to the definition of an object using the F3 key, ADT opens a new tab.
These tabs do not close automatically when you leave them. Consequently, you can end up
with lots of tabs open, which can be confusing. To close all open tabs in one go, use the key
combination Ctrl + Shift + W. If any tabs contain unsaved changes, ADT will prompt you to
save them first.
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Lesson: Working with Development Objects
Figure 34: Testing a Program
In order to test your program, you must first activate it. This creates a version of the program
that the ABAP runtime system can understand. To activate the program, use the lighted
match icon or the keyboard shortcut Ctrl + F3. You can only do this if your program is correct.
The next section shows you what to do if there are errors.
To run the program, use the
Run icon or press F8. This starts a SAPGUI window in a new tab in
ADT. The methods of class cl_s4d_output display various kinds of output in a dialog box. To
close the dialog box, use the Close button in the top right-hand corner.
Note that the SAPGUI window that opened automatically when you ran the program remains
open even once you have finished. You will need to close this tab by hand (or use the
keyboard shortcut Ctrl + Shift + W to close all open tabs).
Figure 35: Correcting Errors
If there are errors in your program, you will not be able to activate it. ADT signals errors in
your code by placing a white cross on a red background at the beginning of the line containing
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Unit 2: The ABAP Development Environment
the error. Each error is described in the
Problems tab, which appears below the
Editor pane.
Equally, if you position the mouse over the error icon in the Editor, the same message will pop
up.
In ADT, you can decide how the Editor should handle errors. By default, errors are signaled
automatically. This is helpful, but also has the disadvantage that the error symbol appears
even when you are typing a correct statement just because you haven't typed the period at
the end yet. If you find this annoying, you can tell the Editor not to display errors
automatically. In this case, you must choose the Check function yourself in order to display
any errors that you have made.
To switch off the automatic error display, choose
Window → Preferences , followed by ABAP
Development → Editors → Source Code Editors . Now deselect the option Automatic syntax
check on all open ABAP source editors.
Figure 36: Using Code Completion
A useful tool in ADT is the code completion utility. If you enter the beginning of the name of a
class and press Ctrl +Space, ADT shows a list of all classes whose names begin with that
sequence of characters. After the component selector, you can do the same to see a list of
methods.
Once you have highlighted the method that you want to call, press Shift + Enter. Instead of
entering just the name of the method in your source code, this key combination inserts the
method name and the full signature, that is, all of the – as parameters – that you can
exchange with the method.
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Lesson: Working with Development Objects
Figure 37: Calling a Method with an Importing Parameter
In this example, the method that we call has a parameter called iv_text. The first code snippet
in the figure shows the result of the code completion. However, we must also pass a value to
iv_text to tell the method what text should be displayed. The second code snippet shows how
we can do that to make the method display the text Hello World.
The vertical pipe characters | denote the beginning and end of a string template. In their
simplest form (as here) string templates just contain a static string of characters. However,
as you will see later, they are actually very powerful and can contain variables and embedded
functions as well.
Figure 38: Method Signature
Here we have a further example for a method. Its task is to work out on which day of the week
a particular date falls. In order to do this, it needs the calling program to tell it which date is
required. This information is passed to the method, so the method regards it as an importing
parameter.
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Unit 2: The ABAP Development Environment
The method also provides a result, namely the day of the week on which the specified date
falls. This information comes from the method and is given to the calling program.
Consequently, the method regards it as an exporting parameter.
Now, look at the method call on the left-hand side of the figure. From the point of view of the
program, a piece of information that is passed to the method is an exporting value, while
results, which are exported from the method, are imported into the program.
Note that you can leave out the word EXPORTING in the method call if there are no other
kinds of parameters involved. This is exactly what happened in the previous figure.
LESSON SUMMARY
You should now be able to:
●
Understand software logistics in ABAP
●
Create a simple ABAP program
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34
Unit 2
Learning Assessment
1. In order to log onto an SAP system using ABAP Development Tools for Eclipse, you must
first create a corresponding entry in the SAP Logon.
Determine whether this statement is true or false.
X
True
X
False
2. Which of the following details must you supply to log onto an SAP system using ADT?
Choose the correct answers.
X
A Client
X
B User
X
C Password
X
D Package
3. Which of the following are characteristics of a package in the ABAP Workbench?
Choose the correct answers.
X
A Packages group logically-related objects.
X
B All of the objects in a package have the same original language
X
C Packages determine how the objects contained in them will be transported
X
D Objects in a package can only be used by other objects from the same package.
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Unit 2: Learning Assessment
4. Which of the following statements about development objects is correct?
Choose the correct answer.
X
A A development object is always assigned to exactly one package at any one time.
All of the changes during its lifetime are assigned to the same change request.
X
B A development object may be assigned to more than one package at any one time.
All of the changes during its lifetime are assigned to the same change request.
X
C A development object is always assigned to exactly one package at any one time.
Changes to the object at different times are assigned to different change requests.
X
D A development object may be assigned to more than one package at any one time.
Changes to the object at different times are assigned to different change requests.
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Unit 2
Learning Assessment - Answers
1. In order to log onto an SAP system using ABAP Development Tools for Eclipse, you must
first create a corresponding entry in the SAP Logon.
Determine whether this statement is true or false.
X
True
X
False
That is correct. You can use an SAP Logon entry to connect to a system, but you may also
enter the details manually.
2. Which of the following details must you supply to log onto an SAP system using ADT?
Choose the correct answers.
X
A Client
X
B User
X
C Password
X
D Package
3. Which of the following are characteristics of a package in the ABAP Workbench?
Choose the correct answers.
X
A Packages group logically-related objects.
X
B All of the objects in a package have the same original language
X
C Packages determine how the objects contained in them will be transported
X
D Objects in a package can only be used by other objects from the same package.
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Unit 2: Learning Assessment - Answers
4. Which of the following statements about development objects is correct?
Choose the correct answer.
X
A A development object is always assigned to exactly one package at any one time.
All of the changes during its lifetime are assigned to the same change request.
X
B A development object may be assigned to more than one package at any one time.
All of the changes during its lifetime are assigned to the same change request.
X
C A development object is always assigned to exactly one package at any one time.
Changes to the object at different times are assigned to different change requests.
X
D A development object may be assigned to more than one package at any one time.
Changes to the object at different times are assigned to different change requests.
© Copyright. All rights reserved.
38
UNIT 3
Working with Variables
in ABAP
Lesson 1
Working with Variables
40
UNIT OBJECTIVES
●
Declare appropriately typed variables in ABAP
●
Work with variables in ABAP
●
Work with non-variable data objects
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Unit 3
Lesson 1
Working with Variables
LESSON OBJECTIVES
After completing this lesson, you will be able to:
●
Declare appropriately typed variables in ABAP
●
Work with variables in ABAP
●
Work with non-variable data objects
Declaring Variables
Figure 39: Declaring Variables
A variable in an ABAP program is a reserved section of the program memory. It has a given
size and type, which determine how much information can be stored in it and what kinds of
information are allowed. It also has a name that allows you to store information in it and
retrieve information from it during the program.
The name of a variable may be up to 30 characters long. It may contain the characters A-Z,
the digits 0-9, and the underscore character. The name must begin with a letter or an
underscore.
You can declare variables at the beginning of a program. These are called global variables,
because they are then available throughout the program. This is, however, not always good
style, and later on you will learn about other ways of declaring variables. The ABAP
programming guidelines recommend that you prefix the names of global variables with the
letter g to make them easily recognizable in your program.
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Lesson: Working with Variables
Depending on the type that you choose, you may also need to specify the length of the
variable and, for some numeric types, the number of decimal places, explicitly.
Figure 40: Sources of ABAP Types
You can specify the type of a variable in different ways:
●
ABAP has a set of 13 predefined simple types.
●
If you want to define a specific type and reuse it in different places in the same program,
you can create a type definition within a program.
●
If you want to define a specific type and reuse it in multiple programs, you could define it in
a global class or a global interface.
These kinds of type definitions are purely technical – they consist of a type, a length, and a
number of decimal places. However, in an SAP System you often need a complete definition
of the type of a particular business entity – for example, a material number. As with any other
type, this has a type and a particular length. However, in addition, these entities also have a
semantic definition, including:
●
Field labels that appear on the user interface
●
A mechanism for displaying possible input values to the user
●
A mechanism for validating entries
These types are defined as Data Elements within the ABAP Dictionary.
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Unit 3: Working with Variables in ABAP
Figure 41: Predefined Complete ABAP Types
The table shows a list of the predefined complete ABAP types. These types are known as
complete because you cannot override their set length of number of decimal places.
The types I and INT8 are for whole numbers. As a rule, you should use type I, but if you need
integers outside of its value range, you can use INT8.
The types DECFLOAT16 and DECFLOAT34 are decimal floating point numbers. You should
use them when you need an extremely large value range or very high precision (16 or 34
decimal places respectively). For normal business logic, you should use type P (see the
following section).
Type D represents a date. In ABAP, the date always has the format YYYYMMDD (without
separators). The system converts this format according to the current locale before the value
is displayed on the user interface. Likewise, when the user enters a date, the system converts
it into the ABAP format before you start to process it.
Type T represents the time. In ABAP, this has the format HHMMSS (without separators in 24
hour format). If the current locale uses 12 hour format, the system converts the values
automatically.
Type STRING is a character string of variable length. The ABAP runtime system allocates and
releases memory to optimize the management of string variables. You cannot influence this
behavior directly. Type XSTRING is similar, but is a byte string instead of a character string.
The maximum length of a string is governed by settings that the system administrator makes.
However, in practice, we talk about strings being unlimited in length.
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Lesson: Working with Variables
Figure 42: Predefined Incomplete ABAP Types
The types listed above are incomplete types. This means that you can specify the length as
part of the declaration. In the case of type P, you may also specify a number of decimal
places.
Type C is a character field. You specify its length in characters; the runtime system then
assigns double the number of bytes in order to accommodate the field.
Type N is a numeric string. This is a sequence of digits that you do not want to regard as a
number and perform calculations with. For example, this could be a personnel number or cost
center.
Type X contains a sequence of bytes.
Type P is a packed number and is explained in more detail in the next figure.
Note that in older coding you may still see the following syntax:
DATA gv_char(5) type C.
The number in parentheses is, in this case, the length of the variable.
You may also see this:
DATA gv_var.
The variable seemingly has neither a type nor a length, but this is not allowed in ABAP.
Variables without a type and length have the default type C and default length 1.
These syntax variants are still allowed for compatibility reasons, but do not use them in your
own code.
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Unit 3: Working with Variables in ABAP
Figure 43: Packed Numbers in Detail
Type P is a packed number. Packed numbers are known as such because each byte contains
two digits. One half byte is required to store the information as to whether the number is
positive or negative. Thus the length of a packed number is twice its length minus 1. A packed
number that is 8 bytes long can contain up to 15 digits.
Packed numbers may also have up to 14 decimal places. You specify these using the
DECIMALS addition in the DATA statement. The statement DATA gv_number TYPE P
LENGTH 3 DECIMALS 2. declares a packed number with 5 digits. Of these, two come after the
decimal point. Consequently, the maximum value of this variable would be 999.99.
The decimal separator in a packed number is always the period. If the current locale calls for a
comma, the runtime system formats the variable automatically for the user interface.
Figure 44: Types in the ABAP Dictionary – Data Elements
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Lesson: Working with Variables
In an SAP system, there are thousands of business entities, such as country code, plant,
material number, fiscal year, cost center, and so on. Technically, it would be possible to define
these entities in every single program using the built-in ABAP types that you have just seen.
However, this would be extremely labor-intensive and error-prone. Instead, SAP provides the
ABAP Dictionary, which is a central store for important data types and also the tool that you
use to create database tables.
In the ABAP Dictionary, single business entities are described by data elements. Above, you
can see the data element that describes an airline. The type is defined just once in the entire
system, and every program or method parameter that needs the definition of an airline can
use it. This ensures consistency throughout the system.
Before you change one of these central objects, you must take care to find out what effect the
changes will have on other objects and programs (and the effects will often be far-reaching).
However, once you have decided that the change is necessary, you only need to alter one
object and the change will be reflected in every other object in the system that uses it. An
example of this is the lengthening of the material number type (MATNR) in SAP S/4HANA: In
the classic Business Suite, material numbers were 18 characters long. In SAP S/4HANA, they
are 40 characters long. Without centrally-defined data elements, SAP would not even have
been able to consider making this change.
Figure 45: Types in Classes
As well as built-in types and ABAP Dictionary types, you will also find type definitions in
classes. To find out if a class contains type definitions, use the Overview function (F2) or open
the class definition and search for the TYPES keyword.
To define a variable in a program with reference to a type from a class, use the syntax <class
name>=><type name> as shown above.
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Unit 3: Working with Variables in ABAP
Figure 46: Initial Values of Variables
Every variable in ABAP has an initial value that depends on its type:
●
The initial value of a numeric variable is always zero.
●
The initial value of a character field is a string of spaces.
●
The initial value of a string is a single space.
●
The initial value of byte fields are analogous to character fields, but with the character Hex
00 instead of space.
●
The initial values of types D, T, and N have the digit zero at every position.
Figure 47: Which Numeric Type to Use?
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Lesson: Working with Variables
Figure 48: ABAP Statements with Keywords
The DATA statement is an example of an ABAP statement that uses a keyword. In ABAP,
there is a set of reserved ABAP words that each have a particular function within the
language. The ABAP words in the example above are DATA, TYPE, LENGTH, and DECIMALS.
An ABAP word that is used at the beginning of a statement is called a keyword. ABAP words
that occur later on in the statement are called additions. Additions may be either obligatory or
optional. Sometimes, using one particular addition in a statement will mean that you cannot
subsequently use certain other additions within the same statement.
Not all ABAP statements use keywords. Often, you can use an expression instead. An
expression is a combination of values, functions, and operators that are processed by the
system and yield a particular result. You can assign this result directly to a variable, or reuse it
in a larger, nested expression. There are, however, tasks that you can only perform using
statements. Examples of these are declaring variables (using the DATA statement) and
communicating with the database using Open SQL.
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Unit 3: Working with Variables in ABAP
Figure 49: Using the ABAP Documentation
ABAP has comprehensive documentation that you can access directly from the ABAP Editor.
To do so, place the cursor on a keyword and press F1. ADT then displays the appropriate
context-sensitive documentation.
Figure 50: Chained Statements
One of the syntactical features of ABAP is the way in which you can write chained statements.
If you have more than one statement that begins in the same way, you can write the common
opening part once only, followed by a colon. You then write the rest of each statement, with
each part separated by a comma. The syntactical and runtime behavior of both variants is
identical.
If you want to write a chained statement, you must use a colon after the opening part of the
statement. However, using a colon does not mean that you must then use a chained
statement. So, for example, the following is syntactically correct:
DATA: gv_var TYPE string.
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Lesson: Working with Variables
Figure 51: Using Variables in Method Calls
This example shows a method that has an exporting parameter. The documentation of the
method shows that there is a parameter ev_text with type string.
It is important to note here that the direction of the parameter depends on the point of view of
the method or the caller. From the point of view of the method, ev_text is a value that is
passed from the method to the caller and is therefore exported. The caller sees this the other
way round – a value that is passed from the method to the program will be imported by the
program. Hence the exporting parameters of the method correspond to importing
parameters of the method call in the program.
When you call the method, the system displays a dialog box in which you can enter your
name. The text that the user enters is the export parameter of the method.
Figure 52: Declaring a Variable for Use in a Method Call
If you want to use the text returned by the method in your program, you must declare an
appropriately typed variable. Since the export parameter of the method has type string, you
must use the type string in your program as well.
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Unit 3: Working with Variables in ABAP
Once you have assigned the method parameter to the variable, you can reuse it in the
program – for example, by passing it to another method, as shown here.
Assigning Values to Variables
Figure 53: Initializing a Variable with a Set Value
When you declare a variable it has the correct initial value for its data type. However, if you
want a variable to have a different value when it is created, you can specify it in the DATA
statement using the VALUE addition. In this example, gv_int2 has the value 12 instead of zero.
Figure 54: Assigning a Value to a Variable
You can assign a value to a variable using the expression <variable> = <value>. The value can
be static – as in the example gv_Int1 = 10 – or you can specify the name of a variable whose
value should be copied into the target variable.
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Lesson: Working with Variables
Figure 55: Deleting the Contents of a Variable
The CLEAR statement resets the contents of a variable to the initial value appropriate to its
type.
Note that the type is always reset to the initial value, even if you created the variable using
VALUE and assigned it a different starting value.
Figure 56: Simple Expressions
The value of a variable can also be the result of an expression. An expression is a combination
of values, operators, and functions that the runtime system processes to calculate a result.
This example is a simple addition: The contents of gv_Int2 and gv_int3 are added together and
the result placed in gv_int1.
For basic arithmetic, ABAP provides the operators + for addition, - for subtraction, * for
multiplication and / for division. In complex expressions involving more than one operator,
multiplication and division take precedence over addition and subtraction. Expressions with
identical precedence are processed from left to right.
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Unit 3: Working with Variables in ABAP
Additionally, you can use the operators DIV for whole-number division and MOD for the wholenumber remainder of a division. Thus, 6 DIV 4 is 1, and 6 MOD 4 is 2.
Figure 57: Built-In ABAP Functions
ABAP has a range of built-in functions for various tasks. Many of these are used for string
processing, but here you can see an example of a numeric function. You use the ipow function
to raise a number to a whole-numbered power. The function has two arguments – base for
the base number and exp for the exponent.
The syntax for using these functions is similar to that of a method call – the function name is
followed by parentheses, and after the opening parenthesis, you must type at least one space.
Then follow the named arguments, to which you assign values using the notation argument =
value. The result of the function is a value, which is then placed into the variable on the lefthand side of the expression.
Using Control Structures in ABAP
Figure 58: Conditional Branching with IF
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Lesson: Working with Variables
Sometimes, you only want to process a certain section of your program under a certain
condition. To do this, you use the IF statement. In its simplest form, you test a condition –
here, for example, the variable gv_value must have the value 1 – and if the condition is true,
the statements between IF and ENDIF are processed. If not, the system leaves them out and
jumps to the statement following ENDIF.
You can refine this behavior further by using the keyword ELSE. In the second example, the
same condition is tested, and if it is true, the statements between IF and ELSE are processed.
After this, the system jumps to ENDIF. However, if the condition is not true, the system jumps
straight to ELSE and, instead, processed the statements between ELSE and ENDIF.
Figure 59: Conditional Branching with IF and ELSEIF
IF blocks can be complex. In this case, we test a first condition with IF. If the condition is true,
then the statements directly following it are processed up to the next ELSEIF statement. After
this, the system jumps to ENDIF.
If the first condition is not true, the system jumps directly to the next ELSEIF statement. If this
condition is true, the statements directly following it are processed until the next ELSEIF,
ELSE or ENDIF statement.
For as long as the conditions specified after IF and ELSEIF are not true, the system keeps
jumping to the next condition and testing it.
As soon as the system has processed one block within the structure, it jumps to ENDIF and
does not process any further blocks.
If there is an ELSE block, it must be the last block in the structure and it will be processed if no
preceding condition was true.
If there is no ELSE block, the system may exit the structure without having processed a single
block.
You can nest IF structures. This means that within an IF, ELSEIF, or ELSE block, you can have
any number of further IF structures. Each inner structure must conclude with ENDIF before
the block in which it is contained ends.
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Unit 3: Working with Variables in ABAP
Figure 60: Logical Expressions in ABAP
When you use IF conditions, you are not restricted to testing whether one value is equal to
another. Instead, you can use logical expressions. A logical expression is a combination of
operands and operators that the runtime system evaluates when the program is executed.
The result is either true or false.
In ABAP, you can use the common symbols for logical operators. There is also a set of
abbreviations that you can use. These are:
EQ - Equals
NE - Not Equal
LT - Less Than
LE - Less Than or Equal
GT - Greater Than
GE - Greater or Equal
A particularly useful expression is IS INITIAL. With it, you can test whether a variable has the
initial value appropriate to its type.
You can combine more than one logical expression using the expressions AND and OR. You
can also invert a condition using NOT. In the last example on the illustration, the runtime
system first checks that gv_value does not have an initial value. If the value is not initial, it
goes on to check that gv_carrid has the value LH. If both expressions are true, then the whole
expression is true. If the first expression that the system evaluates is not true, it does not
process any further parts of the same expression.
If you link expressions using OR then only one of the linked expressions must be true.
However, all of the linked expressions may be true. ABAP does not contain an exclusive OR
function.
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Lesson: Working with Variables
Figure 61: Case Distinction in ABAP
The CASE statement allows you to test the value of a variable quickly and efficiently for
equality with another value. It thus replaces a long string of IF … ELSEIF statements.
After CASE, you write the variable whose value you want to test.
The first statement after CASE must always be WHEN. After WHEN comes a value. If the
variable in the CASE statement has the value in the WHEN statement, the system processes
the corresponding block.
Similarly to IF, only one block can be processed in a CASE structure. You can provide a WHEN
OTHERS block, which will be processed if none of the other values for which you tested match
up. If you use a WHEN OTHERS block, it must be the last block in the structure.
CASE only tests for equality.
You cannot use AND or NOT with CASE. You may, however, group values together using OR,
as shown in the figure above.
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Unit 3: Working with Variables in ABAP
Figure 62: Loops in ABAP
There are two kinds of loop in ABAP. A DO loop is repeated indefinitely. Since endless loops
cause the program to hang, you must provide a way out of the loop. To do this, test an
attainable exit condition, and if it is met, break out of the loop with the EXIT statement. EXIT
terminates the loop processing and resumes the program after the ENDDO statement.
If you know how many loop repetitions you need, you can specify the number in the statement
DO n TIMES. n may be either a literal (as in the illustration above) or an integer variable
containing the number of repetitions. The loop will then be repeated up to this number of
times. (You can still exit the loop processing prematurely using the EXIT statement.)
DO loops are always processed at least once (at least up to the point at which you test the exit
condition). By contrast, ABAP also has WHILE loops. In this case, a logical condition is tested
at the entry to the loop. If the condition is met, the loop is processed. If it is not met, the entire
loop is skipped. After each repetition of the loop, the system tests the condition again. If the
condition is met once again, the loop processing continues. This process is repeated until the
condition is not met, when the loop processing terminates and the program resumes after the
ENDWHILE statement.
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Lesson: Working with Variables
Figure 63: System Variables
System variables are a set of variables in ABAP that provide information about the current
state of the system. They are managed by the runtime system and available to all programs at
all times. You do not have to declare the system variables in your program.
Every system variable begins with the prefix sy- . You can see the whole list in the ABAP
Dictionary if you display the data type SYST. Although there are 171 different system fields,
you do not need very many of them on a day-to-day basis. A lot of system fields are obsolete,
while many more are reserved for internal use in the runtime system. The figure shows some
of the system fields that you might use.
You can read data from the system fields at any time. You must not write your own data into
system fields. If you do, you may adversely affect the system behavior. You may also lose
your data as it could be overwritten by the runtime system at any time.
Some system fields are counters or indexes – for example, sy-index is the index of the current
loop pass in a DO or a WHILE loop. All index fields in ABAP start at 1.
The field sy-subrc is the return code field. Return codes are set by many ABAP statements to
indicate whether the statement was successful or not. When a statement is executed
successfully, sy-subrc has the value 0. If an error occurs when the statement is processed, sysubrc has a value unequal to zero. The documentation of ABAP statements describes
whether the statement sets a value of sy-subrc (and, indeed, of other system fields), and what
values are set in what situations.
As an example, let us take the SELECT statement, which reads data from the database. When
the runtime system executes a SELECT statement, it sets a value of sy-subrc. If sy-subrc is
zero, it means that at least one row was returned by the query. If there are no rows in the
database that match the query, sy-subrc has the value 4. At the same time, the statement
sets a value for the further system field sy-dbcnt. This value of this field corresponds to the
number of rows read from the database by the SELECT statement.
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Unit 3: Working with Variables in ABAP
Figure 64: Errors During Methods
So far we have seen how methods can have importing and exporting parameters so that we
can supply them with information and receive results from them. However, sometimes a
method cannot do what we asked it to do. In this case, it triggers an exception.
In the example, we called a method whose task was to divide iv_int1 by iv_int2. Since iv_int2 is
zero, this will not be possible. Dividing by zero in ABAP leads to a runtime error. To prevent
this happening, the method correctly and sensibly checks the import parameters before it
starts to do anything. When it finds out that the second value is zero, it triggers an exception.
This terminates the method, and communicates to the calling program that an error has
occurred.
If the calling program ignores the exception, a runtime exception will occur.
Figure 65: Finding Out About Errors
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Lesson: Working with Variables
To find out which exceptions a method can raise, use the object documentation in ADT and
look for the RAISING clause. This is the name of the exception, and you will need it in order to
react to the error in your program.
Figure 66: Reacting to Errors with TRY and CATCH
To react to exceptions that methods raise, use a TRY…ENDTRY block.
You write the TRY statement before the method call that could potentially raise an exception.
Next comes the method call itself and any further statements that should only be processed if
the call is successful (for example, displaying a valid result).
After that comes the CATCH statement. In it, you write the name of the exception to which
you want to react. If the method triggers this exception, the program will jump into the
corresponding CATCH statement and process all of the statements from there until the end
of the block, which is marked in this case by the ENDTRY statement.
You can write more than one CATCH block to react differently to different errors.
Note that not all methods use this technique for processing their exceptions. There is another,
older, technique that you will learn more about later on.
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Unit 3: Working with Variables in ABAP
Simple String Processing
Figure 67: String Templates
String templates are similar to literals but provide you with enhanced features. In their
simplest form, they are a string literal, enclosed between vertical pipes. However, you can also
use variables as placeholders, which are then replaced at runtime by the value of the variable.
To use a variable as a placeholder in a string template, use curly braces to enclose the name
of the variable. The spaces after the opening brace and before the concluding brace are
mandatory.
The space before the opening brace in the example is optional, and will be displayed as a
blank space at runtime.
There is a wide range of formatting directives that you can use in string templates. For full
information, place the cursor inside a string template in the Editor and press F1. This displays
the full documentation for string templates.
Figure 68: Joining Strings
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Lesson: Working with Variables
You can join fields together using the concatenation operator &&. You can join any
combination of variables and literals, and the system assigns the result to the variable that
you entered on the left-hand side of the equals sign.
The constituent parts of the expression are joined with no space or other separator between
them. If you need spaces or another separator character, you must remember to insert it
yourself as part of the expression as shown in the second example.
If you want to join several elements together and insert a separator character between each
pair, you can use the older CONCATENATE statement. In this example, you would write:
CONCATENATE gv_part1 gv_part2 INTO gv_string SEPARATED BY space.
If you use the CONCATENATE statement, you may not use numeric fields (types I, P,
DECFLOAT16, DECFLOAT34 or F). This restriction does not apply to the && operator.
Figure 69: Splitting Strings
To split a string into its constituent parts, use the SPLIT statement. In it, you specify a
separator character at which the string should be split. This splitting Hello World at the space
character yields two results Hello and World.
In order to use SPLIT correctly, you must know how many fragments will result from the
operation and specify exactly this number of fields. If you specify too few fields, the entire
remainder of the source string will be placed into the last field. If the last field has a fixed
length and is not long enough, the string will be truncated. If you specify too many fields, the
surplus fields remain untouched by the operation.
You may not use numeric fields in the SPLIT statement.
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Unit 3: Working with Variables in ABAP
Figure 70: Finding Text
To search a variable for a particular pattern, use the find function. This has the following
arguments:
val: The field in which you want to search
sub: The string for which you want to search
occ: The occurrence of the string for which you are searching. 1 is the first occurrence, 2 the
second, and so on. If you want to search backwards from the end of the string, use negative
values.
case: Use this argument to specify whether the case of the search string should match
exactly. X means match case, space means that the case is not important.
The function returns the offset of the position at which the search string begins. This is its
position in the string. An offset of 0 means that the search string occurs at the very beginning
of the string. Here, the offset is 6. An offset of -1 indicates that the search string was not
found.
Figure 71: Replacing Text
The replace function is similar to find. However, the return value in this case is the modified
string.
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Lesson: Working with Variables
You use the occ argument to specify which occurrence of the search string should be
replaced. Once again, positive values count from the beginning of the string, negative values
from the end. A third option with replace is to specify occ = 0. This replaces all occurrences of
the search string.
Working wIth Non-Variable Data Objects
Figure 72: Literals
Not all of the information that you use in a program is contained in variables. In the figure are
some examples of literals. Literals have no name, which means there is no way of changing
their value when the program is running.
To use an integer as a literal, you just write the number. However, numbers containing
decimal places must be enclosed in quotation marks, otherwise the decimal point would be
interpreted by the system as the end of the statement.
Character strings that you use as literals must be enclosed in quotation marks to distinguish
them from the names of variables.
Literals must begin and end in the same line of code.
Figure 73: Constants
Constants allow you to create a named data object that represents a particular value.
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Unit 3: Working with Variables in ABAP
The advantage of this is that the name can reflect the particular purpose of the value, making
the program easier to understand.
You define constants using the CONSTANTS statement. The syntax of CONSTANTS is almost
identical to that of DATA. You must, however, set a value for the constant using the VALUE
addition.
You may not change the value of a constant during the program.
Figure 74: Text Elements
If you use text literals directly in your source code, they cannot be discovered and translated
using SAP's built-in translation tools. Consequently, if you are working in a multilingual
environment, you must store texts so that they can be translated. In ABAP programs, this
means using the text pool.
In the text pool, you assign a three-character ID to a text and store it in the original language
of your program. The text pool is accessible to the translation tools, so your texts can then be
translated into other languages as required.
To open the text pool of an ABAP program, right-click on the source code in the Editor and
choose Open Other -> Text Elements . As there is no native Eclipse editor for the text pool, the
text elements appear in a SAP GUI window.
Text elements are only visible once you have activated them.
LESSON SUMMARY
You should now be able to:
●
Declare appropriately typed variables in ABAP
●
Work with variables in ABAP
●
Work with non-variable data objects
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64
Unit 3
Learning Assessment
1. In ABAP, every variable has a fully-defined type.
Determine whether this statement is true or false.
X
True
X
False
2. [Question]Which of the following data types are incomplete (and therefore support the
LENGTH addition to the DATA statement)?
Choose the correct answers.
X
A STRING
X
B P
X
C I
X
D C
3. Below you will see some data types and the initial values of variables that are declared
with that type. Which of the combinations of data type and initial value are correct?
Choose the correct answers.
X
A Type P. Initial value undefined.
X
B Type I. Initial value 0
X
C Type C. Initial value a single space.
X
D Type D. Initial value ‘19700101’.
4. In an IF structure with multiple ELSEIF statements, more than one branch will be
processed if more than one prerequisite condition is met.
Determine whether this statement is true or false.
X
True
X
False
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65
Unit 3: Learning Assessment
5. Which of the following statements are true about CASE structures?
Choose the correct answers.
X
A You can link values with AND.
X
B You can only test values for equality.
X
C You can link values with OR.
X
D WHEN OTHERS is compulsory.
6. You are using TRY..ENDTRY to handle errors. If you want to handle any error in a single
CATCH statement, which statement would you use?
Choose the correct answer.
X
A CATCH OTHERS.
X
B CATCH cx_root.
X
C CATCH cx_static_check.
X
D CATCH cx*.
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66
Unit 3
Learning Assessment - Answers
1. In ABAP, every variable has a fully-defined type.
Determine whether this statement is true or false.
X
True
X
False
2. [Question]Which of the following data types are incomplete (and therefore support the
LENGTH addition to the DATA statement)?
Choose the correct answers.
X
A STRING
X
B P
X
C I
X
D C
3. Below you will see some data types and the initial values of variables that are declared
with that type. Which of the combinations of data type and initial value are correct?
Choose the correct answers.
X
A Type P. Initial value undefined.
X
B Type I. Initial value 0
X
C Type C. Initial value a single space.
X
D Type D. Initial value ‘19700101’.
4. In an IF structure with multiple ELSEIF statements, more than one branch will be
processed if more than one prerequisite condition is met.
Determine whether this statement is true or false.
X
True
X
False
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67
Unit 3: Learning Assessment - Answers
5. Which of the following statements are true about CASE structures?
Choose the correct answers.
X
A You can link values with AND.
X
B You can only test values for equality.
X
C You can link values with OR.
X
D WHEN OTHERS is compulsory.
6. You are using TRY..ENDTRY to handle errors. If you want to handle any error in a single
CATCH statement, which statement would you use?
Choose the correct answer.
X
A CATCH OTHERS.
X
B CATCH cx_root.
X
C CATCH cx_static_check.
X
D CATCH cx*.
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68
UNIT 4
Debugging in ABAP
Lesson 1
Debugging in ABAP
70
UNIT OBJECTIVES
●
Debug ABAP code
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69
Unit 4
Lesson 1
Debugging in ABAP
LESSON OBJECTIVES
After completing this lesson, you will be able to:
●
Debug ABAP code
Debugging an ABAP Program
Figure 75: The Need for Debugging
There is no way around the fact that errors occur in programs. However, they manifest
themselves in different ways. When a user starts a faulty application, it may crash, something
unexpected may happen, or nothing at all may happen. From the user's point of view, at user
interface level, it is impossible to say just how and why this error occurred.
As a developer, you now need to examine the program more closely – line-by-line in fact – to
establish just what statements and combinations of values in the different program variables
caused the error. This is where the Debugger comes in.
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Lesson: Debugging in ABAP
Figure 76: Starting the Debugger
To debug an ABAP program, you set a breakpoint then run the program normally. When the
program reaches the breakpoint, the system interrupts it and opens the ABAP Debug
perspective in ADT. You can then execute each subsequent statement individually to see what
effect it has on the program. You can also inspect the contents of all of the variables in the
program to see if any of the values are unexpected.
Breakpoints are user-specific and are persistent – they remain active even after you have
logged off from ADT and back on again. To prevent the debugger from starting at a breakpoint
you must either delete it (using the Toggle Breakpoint function) or deactivate it using the
corresponding function in the context menu.
Figure 77: The Debug Perspective in ADT
When you debug an ABAP program using ABAP Development Tools, you use the Debug
perspective. This is a customized version of the standard Eclipse Debug perspective, and it
contains views and functions that are particularly important for debugging.
The most important is, of course, the Source Code view. This displays the source code of your
program and highlights the current position in the program. Also crucial is the Variable view
(not selected in the figure, The Debug Perspective in ADT), which shows the current values of
variables. Next to it above, you see the Breakpoints view. Breakpoints are points in the
program at which normal processing is interrupted and the system shows you the Debugger
so that you can analyze the state of the program at exactly that moment.
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Unit 4: Debugging in ABAP
Figure 78: Using the Debugger
When the debugger starts, you will see the current line highlighted in the source code display.
This is the next line that will be executed when you restart the program.
In the top right-hand corner of the screen are two important views – variables and
breakpoints. In the variable view, you can see all of the global variables in the program listed
under Globals. As there may be a lot of these variables, you can also display the values of
selected variables that you need to inspect by entering the name of the variable in the <Enter
variable> field. You can also display a variable in this area by placing the cursor on it anywhere
in the source code display and pressing Ctrl + Shift + I.
As well as setting breakpoints in the Editor, you can also set more in the Debugger. Place the
cursor in the left-hand margin of the source code editor, right-click, and choose
Toggle
Breakpoint .
To continue processing the program you have two options:
●
Execute the next statement by pressing the F5 key. This executes the next statement and
stops again.
●
Continue the program by pressing the F8 key. This continues running the program until it
encounters another breakpoint. If there are no more breakpoints, the program continues
until it is finished.
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Lesson: Debugging in ABAP
Figure 79: Special Breakpoints
Breakpoints that stop at a particular line are not always useful. Sometimes you know that you
want to stop at a particular statement without necessarily knowing where the statement
occurs in your source code. You can do this by setting a statement breakpoint. To do this,
switch to the Breakpoints tab in the Debug perspective and click on the menu button
highlighted in the figure. Choose
Add Statement Breakpoint .
A dialog box appears in which you can choose the relevant ABAP statement or function. The
statement for an authorization check is AUTHORITY-CHECK, so you would highlight this and
choose OK. Your program will then stop whenever it encounters an authorization check.
Significantly, this does not only apply to the program that you are currently working in, but to
all modularization procedures (like methods) that it calls.
Figure 80: Watchpoints
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Unit 4: Debugging in ABAP
If an unexpected value of a variable is causing you problems, you can track its value during the
course of the program using a watchpoint.
To set a watchpoint on a variable, right-click it in the source-code display and choose
Watchpoint . This creates a watchpoint, which you can then see in the Breakpoints view.
Whenever the value of the variable changes, the program will stop in the Debugger.
Set
If you only want to stop when the variable reaches a particular value, you can set a condition.
For example, you could enter > 200, and the program would only be interrupted whenever the
value of gv_result changed to a value greater than 200.
Figure 81: Altering the Program Flow
When you are debugging a program, you will sometimes want to see what the program would
have done under different circumstances. There are two ways that you can do this – by
changing the value of a variable and by jumping to a particular line in the program.
Suppose you have just identified the source of an error in your program – you realize that you
should have initialized the value of a variable using CLEAR, but you forgot. To see whether the
program would have worked properly in that case, you can change the value of the variable
directly in the Debugger. The program then continues with the value that you set.
To change the value of a variable, right-click it in the variable display and choose
Value. A dialog box appears in which you can then enter the new value.
Change
In another example, you may have written a branch of code that is processed in response to a
particular error. However, you cannot accurately produce the error situation in your test. You
can, however, still execute the code to see how it reacts. To do this, position the cursor on the
line at which you want to start and choose the Jump To Line function. The system resumes
processing at the line in which you positioned the cursor.
You should only ever use these functions in a development system, as theoretically they can
cause data inconsistency and present a security risk.
LESSON SUMMARY
You should now be able to:
●
Debug ABAP code
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74
Unit 4
Learning Assessment
1. Which of the following tasks can you perform both in the ABAP Perspective and in the
Debug Perspective?
Choose the correct answers.
X
A Set a watchpoint.
X
B Set a breakpoint for all occurrences of a particular statement.
X
C Set a breakpoint on a particular line.
X
D Activate changes to your program.
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75
Unit 4
Learning Assessment - Answers
1. Which of the following tasks can you perform both in the ABAP Perspective and in the
Debug Perspective?
Choose the correct answers.
X
A Set a watchpoint.
X
B Set a breakpoint for all occurrences of a particular statement.
X
C Set a breakpoint on a particular line.
X
D Activate changes to your program.
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76
UNIT 5
Calling Reusable
Procedures
Lesson 1
Understanding Reuse
78
Lesson 2
Using Classes
82
Lesson 3
Using Function Modules
86
UNIT OBJECTIVES
●
Understand the benefits of reusing procedures
●
Use an existing global class
●
Use existing function modules
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77
Unit 5
Lesson 1
Understanding Reuse
LESSON OBJECTIVES
After completing this lesson, you will be able to:
●
Understand the benefits of reusing procedures
The Benefits of Reuse
Figure 82: Software Development Without Reuse
Software development today attaches great importance to reusing components wherever
possible. The figure above shows what two programs would look like if they do not reuse
anything. In program 1, code block B occurs identically in two different places in which it is
required. To make matters worse, it is also duplicated at other points in other programs.
Programs that are developed in this way are larger than they need to be. Each time the coding
block is duplicated, the program size increases by the size of the block. There is also no way of
knowing just how often and at what points the block has been used. Consequently, if the
behavior of "code block B" needs to be changed, a developer must examine a large number of
programs to establish just which parts of which programs need to be adjusted.
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Lesson: Understanding Reuse
Figure 83: Software Development with Local Reuse
Now let us look at what happens if code block B is stored as a reusable procedure within a
single program. Instead of duplicating the coding, it is stored once only in the program and
called when required. This makes the code easier to maintain, because it only exists once, and
makes the program itself easier to understand because it contains fewer lines of code.
Figure 84: Software Development with Global Reuse
If a particular block of code is required by more than one program, you can store it centrally in
a special container. From here, it is available to all other programs in the system. Global reuse
in ABAP is implemented using methods and function modules. The container for a method is
a global class, and the container for a function module is a function group.
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Unit 5: Calling Reusable Procedures
Figure 85: Calling Reusable Procedures
When you use reusable procedures, there are certain principles that apply:
The container is loaded into the memory area of the calling program. Each program that calls
a reusable procedure has its own copy of the container in its own program memory. This
makes it impossible for one program to interfere with the running of another.
One side cannot see the data of the other. A program that calls a reusable procedure has its
own data. Equally, the called procedure – and its container – may have their own data.
Importantly, neither side can access the data of the other side directly. Were this to be
allowed, it would become impossible to guarantee the consistency of the data. Keeping data
consistent requires that access to that data is restricted as much as possible.
Data can only be exchanged using parameters. In order to perform a function, reusable
procedures require information from their caller. They also need to deliver results that they
have calculated. Data exchange between a procedure and its caller is regulated by the
definition of parameters as part of the procedure definition. This is a kind of contract between
the procedure and a potential caller that describes the information that a procedure requires
and the results that it can provide. Only this information can be exchanged between the caller
and the called procedure.
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80
Lesson: Understanding Reuse
Figure 86: Global Reusable Procedures in ABAP
In ABAP, for historical reasons, there are two different kinds of global reusable procedure.
Methods are functions that belong to a global class.
Function modules are functions that are logically related and belong to the same function
group.
As a rule, if you are creating a new application, you should use classes and methods. However,
for certain tasks, you cannot replace function modules. See the documentation of the CALL
FUNCTION statement for full details.
LESSON SUMMARY
You should now be able to:
●
Understand the benefits of reusing procedures
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81
Unit 5
Lesson 2
Using Classes
LESSON OBJECTIVES
After completing this lesson, you will be able to:
●
Use an existing global class
Understanding the Definition of a Class
Figure 87: Identifying Global Classes
There are different ways to locate a particular class. If the class belongs to one of your favorite
packages, you can expand the object list until you find it. You will find classes in the branch
Source Code Library.
If you know part of the name of the class, you can use the key combination Ctrl + Shift + A to
open the Open ABAP Repository Object dialog box. Alternatively, you can use Ctrl + H to open
the Search dialog. In the search dialog box, you have the added feature that you can restrict
the search to your favorite packages.
If you have a program open in the Editor that uses the class, you can navigate to it using the
F3 key.
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Lesson: Using Classes
Figure 88: Inspecting the Structure of a Class
When you open a class in ADT, you see its source code in the Editor. Later on, you will learn
how to create your own classes. However, for now you are just going to use functions
provided by an existing global class in the form of static methods.
One of the main principles of object-oriented design is to restrict access to the components of
classes. By doing so, a class can, for example, stipulate that data may only be changed from
within the class itself and not from outside. The aim of this is to ensure data consistency.
Likewise, access to functions may also be restricted.
If you look at the class definition, you will see three statements: PUBLIC SECTION,
PROTECTED SECTION, and PRIVATE SECTION. When you write a program that is going to
use the class, you only have access to the PUBLIC SECTION.
In the public section of this class, there are four CLASS-METHODS statements. Each of these
defines a static method, which is a function like the ones you have used already in this class.
The four methods in this class implement the four arithmetic functions addition, subtraction,
multiplication, and division.
You can see the components of the class both in the Editor and in the Outline view. The green
dot in the outline view indicates that the methods are public. To navigate from an element in
the outline view to the actual method definition in the source code, right-click the component
name and choose Navigate To Declaration.
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Unit 5: Calling Reusable Procedures
Figure 89: Signature of a Method
Methods can have parameters and exceptions which, together, make up their signature.
IMPORTING parameters are values that are passed from the caller to the method. They may
be obligatory (default) or optional. The method is not allowed to change the values of
importing parameters. A method may have any number of importing parameters.
EXPORTING parameters are values that are passed from the method back to the caller. They
are always optional. A method may have any number of exporting parameters.
CHANGING parameters are values that are passed from the caller to the method. They may
be obligatory (default) or optional. The method is allowed to change the values and, at the end
of the method, the changed value is passed back to the program. A method may have any
number of changing parameters.
A RETURNING parameter is a special return value that can be passed from a method back to
its caller. The caller can assign the value directly to a variable or use it in an expression
without first having to receive it via the signature in the method call. A method may only have
one returning parameter.
RAISING is the section of the method signature that lists the exceptions that the method can
trigger in case of an error.
Note that instead of RAISING, method can – for historic reasons – have an EXCEPTIONS
section. This is for methods that use an older form of exception handling. Since this is very
commonly found in function modules and works in exactly the same way for function modules
and methods alike, we will deal with this technique in the section on function modules.
When a program and a method exchange information, the system does not normally pass
copies of the data. Instead, it merely informs the method of the address in memory of the
corresponding variables. This approach is more efficient in terms of performance and
memory consumption and is called call by reference. Methods can pass copies of the data if
necessary. This is called call by value.
Note that returning parameters are always called by value.
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Lesson: Using Classes
Figure 90: Calling a Public Static Method
This code example shows how to call the method defined in the previous figure by observing
the following rules:
You must supply all obligatory parameters: Since none of the importing parameters of the
method were defined with the words OPTIONAL or DEFAULT, you can infer that they are
obligatory and must therefore be supplied in the EXPORTING section of the method call. The
parameter name is on the left-hand side of the equals sign, the variable that you want to pass
to the method on the right.
You must use corresponding types in the method call: The importing parameters iv_int1 and
iv_int2 are integers, therefore the variables that you pass to them must also be integers. The
parameter ev_result has type P with length 16 and 2 decimal places, and the variable gv_result
must also have this type. You could define the variable using the built-in ABAP type P, but
since the class already provides a corresponding type, it is easier to use it.
You should take all possible exceptions into account: The signature of the method indicated
that the exception cx_s4d400_zerodivide could occur. Therefore you should write a TRY…
CATCH…ENDTRY block so that the error will be handled if it occurs. If there is no
corresponding catch block and the method triggers the exception, the program will terminate
with a runtime error.
LESSON SUMMARY
You should now be able to:
●
Use an existing global class
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Unit 5
Lesson 3
Using Function Modules
LESSON OBJECTIVES
After completing this lesson, you will be able to:
●
Use existing function modules
Function Groups And Function Modules
Figure 91: Function Groups and Function Modules
A function group is a container program in which you can create function modules. Each
function module provides a single reusable function that can be called by any other program,
function module, or method in the system. Function groups can also contain data that can be
accessed by all of the function modules in the group. The data is, however, not visible outside
the function group.
People often ask why ABAP has both function groups with function modules and classes and
methods. The answer, as so often in ABAP, is historical. Function modules predate classes
and methods by several years and were introduced to allow developers to create globally
reusable units of code. Classes and methods came along later when object-orientation was
introduced into ABAP. At one level, you can use methods just like function modules (a
function module is almost identical to a public static method in a class). However, objectorientation allows you to do much more than you can with function modules, and you will
learn about this later on.
Even though SAP would recommend today that you work with classes and methods, you do
not always have the choice – many essential functions in the SAP System are implemented
using function modules, and where these exist, it makes sense to reuse them. This applies
both to simple tasks in ABAP programs (for example, there is a function module that allows
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Lesson: Using Function Modules
you to find out on which day of the week a particular date falls, another for performing
currency conversions, and so on) and to the definition of OData Services in SAP Gateway,
where an existing function module might provide a useful Create or Update operation that you
would otherwise have to code yourself.
Figure 92: Interface of a Function Module
Function modules can have parameters and exceptions which, together, make up their
interface. Interfaces of function modules and signatures of methods are essentially the same,
but use different terms. This is because the term interface was already in use for function
modules when classes were introduced into ABAP. Unfortunately, the term interface has a
different meaning in object-orientation and SAP adopted the industry standard term
signature for method parameters.
IMPORTING parameters are values that are passed from the caller to the function module.
They may be obligatory (default) or optional. The function module is not allowed to change
the values of importing parameters. A function module may have any number of importing
parameters.
EXPORTING parameters are values that are passed from the function module back to the
caller. They are always optional. A function module may have any number of exporting
parameters.
CHANGING parameters are values that are passed from the caller to the function module.
They may be obligatory (default) or optional. The function module is allowed to change the
values and, at the end of the module, the changed value is passed back to the program. A
function module may have any number of changing parameters.
EXCEPTIONS is the section of the interface in which the function module lists the errors that it
can trigger. These are not exception classes that you handle with TRY… CATCH… ENDTRY
control structures, but an older form of exception handling. Function modules usually have
this kind of exception. It is possible (but unlikely) that you will encounter more modern
function modules that make use of the newer exception handling concept.
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Unit 5: Calling Reusable Procedures
Figure 93: Calling a Function Module
You call a function module using the CALL FUNCTION statement. In it, you must specify the
name of the function module in quotes, and in upper case. The code completion is available to
help you – type the beginning of the name of the function module and press Ctrl + Space to
see a list of function modules that fit the pattern you entered. From the list, press Shift +
Enter to insert the rest of the name of the function module and the full interface definition into
your program.
Since the name of a function module has to be unique system-wide, you do not need to use
the name of the function group in the CALL FUNCTION statement.
Figure 94: The System Field sy-subrc
One very important system field is sy-subrc. This is the return code field, which tells us if a
particular statement was successful. If sy-subrc has the value zero, the statement was
successful. If the value if sy-subrc is unequal to zero, this means that an error has occurred.
You can query the value of sy-subrc using an IF statement to find out if the preceding
statement was successful or not.
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Lesson: Using Function Modules
To find out whether a particular statement sets a value of sy-subrc, and what values are set in
what circumstances, consult its documentation.
Figure 95: sy-subrc and Function Modules
Function modules also use sy-subrc to communicate to their caller that an error occurred.
When a function module ecounters an error, it reacts by stopping processing. This is because
it cannot normally know how to resolve the error by itself. It then communicates the fact that
an error occurred to its calling program. It does this using the system field sy-subrc.
When you call a function module, you list its possible exceptions in the EXCEPTIONS section
of the CALL FUNCTION statement. Here, you assign a numeric value to each exception that
you need to process. If the corresponding exception occurs, the system assigns that numeric
value to sy-subrc. Thus, by finding out the value of sy-subrc after the function call, you will
know whether the statement was successful – in this case, sy-subrc will have the value zero –
or that it contained an error, in which case sy-subrc will have the corresponding numeric
value.
When you assign values to exceptions, you can use the same value for more than one
exception. This allows you to define groups of exceptions that should all be processed in the
same way.
There is also a generic exception OTHERS. It does not occur in the interface of function
modules, but you can always use it in the EXCEPTIONS part of the CALL FUNCTION
statement. OTHERS stands for any exception of the function module that you have not listed
explicitly.
If a function module raises an exception and you do not catch it, either explicitly by name or
implicitly using OTHERS, the system will trigger a runtime error.
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Unit 5: Calling Reusable Procedures
Figure 96: Error Handling Using Classic Exceptions
If a function module triggers an exception, the same thing happens as with class-based
exceptions, namely that the function module terminates and control returns to the calling
program.
The remaining code in the function module is not processed. The system places the numeric
value corresponding to the exception (you did this in the CALL FUNCTION statement) in the
system field sy-subrc. By finding out the value of sy-subrc, you can find out whether the
function call was successful or not.
LESSON SUMMARY
You should now be able to:
●
Use existing function modules
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Unit 5
Learning Assessment
1. Which of the following are modularization techniques in ABAP?
Choose the correct answers.
X
A Stored procedures.
X
B Function modules
X
C Include programs.
X
D Methods of global or local classes.
2. Programs exchange data with modularization units by means of an interface. It is not
possible for one side to directly access the data of the opposite side.
Determine whether this statement is true or false.
X
True
X
False
3. The name of a function module need only be unique within its function group.
Determine whether this statement is true or false.
X
True
X
False
4. Which of the following statements regarding the interface of a function module are true?
Choose the correct answers.
X
A All of the changing parameters are mandatory.
X
B There may be any number of importing parameters.
X
C There must be exactly one exporting parameter.
X
D Exporting parameters are always optional.
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Unit 5: Learning Assessment
5. Which of the following happens when a function module triggers an exception?
Choose the correct answer.
X
A The function module continues processing and triggers the exception at the end of
the module.
X
B The function module triggers the exception and, at the same time, a runtime error.
X
C The function module terminates and returns control to its calling program.
X
D The function module checks whether any other exceptions need to be triggered
along with the first error that has already been discovered.
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Unit 5
Learning Assessment - Answers
1. Which of the following are modularization techniques in ABAP?
Choose the correct answers.
X
A Stored procedures.
X
B Function modules
X
C Include programs.
X
D Methods of global or local classes.
2. Programs exchange data with modularization units by means of an interface. It is not
possible for one side to directly access the data of the opposite side.
Determine whether this statement is true or false.
X
True
X
False
3. The name of a function module need only be unique within its function group.
Determine whether this statement is true or false.
X
True
X
False
4. Which of the following statements regarding the interface of a function module are true?
Choose the correct answers.
X
A All of the changing parameters are mandatory.
X
B There may be any number of importing parameters.
X
C There must be exactly one exporting parameter.
X
D Exporting parameters are always optional.
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Unit 5: Learning Assessment - Answers
5. Which of the following happens when a function module triggers an exception?
Choose the correct answer.
X
A The function module continues processing and triggers the exception at the end of
the module.
X
B The function module triggers the exception and, at the same time, a runtime error.
X
C The function module terminates and returns control to its calling program.
X
D The function module checks whether any other exceptions need to be triggered
along with the first error that has already been discovered.
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94
UNIT 6
Using Structures
Lesson 1
Using Structures
96
UNIT OBJECTIVES
●
Use structure types
●
Use structure variables
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Unit 6
Lesson 1
Using Structures
LESSON OBJECTIVES
After completing this lesson, you will be able to:
●
Use structure types
●
Use structure variables
Working with Structure Types
Figure 97: The Story So Far – Simple Variables
Up until now, you have been using simple variables, each of which can store a single piece of
information. Here, for example, there are two variables, one for an airline code, the other for a
flight number.
These two variables are completely independent of one another, and are therefore not
suitable for storing different pieces of information that belong together. Here, the flight
number only makes sense in conjunction with the airline. In other examples, you may need to
read a record from the database and need to hold all of this information together. In ABAP,
the solution is to use a structure.
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Lesson: Using Structures
Figure 98: Using a Structure
A structure is an ABAP variable with a name and a structured type. It is subdivided into
components, each of which also has a name and a type. You can address both the structure
as a whole and the individual components. Importantly, you can use each component in
exactly the same way that you would use a standalone elementary variable.
You will often find structured types in the ABAP Dictionary, and sometimes in global classes.
You may also define them locally in a program.
Figure 99: Types in the ABAP Dictionary
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Unit 6: Using Structures
Figure 100: Type Definition in the ABAP Dictionary
There are two ways to edit a type definition in the ABAP Dictionary: You can either use the
source-code based Eclipse editor or the classic SAPGUI editor. In ABAP systems with Release
7.50 or higher, the source code editor appears by default. If you want to switch to the
SAPGUI-based editor, right-click in the editor and choose
Open With -> SAPGUI .
In either case, the structure definition consists of a series of components. Each component
has a name, which must be unique within the structure, and a type. The type is usually a data
element, but can be a built-in ABAP Dictionary type.
A structure can have any number and combination of components according to your
requirements. Structures can contain other structures as components (nested structures).
Figure 101: Type Definition in an ABAP Program
You can also define structured types in an ABAP program using the TYPES statement. The
structure definition begins with the statement TYPES BEGIN OF <name> and ends with
TYPES END OF <name>. In between, you name each component and specify its type. Here,
each of the types is a data element from the ABAP Dictionary. However, you could also use
built-in ABAP types.
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Lesson: Using Structures
Declaring and Using Structures
Figure 102: Working with Structure Components
To declare a structured variable, use the DATA statement and specify a type that is a
structure. This may come from the ABAP Dictionary (as in this case) or be a local typed
defined in your program. Note that if you use a local type, the type definition must come
before the point in your program at which you attempt to use it.
In a freshly declared structure, each component has the initial value that corresponds to its
type.
To address a single component of a structure, you write the name of the structure variable (in
this case gs_flight), followed by a dash (more correctly known as the component selector),
followed by the name of the component. You can do this anywhere in an ABAP program where
a variable is allowed.
ADT provides code completion for structures: After the dash, you can press Ctrl + Space to
open the code completion dialog and see what components the structure has.
The statement CLEAR <structure>-<component> resets the component to its own initial
value. The statement CLEAR <structure> resets the value of each individual component to its
own initial value.
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Unit 6: Using Structures
Figure 103: Filling a Whole Structure Using VALUE
If you want to fill a whole structure, you can address each component individually as you saw
in the previous figure. However, you can also use a VALUE expression to fill the structure. The
expression fills the structure and assigns the filled structure to a variable – in this case
gs_flight. The pound sign (#) tells the ABAP system to create a structure with the same type
as the target variable gs_flight. In the parentheses, you list the components of the structure
that you want to fill (it does not have to be all of them) and assign a value to them. The value
can be either a literal or the contents of a variable.
When you fill a structure in this way, the runtime system deletes all existing values from the
structure before refilling it with the values from your expression.
Figure 104: Using the BASE Expression
To avoid the entire structure being deleted when you use the VALUE expression, you can add
the BASE expression to it. The meaning of this expression is: "Take the structure gs_flight,
change the value of the seatsocc component, and assign the reult back to the structure
gs_flight".
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Lesson: Using Structures
Figure 105: Copying Data Between Structures
In ABAP, you may only copy the contents of one structure directly into another structure
using the notation <target structure> = <source structure> if the two structure types are
compatible. This is generally only the case if both structures have the same type. If the
structures have different types, as is the case here, you will see a syntax error if you attempt
the assignment.
When you copy data between structures, you usually want to copy information from one field
into the corresponding field of the target structure – carrid to carrid, connid to connid, and so
on. To achieve this in ABAP, use the CORRESPONDING expression. This assigns values from
gs_flight to the corresponding, that is, identically named components of gs_conn. You must
remember the following points:
●
The fields must have identical names.
●
The components do not have to be in the same position or sequence in the two structures.
●
If the fields have different types, ABAP converts the value according to a predefined set of
rules. The possible outcomes of this conversion are:
●
The conversion is successful and no data is lost (for example, when an integer is converted
into a string).
●
The conversion is technically successful, but data gets lost on the way (for example, if you
assign the contents of a character field to another, shorter, character field, the contents of
the field will be truncated).
●
The conversion fails (for example, if you assign a sequence of letters and symbols to a
numeric field).
When you use the CORRESPONDING expression in this way, the target structure is initialized
before being refilled with the result of the expression. If you want to keep the contents of the
target structure and just merge new values into it, you use the BASE expression in exactly the
same way as you learned with VALUE.
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Unit 6: Using Structures
Note that in older programs you will also see the statement MOVE-CORRESPONDING
<source> TO <target>. This statement has the same effect as <target> = CORRESPONDING
#( BASE <target> SOURCE ).
Figure 106: Merging Data from Multiple Structures
Using the CORRESPONDING and BASE functions, you can combine data from multiple
structures into a single large structure. This example fills the structure gs_info first with the
components carrid and carrname from gs_carrier, then with carrid, connid, and fldate from
the structure gs_flight. Note that CARRID is filled twice, because it occurs in both source
structures.
Figure 107: Structures In The Debug Perspective
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102
Lesson: Using Structures
To display a structure in the debug perspective, you double-click the name of the structure as
you would any normal variable. When the structure appears in the Variables view, click the
arrow to expand the display and show the individual components.
You can change the value of individual components in exactly the same way as you would with
any other variable.
If you double-click the name of a structure component such as gs_connection-carrid, the
individual component appears by itself in the Variables view. You can then treat it like any
other variable.
LESSON SUMMARY
You should now be able to:
●
Use structure types
●
Use structure variables
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103
Unit 6
Learning Assessment
1. You can declare structure types both in the ABAP Dictionary and in an ABAP program.
Determine whether this statement is true or false.
X
True
X
False
2. You want to copy data between two structures. The structures have different types, but
some of the components have identical names. The target structure already contains
data, which you do not want to lose during the copy operation. Which of the following
functions would you use?
Choose the correct answer.
X
A VALUE with BASE.
X
B VALUE without BASE.
X
C CORRESPONDING with BASE.
X
D CORRESPONDING without BASE.
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104
Unit 6
Learning Assessment - Answers
1. You can declare structure types both in the ABAP Dictionary and in an ABAP program.
Determine whether this statement is true or false.
X
True
X
False
2. You want to copy data between two structures. The structures have different types, but
some of the components have identical names. The target structure already contains
data, which you do not want to lose during the copy operation. Which of the following
functions would you use?
Choose the correct answer.
X
A VALUE with BASE.
X
B VALUE without BASE.
X
C CORRESPONDING with BASE.
X
D CORRESPONDING without BASE.
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105
UNIT 7
Using Internal Tables
Lesson 1
Internal Table Types
107
Lesson 2
Working with Internal Tables
113
UNIT OBJECTIVES
●
Understand internal table types
●
Work with internal tables
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Unit 7
Lesson 1
Internal Table Types
LESSON OBJECTIVES
After completing this lesson, you will be able to:
●
Understand internal table types
Attributes of Internal Tables
Figure 108: Structures and Internal Tables
While structures are useful in ABAP for managing a single data record, you often need to
manage several related data records in a single variable. This is necessary, for example:
●
When you read multiple records from the database and need to store them in your
program.
●
When the user has entered more than one line of data on the user interface (for example,
line items in a purchase order).
●
When you want to display multiple records in a table on the user interface.
●
When you need to export data to a file.
In ABAP, you use internal tables whenever you need to manage more than a single line of
data. As such, they take on the role that arrays, chained lists, recordsets, or collections play in
other languages.
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Unit 7: Using Internal Tables
One of the biggest advantages of internal tables is that the runtime system takes care of
memory management automatically. When you declare an internal table, the runtime system
calculates an appropriate initial memory allocation for it. When you add more data to the
table, the table grows automatically. When you empty the table, the system automatically
releases excess memory. In other words, you do not have to worry about managing the size of
the table yourself. However, this does not mean that the size of an internal table has no limits.
If the table requires more memory than the program session is allowed to use, a runtime error
will occur.
Every internal table is characterized by its line type, its access type, and its key attributes.
Figure 109: Line Type of an Internal Table
The line type of an internal table describes what columns the table has. In the majority of
cases, the line type is a structure, which means that every line in the internal table contains a
column with the name and type of the corresponding structure component. As well as being a
simple field, a column of an internal table could be a nested structure or even a nested
internal table.
Sometime, instead of using a structure, you will need an internal table with just a single
column. In this case, the column does not have a name in the same way that a component of a
structure does, but ABAP assigns it the pseudo-name table_line.
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Lesson: Internal Table Types
Figure 110: Access Type of an Internal Table
Every internal table has one of three access types. The access type determines how data is
stored in the table and, based on how it is stored, how the system reads the table to retrieve
the data. The different approaches to retrieving the data can make significant performance
differences.
In a standard table, the contents are not stored in a particular sort order. By default, new
records are appended to the end of the table. In order to retrieve data from the table, the
system must read it sequentially, which can lead to long retrieval times if the table is very
large.
In a sorted table, the contents of the table are always sorted according to the table key in
ascending order. When you insert a new record into the table, the system ensures that it is
placed at the correct position. Since the data is always sorted, the system can retrieve
records more efficiently than from a standard table (as long as you follow particular rules).
Hashed tables are managed using a special hash algorithm. This ensures that the system can
retrieve records very quickly even if the table is extremely large. However, this performance
gain only works in very particular cases.
In this class, we will concentrate on standard tables. The other kinds of tables and how to
program efficiently with internal tables is covered in the Advanced ABAP class.
Figure 111: Key Attributes of Internal Tables
Every internal table has a key. In standard tables, the key does not play a particularly
significant role. For sorted and hashed tables, the key is very important as it determines the
way in which the data will be managed in the table. Crucially, sorted and hashed tables are
only faster for data retrieval if you use the key properly.
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Unit 7: Using Internal Tables
If you do not explicitly specify the key components in your table definition, it will have a default
key. This consists of all non-numeric columns of the table line type.
A further attribute of the table key is its uniqueness. You will sometimes want to allow
duplicate entries in an internal table, and sometimes you will want to ensure that the key is
unique. Here, the following rules apply:
●
Duplicates are always allowed in standard tables.
●
Duplicates are never allowed in hashed tables.
●
In a sorted table, you must choose when you define it whether the key is to be unique or
non-unique.
Internal tables may also have secondary keys. Secondary keys are a way of improving the
performance of data retrieval from internal tables. You will find more information about
secondary keys in the ABAP syntax documentation or the Advanced ABAP class.
Figure 112: Addressing Internal Tables – Key and Index Access
There are two ways to access a line of an internal table – by index and by key.
Index access means addressing a line of the internal table by its line number.
Key access means addressing a line of the internal table by looking for particular values in
particular columns. The columns in which you search may be key columns, but can just as
easily be non-key columns. In either case, we still speak of "key access".
Index access is the most frequent way to access a standard table.
Key access is the more common way to access a sorted table.
Hashed tables do not have an index, therefore index access is not allowed.
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110
Lesson: Internal Table Types
Internal Table Types
Figure 113: Internal Table Type in the ABAP Dictionary
You can define internal table types in the ABAP Dictionary. This makes them reusable in any
other object, such as a program or as the type of a parameter of a function module or method.
There is no native Eclipse editor for creating internal table types. Consequently, the SAPGUI
editor opens when you create, edit, or display a table type.
There are five tabs in the editor:
Attributes: This contains the package assignment and the date of the last change to the
object.
Line Type: Here you enter the name of the ABAP Dictionary structure that you want to use to
define the line type of the internal table. To create a single-column table, you would enter the
name of a data element instead.
Initialization And Access: On this tab, you specify whether you want to create a standard,
sorted, or hashed table.
Primary Key: Here you specify the key components of the table line. Normally you enter a list
of column names. In exceptional cases, you can create either a default key or specify that the
entire table line should make up the key.
Secondary Keys: On this tab, you may define secondary keys for the internal table type.
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Unit 7: Using Internal Tables
Figure 114: Internal Table Type in an ABAP Program
If you only need your internal table type within a single program, you can create it locally using
the TYPES statement. The definition in this figure is identical to the previous definition from
the ABAP Dictionary.
It is good programming style to define the type first and then to create a variable that refers to
the type. However, for compatibility reasons in ABAP, you will also see the following:
DATA gt_flights TYPE STANDARD TABLE OF d400_s_flight WITH NON-UNIQUE KEY carrid
connid fldate.
This incorporates the complete type definition into the declaration of the variable. Although
you "save" a line of code by rolling the type definition and the variable declaration into one
statement, this approach has the disadvantage that you cannot reuse the type definition in
other variables or in parameter definitions.
A popular short notation for declaring internal tables is:
DATA gt_flights TYPE TABLE OF d400_s_flight.
At first glance, there is much information that is missing from this declaration – there is no
explicit definition of the access type or the key of the internal table. The shortened syntax
defines a standard table with a default key (comprising all non-numeric components in the
table line). The key is non-unique, as is always the case with standard tables.
In very old ABAP coding, you may also see the following obsolete form for declaring internal
tables:
DATA <table> TYPE <line> OCCURS 0.
LESSON SUMMARY
You should now be able to:
●
Understand internal table types
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Unit 7
Lesson 2
Working with Internal Tables
LESSON OBJECTIVES
After completing this lesson, you will be able to:
●
Work with internal tables
Filling an Internal Table
Figure 115: Filling an Internal Table
There are various ways in which you can fill an internal table with data. Sometimes, the data
will be returned as the result of a call to a function module or method. Very often, the table is
filled with data when you execute a SELECT statement to retrieve data from the database
(you will learn about this in the next unit). The third way is to add individual lines to an internal
table in a program.
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Unit 7: Using Internal Tables
Figure 116: Adding a Line to an Internal Table – Use a Structure
The simplest way to add a new line to an internal table is to take a variable whose type
corresponds to the line type of the internal table and fill it with the appropriate values.
Normally, this variable will be a structure. In the example, we have used the VALUE operator
to fill the structure. It would equally be possible to fill the individual components of the
structure using the syntax <structure>-<component> = <value>.
Once you have filled the structure, you can add it to the internal table.
Figure 117: Adding the Structure to an Internal Table
To insert the contents of the structure into the internal table, use the VALUE operator again.
When you use VALUE to fill an internal table, there are at least two sets of parentheses. The
outer parentheses enclose the entire expression. The inner parentheses describe the
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Lesson: Working with Internal Tables
contents of a single line of the table. In this case, the line is described by the structure that we
filled in the last code example.
The result of this expression is an internal table with a single line. Before filling the table, the
system deletes any existing contents from the table.
Figure 118: Using BASE to Preserve the Existing Contents
In order to add new lines to an internal table without deleting the existing contents, you use
the BASE addition. This tells the system to use the contents of (in this case) gt_flights and to
add the contents of the structure gs_flight to it.
When you use VALUE in this way, the system always ensures that the new record is inserted
at the correct position in the table. In a standard table, the system appends the record to the
end of the table. In a sorted table, the system inserts the new line in the correct sort order.
Figure 119: Adding Lines Without Using a Structure
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Unit 7: Using Internal Tables
It is also possible to add lines to an internal table without using a structure. In the example on
the left-hand side of the figure, we fill a structure and use it in the VALUE expression.
However, at the position where we used gs_flight, we can also use a component-bycomponent description of a structure, which is what we have done on the right-hand side.
The effect on the internal table is the same in both cases. There is, however, one difference in
the program: In the left-hand example, we can carry on using the structure by addressing its
name gs_flight. The structure that we have described on the right-hand side has no name, so
we cannot subsequently use it later on in the program.
Neither of these variants can be considered "better" than the other. As is so often the case in
programming, they are merely different ways to achieve the same result.
Figure 120: Filling One Internal Table from Another
To copy data between identically named fields of two internal tables, use the
CORRESPONDING operator. This works similarly to CORRESPONDING for structures – in
each line of the internal table, the system copies data between identically named fields.
If the target internal table contains data before the statement, the system deletes it. If you
want to retain the data and add the contents of the source table to the target, use the form
<target> = CORRESPONDING #( BASE ( <target> ) <source> ).
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Lesson: Working with Internal Tables
Reading an Internal Table
Figure 121: Reading a Line By Index
When you read a single line of an internal table that has a structure as its line type, the result
is a structure of the same type. In ABAP, this is often referred to as the work area.
The way in which you access an internal table in ABAP depends on whether you want to read a
single line or multiple lines, and whether you want to identify the lines by their index (line
number) or key (particular values in particular columns).
To access a single line of a table, you use a table expression. This consists of the name of the
internal table and, in brackets, the identification of the line that you want to read. The result of
a table expression is a single line of the table.
In the table expression, there is no space between the name of the internal table and the
opening bracket. There must, however, be at least one space after the opening bracket and
before the closing bracket.
To identify a line of the table by its line number, write the line number in the brackets. You
may give the line number as a literal as displayed here or you could use an integer variable.
If the corresponding line of the internal table does not exist, the system triggers an exception.
If you do not handle the exception, a runtime error occurs. You will see in a moment how the
exception handling works.
This kind of table expression is relatively new in ABAP. In older programs, you will see the
statement READ TABLE <table> INTO <target> INDEX <line>, which corresponds exactly to
<target> = <table>[ <line> ].
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Unit 7: Using Internal Tables
Figure 122: Reading a Line by Key
To identify a line by its key (by values in particular columns), you also use a table expression,
but in the brackets you specify the value for which you want to search. Once again, you must
leave a space after the opening bracket, after which you list the key specification in the form
<column> = <value>. You can specify any number of columns in any combination. Note that
there is no "AND" and no comma after each column and value.
The result of the table expression is either an exception (because the line does not exist in the
table) or a single line. It is, of course, possible that more than one line of the table satisfies the
search criteria. In this case, the system returns the first entry that it finds in the table and then
stops the search. There is no indication to you in the program that other matching entries
exist. If you suspect that more than one line of the internal table will match your search
criteria, you must use a technique that can process multiple lines. You will learn about this in a
moment.
This kind of table expression is relatively new in ABAP. In older programs, you will see the
statement READ TABLE <table> INTO <target> WITH KEY <key_specification> or READ
TABLE <table> INTO <target> WITH TABLE KEY <key_specification> which both correspond
to <target> = <table>[ <key_specification> ].
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Lesson: Working with Internal Tables
Figure 123: Reading a Single Component of a Line
A table expression of the form <table>[ <line specification> ] identifies a single line of an
internal table. In the previous examples, we have assigned the result of the expression directly
to a structure variable that has the same type as the line type of the internal table. However,
you do not always need the entire line. If this is the case, you can use the table expression to
address a single component of the result structure. You do this simply by adding a dash after
the table expression, then specifying the name of the component that you want to address.
Once you have typed the dash, you can use the code completion to display a list of
components.
Figure 124: Correct Error Handling
If the line that you want to read using a table expression does not exist, the runtime system
triggers an exception cx_sy_itab_line_not_found. You should always use a TRY…CATCH…
ENDTRY block to process this exception, otherwise a runtime error will occur.
If you just want to test whether a line exists, do not use this technique – there is a special
function for this purpose.
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Unit 7: Using Internal Tables
Figure 125: Checking Whether a Line Exists
The line_exists( ) function allows you to check whether a particular line exists in an internal
table as part of a logical expression. In the parentheses, you write a table expression in exactly
the same way as if you wanted to read the line. The system checks whether the line exists and
sets the expression to true or false accordingly.
This function only checks whether or not the line exists. It does not read the line.
Figure 126: Reading Multiple Lines by Index
Table expressions only read a single line. To read multiple lines of an internal table, you use a
LOOP…ENDLOOP structure. During the loop, the lines of the internal table are placed
successively into a work area that has the same type as the line type of the internal table
(normally a structure). Between LOOP and ENDLOOP, you write the statements that process
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Lesson: Working with Internal Tables
each table line. When the system reaches the ENDLOOP statement, it jumps back to LOOP
and processes the next line.
The simplest form of LOOP … ENDLOOP is LOOP at <internal table> INTO <work area>. with
no further additions. This implements a loop over the entire internal table in which each line is
processed sequentially.
If you want to loop over a part of the internal table, you can specify a starting and a finishing
line. LOOP AT <internal table> into <work area> FROM <start> TO <finish> starts processing
at the given line and finishes at the line specified in <finish>. If you leave out <start>, the
system starts at the beginning of the internal table. If you leave out <finish>, the system
continues processing to the end of the internal table.
Figure 127: Reading Multiple Lines by Key
To process multiple lines of an internal table by specifying key fields, you use LOOP AT
<internal table> INTO <target> WHERE <condition>. The where condition can contain any
number of constituent expressions joined using AND and OR. Within the expressions, you can
use not just the equals operator but also greater than, greater than or equal to, less than, less
than or equal to, unequal, and between.
Figure 128: How Many Lines Are in the Table?
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Unit 7: Using Internal Tables
To find out how many lines an internal table contains, use the lines( ) function. In the
parentheses, you specify the name of the internal table that you want to inspect. The return
value of the function is an integer.
Modifying and Deleting Table Entries
Figure 129: Modifying a Whole Line by Key
If you want to write changes from a structure back into an internal table, use the MODIFY
TABLE <internal table> FROM <work area> statement. The system reads the key values of
the table line from the work area and uses them to identify the line of the table that needs to
be modified. It then overwrites the table line with the contents of the work area.
Figure 130: Modifying a Whole Line by Index
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Lesson: Working with Internal Tables
To overwrite a particular line of the internal table, you can use the statement MODIFY
<internal table> FROM <work area> INDEX <line>. This overwrites the specified line with the
contents of the structure.
Figure 131: Modifying a Table Entry Directly
You can modify the contents of a single field in a single line of an internal table very easily
using a table expression. In the table expression you identify the line of the table with which
you want to work, using either the line number (as shown here) or a key expressions. You then
append the name of the field that you want to change and assign the new value after the
equals sign.
Figure 132: Modifying Table Entries in a Loop
There will often be times when you need to modify the contents of every line of an internal
table. To do this, you implement a loop over the entire table, which places each line of the
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Unit 7: Using Internal Tables
table successively into a work area. Within the loop, you change the contents of the structure
components, and you must then write the changes back into the internal table before the
ENDLOOP statement. If you do not do this, the changes will be lost when the current line is
overwritten by the next line.
When you process an internal table in a loop, the system knows which line it is currently
working on. Consequently, you can use a special short form of the MODIFY statement in this
context, namely MODIFY <internal table> FROM <work area>. The line number of the line that
you want to modify is missing from the statement, but the system knows to modify the
current line within the loop processing.
If you use this short form outside of LOOP…ENDLOOP, the system does not know which line
to modify and triggers a runtime error.
Figure 133: Deleting Table Entries
To delete an entry from an internal table, you must once again choose whether you want to
identify it by its key or its index. In the key access at the top of the figure, the system reads the
key values from the work area and uses them to identify the line of the internal table that
should be deleted.
In the index access, the system simply deletes the line of the internal table that you specify.
Deleting a line from an internal table causes a gap in the line numbering. The system closes
this gap automatically by moving subsequent lines up in the table.
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Lesson: Working with Internal Tables
Figure 134: Deleting the Entire Contents of an Internal Table
To delete the entire contents of an internal table, use the CLEAR statement exactly as you
would with a simple variable or structure. The statement deletes the contents of the internal
table and releases excess memory.
After the CLEAR statement, the table still exists and you can refill it. The system restores the
amount of memory reserved for the table to an initial size. The initial size is calculated so as to
allow you to fill the table again without it taking too long, but at the same time not to take up
too much memory.
LESSON SUMMARY
You should now be able to:
●
Work with internal tables
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Unit 7
Learning Assessment
1. When you declare an internal table type, you must specify the number of lines that the
table should have.
Determine whether this statement is true or false.
X
True
X
False
2. Which of the following attributes are essential parts of the definition of an internal table
type?
Choose the correct answers.
X
A Storage type
X
B Line type
X
C Access type
X
D Key components
3. To add lines to an internal table, you can use the VALUE expression. Which of the following
statements regarding the VALUE expression are true?
Choose the correct answers.
X
A You can only add one line to the table at a time.
X
B You can specify the contents of the line(s) that you want to add either in the form
of a structure or as a series of single values.
X
C The existing contents of the internal table will be overwritten unless you use the
BASE addition.
X
D The lines that you want to add can be taken directly from an internal table.
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Unit 7: Learning Assessment
4. For which operations can you use the table expression itab[ <line-spec> ]?
Choose the correct answers.
X
A To read a line.
X
B To insert a new line.
X
C To delete a line.
X
D To modify a line.
5. What is the effect of the statement CLEAR itab?
Choose the correct answer.
X
A The contents of the table are deleted.
X
B The contents of the table are deleted and surplus memory is released.
X
C The table object is completely destroyed and can no longer be addressed in the
program.
X
D The contents of the first line of the table are deleted.
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Unit 7
Learning Assessment - Answers
1. When you declare an internal table type, you must specify the number of lines that the
table should have.
Determine whether this statement is true or false.
X
True
X
False
2. Which of the following attributes are essential parts of the definition of an internal table
type?
Choose the correct answers.
X
A Storage type
X
B Line type
X
C Access type
X
D Key components
3. To add lines to an internal table, you can use the VALUE expression. Which of the following
statements regarding the VALUE expression are true?
Choose the correct answers.
X
A You can only add one line to the table at a time.
X
B You can specify the contents of the line(s) that you want to add either in the form
of a structure or as a series of single values.
X
C The existing contents of the internal table will be overwritten unless you use the
BASE addition.
X
D The lines that you want to add can be taken directly from an internal table.
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Unit 7: Learning Assessment - Answers
4. For which operations can you use the table expression itab[ <line-spec> ]?
Choose the correct answers.
X
A To read a line.
X
B To insert a new line.
X
C To delete a line.
X
D To modify a line.
5. What is the effect of the statement CLEAR itab?
Choose the correct answer.
X
A The contents of the table are deleted.
X
B The contents of the table are deleted and surplus memory is released.
X
C The table object is completely destroyed and can no longer be addressed in the
program.
X
D The contents of the first line of the table are deleted.
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UNIT 8
Open SQL
Lesson 1
Understanding Open SQL
131
Lesson 2
Creating Database Tables
139
Lesson 3
Reading Data from the Database
141
Lesson 4
Preview: Core Data Services in SAP S/4HANA
149
UNIT OBJECTIVES
●
Understand Open SQL
●
Create a database table
●
Read data from the database
●
Understand the role of Core Data Services
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Unit 8
Lesson 1
Understanding Open SQL
LESSON OBJECTIVES
After completing this lesson, you will be able to:
●
Understand Open SQL
Open SQL Architecture
Figure 135: Database Table
SAP systems run on relational database management systems. In the past, SAP did not
produce a system for a particular database system, so an SAP Business Suite system could
run on Oracle, MS SQL Server, DB2, MaxDB, and others – including latterly, of course, SAP
HANA. The difference with SAP S/4HANA is that it is developed exclusively for SAP HANA as
the underlying database.
In a relational database, information is stored in two dimensional tables, in which each row
represents one data record whose contents are split up into columns. The database is known
as a relational database, as there are also relations between tables. For example, the table in
the figure contains an airline code. In the database table, there would be a relationship to a
further table containing the information that AA stands for American Airlines, LH for
Lufthansa, QF for Qantas, and so on.
A sequence of columns at the beginning of each database table forms its key. The key is a
combination of values that ensures that each row in the table can be identified uniquely.
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Unit 8: Open SQL
Database tables in SAP systems are repository objects, and as such, are cross-client.
However, the vast majority of tables contain business data, which is client-specific. To keep
the data separate, client-specific tables have a client field (often named MANDT) as their first
key field. SAP's own database access statements – Open SQL – ensure that a statement only
manipulates data from the current client.
Figure 136: Structured Query Language – SQL
All relational database systems use a variant of Structured Query Language (SQL) to allow
you to work with them, and SAP HANA is no exception. There are three main pillars of SQL:
Data Manipulation Language (DML): DML comprises the statements that you need to work
with information in database tables. These are INSERT for adding new records, SELECT for
reading data, UPDATE to change existing records, and DELETE to delete table contents.
These statements are reflected in ABAP in the form of Open SQL.
Data Definition Language (DDL): Data Definition Language provides commands for creating
and deleting database tables and other database artefacts such as schemata, table indexes,
and views. In an SAP system, you perform these tasks using the ABAP Dictionary, not with
ABAP programs.
Data Control Language (DCL): Data Control Language is used in SQL to restrict access to
data in the database for a particular user. It is not used in its classic form in ABAP, since the
users at database level do not correspond one-to-one with the end users. Consequently,
ABAP has its own authorization concept.
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Lesson: Understanding Open SQL
Figure 137: Open SQL Architecture
In the past, SAP systems had to support a range of database platforms, and each platform
had a slightly different implementation of the SQL standard. This meant that each platform
needed slightly different commands to achieve a particular task. To avoid the ABAP code
being database-specific, SAP invented Open SQL. This is an abstract set of SQL commands
implemented at ABAP level. As it happens, none of the databases that SAP supports could
understand Open SQL by themselves. To translate Open SQL into a variant that the database
understands, the system uses the database interface.
Even though SAP S/4HANA only runs on SAP HANA, SAP has still retained the concept of
Open SQL and the database interface. This is for the following reasons:
●
Architecture compatibility – Open SQL and the database interface are an integral part of
the system architecture.
●
Code compatibility – Open SQL coding from previous SAP products (including customerspecific development) should run free of side-effects in an SAP S/4HANA system.
●
Built-in features of the database interface – the database interface does not just translate
statements. It is also responsible for automatic client handling and for managing the SAP
table buffer.
In cases where Open SQL proves insufficient, you can use the underlying Native SQL of the
database system to access the database. SAP strongly discourages you from using Native
SQL to perform tasks that can be achieved using Open SQL. In the case of SAP HANA, it can
be beneficial in terms of performance to use built-in features of Native SQL. In this case, we
recommend that you use ABAP Managed Database Procedures (AMDP).
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Unit 8: Open SQL
Figure 138: Database Tables and CDS Views
In the SAP S/4HANA system, all of the data is stored in database tables. From an ABAP
program, you can access the tables directly to read the information. However, in reality, it is
not so simple. To collect all of the data together that belongs to a particular business entity,
you almost always have to read more than one table.
In SAP S/4HANA, these entities are represented in the ABAP Dictionary by Core Data Service
(CDS) Views. Views are a logical collection of information that comes from more than one
database table. In the definition of the view, you find the fields that it contains but also the
conditions for combining the tables (for example, line items from a line item table match an
order header from the headers table when the order number on both sides is the same).
SAP /4HANA contains what is known as a Virtual Data Model (VDM). The VDM is a hierarchy
of CDS views that read, combine, and process data from different database tables in all of the
different ways that the business applications require.
As well as describing how to read data from the database, CDS views also contain important
information that enable the system to generate views for the user interface and the OData
services that allow the frontend and backend systems to communicate. Thus they are a
central pillar in the SAP S/4HANA programming model.
In Open SQL in ABAP, there is no difference in how you read data, regardless of whether you
read it directly from a database table or using a CDS view.
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Lesson: Understanding Open SQL
Figure 139: Database Tables in the ABAP Dictionary
To define a database table in an SAP S/4HANA system, you use the ABAP Dictionary. The
definition contains a list of fields, each one with a type that is described by a data element. A
combination of fields at the beginning of the table structure defines the key, which identifies
each record uniquely.
From the definition of a database table, you can also display its content by choosing the
function highlighted in the figure.
Figure 140: CDS Views in the ABAP Dictionary
CDS views are also ABAP Dictionary objects, but you edit them using a source code editor
rather than a form-based one as is the case with database tables.
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Unit 8: Open SQL
A CDS view can be a selection of fields from a single table, as shown here, or a combination of
fields from different tables.
CDS views also contain annotations. Annotations provide information about the view or
individual fields in it that goes beyond the simple instruction "Select this field from that table".
The system of annotations is open, so that any framework can add its own metadata to the
definition of a CDS view without altering the basic SQL definition.
To open the data preview for a CDS view, press F8.
The Flight Data Model
Figure 141: The ABAP Training Flight Data Model
Figure 142: Table SCARR
The database table SCARR contains a list of airlines, including their airline code and full name.
In other tables in the data model, only the code is used.
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Lesson: Understanding Open SQL
Figure 143: Table SPFLI
Table SPFLI contains a list of flight connections. Each connection is identified by an airline
code (which must be present in table SCARR) and a flight number. These details are followed
by information about the departure and arrival airports, cities, and countries of the flight, and
its departure and arrival times.
Figure 144: Table SFLIGHT
This table contains a list of actual flights for each connection. Whereas SPFLI defines that LH
400 flies from Frankfurt to New York, SFLIGHT contains an entry for each day on which LH
400 actually flies, along with the price of the flight, type of airplane used, and the number of
available seats.
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Unit 8: Open SQL
LESSON SUMMARY
You should now be able to:
●
Understand Open SQL
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138
Unit 8
Lesson 2
Creating Database Tables
LESSON OBJECTIVES
After completing this lesson, you will be able to:
●
Create a database table
Creating a Database Table
Figure 145: Creating a Database Table
There are three stages to creating a database table:
1. Delivery and Maintenance attributes. On this tab, you specify the delivery class for the
table and whether users should be able to display and change table contents using the
Data Browser (transaction SE16). By far the most common delivery class, and the one that
you will use for the vast majority of your own tables, is A for Application Table. This class
covers both master data and transactional data. Choose the appropriate option from the
dropdown list Data Browser/Table View Editing to determine whether users can see the
table contents in transaction SE16.
2. Fields. On this tab, you specify the actual fields that the table has. Remember the client
field, as you need to be able to store client-specific data in a table object that is crossclient. For each field, you need a data element that specifies the type. Mark the Key
column for each field that is to be part of the table key.
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Unit 8: Open SQL
3. Technical Settings. The technical settings of the table are information that is not part of
the table definition itself, but that the system needs (or at least finds useful). In SAP S/
4HANA, the most important attribute is the storage type of the table. As a rule, you should
set this to Column Store. You also need to specify the data class (APPL0 for master data
and APPL1 for transactional data), and the size category of the table. This provides the
system with information about the predicted size of the table. It is definitely not an upper
limit for the table size! In the technical settings, you can also set the buffering options for
the table. The majority of tables are not suitable for buffering, sometimes because they
are too big, but mostly because their contents keep changing.
Once you have maintained all three sections of the table definition, you can activate it.
Common reasons for the activation failing are either that you have forgotten to maintain the
technical settings of the table or that you have forgotten to mark the key fields of the table.
When you activate the definition of a database table in ADT, the system creates the
corresponding physical table object in the database.
LESSON SUMMARY
You should now be able to:
●
Create a database table
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140
Unit 8
Lesson 3
Reading Data from the Database
LESSON OBJECTIVES
After completing this lesson, you will be able to:
●
Read data from the database
The SELECT Statement
Figure 146: The SELECT Statement
To read data from the database, you use the SELECT statement. It makes no difference if you
are accessing a database table directly or reading data using a CDS view.
When you write a SELECT statement in Open SQL, the syntax check compares what you have
written with the definitions of the tables and CDS views in the ABAP Dictionary. If you try to
address tables, views, or fields that do not exist, a syntax error occurs.
When you run your program, the database interface translates the Open SQL statement into a
corresponding Native SQL statement. The database executes the statement and returns the
result to the database interface. The database interface passes the result back to the ABAP
program.
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Unit 8: Open SQL
Figure 147: Syntax of the SELECT Statement
The basic syntax of the SELECT statement contains several sections – called clauses – and
always follows the pattern in the figure. The first thing that you specify after SELECT is the
source of the data, in the FROM clause. This can be either a database table or a CDS view.
Reading data from the database – even an in-memory database like SAP HANA – can be
time-consuming. To ensure that it does not take too long to retrieve the data, and to make
sure that the user sees data that he or she actually needs, rather than the entire contents of a
table, it is important only to transfer data that you need. There are, of course, two ways to do
this – firstly by only transferring the columns that you need, and secondly by restricting the
data selection to rows that meet certain selection criteria.
In the FIELDS clause of the SELECT statement, you list the columns of the database table that
you want to read. The columns in the list must be separated by commas. If you want to read
the entire table line, you can specify FIELDS * instead of a column list. Be aware, however,
that this can cause the SAP HANA database considerably more work than just reading the
columns you need.
In the WHERE clause, you can specify a condition that describes which rows of the table will
be read. For example, the condition WHERE carrid = 'LH' means that only those rows will be
read in which the column CARRID contains the value LH. The WHERE clause can contain
multiple conditions linked with the AND and OR operators. For example, WHERE carrid = 'LH'
and FLDATE > sy-datum would return all Lufthansa flights with a departure date in the future.
You can also negate conditions using NOT.
At the very end of the SELECT statement comes the INTO clause. This specifies the variable in
the ABAP program into which the data should be placed. This is normally a structure or an
internal table and should ideally have the same sequence of components as the column list in
the FIELDS clause.
Note that you will see other forms of SQL syntax in ABAP. These are older and have been
retained to ensure compatibility. You should get used to using the modern syntax, as it
provides far more functions and features than the old form. Furthermore, only the new form
supports selections using CDS views.
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Lesson: Reading Data from the Database
Figure 148: Field Lists and ABAP Variables
Whenever you write a list of fields in a SELECT statement, you must separate the field names
using commas. In the example, this applies to the FIELDS clause. Later, in more advanced
queries, you will find that it also applies to the GROUP BY and ORDER BY clauses.
Wherever you use an ABAP variable in a SELECT statement, you must prefix it with the @
character. This indicates to the SQL parser (part of the ABAP compiler) that the variable
comes from ABAP and not from the database. You need the @ symbol both when you pass a
value to the statement (as in the WHERE clause) and when you receive data from the
statement (as in the INTO clause).
Figure 149: Selecting a Single Row into a Structure
The SELECT SINGLE statement reads a single entry from the specified database table. If the
entry exists, the system places it into the variable listed in the INTO clause. If it does not exist,
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Unit 8: Open SQL
the contents of the variable remain unchanged, and the system sets the return code field sysubrc to 4.
The SELECT SINGLE statement searches the table until it finds one record that meets the
specified condition, at which point it stops searching. Even if there is more than one table
entry that satisfies the condition, the system only returns the first. There is not even a
warning that there were (or might have been) other matches.
SELECT SINGLE is particularly useful if the WHERE clause contains a single value for every
key fields of the table. By specifying a single value for every key fields of a database table, you
identify a unique record.
If you need the capability to process more than one record, use a different variant of the
SELECT statement.
Figure 150: Selecting Multiple Rows into an Internal Table
To select multiple rows from a database table, you can use the SELECT… INTO TABLE variant
of the SELECT statement and place all of the rows directly into an internal table in your
program.
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Lesson: Reading Data from the Database
Figure 151: Selecting Multiple Rows Using a Loop
A further variant of SELECT is the SELECT loop. In this case, you read multiple rows from the
database, but place them successively into a structure. The statements between LOOP and
ENDLOOP are executed for each row that you have read. If you want to keep a row beyond the
end of the loop, you must insert it into an internal table.
Figure 152: Correspondence Between FIELDS and INTO
The INTO or INTO TABLE clause of the SELECT statement will only work correctly if the
number and types of the components of the structure or the line type of the internal table
correspond to the number and types of the columns specified in the FIELDS clause. In the
above example, the statement can only work if the structure gs_connection has four
components with the same type and length as the columns carrid, connid, cityfrom, and cityto
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Unit 8: Open SQL
of the database table. Note that, in this case, the names do not have to be identical – the
system fills the target structure from left to right.
If the field list in the FIELDS clause does not match the structure or table line type in the INTO
clause, a runtime error will occur.
Figure 153: INTO CORRESPONDING FIELDS
If the fields of the FIELDS clause and the components of the structure in the INTO clause do
not match, you can use the variant INTO CORRESPONDING FIELDS or INTO
CORRESPONDING FIELDS OF TABLE. This has the same effect as the CORRESPONDING #( )
operator that you learned about earlier and ensures that data is copied between identically
named components.
Once again, only the names must be identical. If identically named components have different
types, the system attempts to convert the contents of the source field into the type of the
target field. You must bear in mind that even if this operation does not cause a runtime error,
data may still be lost due to the conversion rules.
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Lesson: Reading Data from the Database
Figure 154: Data Transfer from the Database to the Program
When you read data from the database, the database interface requests it from the database,
which transfers it in packages to the application server. The database interface optimizes this
process so as to read the data as quickly as possible. The database interface then passes the
data to the program according to the way you programmed the SELECT statement. For
SELECT SINGLE this means that the single line is transferred into the corresponding
structure. In the case of SELECT INTO TABLE, the internal table is filled in a single transfer. If
you choose SELECT…ENDSELECT, the rows are transferred individually from the database
interface to the program.
The key to efficient Open SQL programming in ABAP is to transfer as little data as possible
from the database to the program. You can do this by using a list of columns in the FIELDS
clause of the SELECT statement – then you do not transfer the entire row, but only the
columns you actually need. The second way to do it is to formulate a WHERE clause that
describes as precisely as possible the rows of the table that you need.
Figure 155: Expressions in the FIELDS Clause
In addition to reading data from the database, the SELECT statement also allows you to
process it in expressions. For example, the database table SFLIGHT contains the number of
occupied seats for each flight and the number of seats that are actually booked. This example
shows how you can get the database to calculate the number of free seats and return it as
part of the result set of the SELECT statement.
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Unit 8: Open SQL
When you perform a calculation like this in the SELECT statement, the system generates a
new column for it in the result set. Because it does not represent a column from one of the
database tables, it initially has no name. It is at least good practice – and in some cases
mandatory – to name the column using the notation AS <name>. The calculated column is
then known by this name in the rest of the SELECT statement and in the ABAP program.
For further information on how to use expressions in the SELECT statement, refer to the
ABAP documentation for SELECT.
Figure 156: Expressions in the WHERE Clause
Another place in which you can use expressions in the SELECT statement is in the WHERE
clause. This is particularly useful for performance reasons, as you can further restrict the data
that you read.
In this example, the SELECT statement only reads the flights with fewer than ten free seats.
Without the possibility to do this directly in the WHERE clause, you would have to read all of
the flights from the database and then decide by performing the calculation in the program
which records to keep and which to discard.
This calculation is a very simple example of code push-down. Code push-down in the context
of SAP HANA and SAP S/4HANA means getting the database to perform calculations rather
than performing them in the program on data that has already been read. The aim is that the
results of the calculations can filter out unwanted data before it is transferred to the program,
thus speeding up the data transfer. You will hear about code push-down in the context of
Open SQL, Core Data Services, and ABAP-Managed Database Procedures.
LESSON SUMMARY
You should now be able to:
●
Read data from the database
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Unit 8
Lesson 4
Preview: Core Data Services in SAP S/4HANA
LESSON OBJECTIVES
After completing this lesson, you will be able to:
●
Understand the role of Core Data Services
Joins and Core Data Services
Figure 157: Understanding a Data Record
In a relational database management system (RDBMS), there are strict design rules that are
intended to save space in the database (a hug consideration in the 1970s and 1980s) and to
preserve the integrity of the data. Even if storage space is no longer such an issue, data
integrity is important enough for these rules to have stuck around!
One important rule is that the system must not store data redundantly. The consequence of
this in a table of flight bookings is that we see the flight number, consisting of airline and
connection number, but not any other information about the flight, such as its departure and
arrival cities and times. The reason for this is simple; the flight number identifies a connection
uniquely, and any other information stored in the flight booking record would have to be
duplicated for each booking and would therefore be redundant and liable to data
inconsistency.
The same applies to the customer information. The booking record contains the number of
the customer, but not his or her name and address.
In order to present the full details of a flight booking to the end user, we therefore need to
consult more than just this table. This is the norm in SQL programming and in this case we
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Unit 8: Open SQL
can consider ourselves fortunate that we only have to read three tables and not a whole lot
more.
Figure 158: Joining Tables
In the SELECT statement, you can read from more than one table in a single operation using a
table join. In it, you specify the columns that you want to read, the tables from which they
come, and also under what conditions a record from one table matches one from another.
In this example, we are reading from the bookings table SBOOK, which contains a customer
number in the column CUSTOMID. In order to find the customer details, we need to use the
customer number to find the corresponding entry in table SCUSTOM. We also need to use the
combination of CARRID and CONNID to find the details of the connection.
While table joins are relatively simple to implement in ABAP, they have the disadvantage that
you need to rewrite them in every program. Furthermore, each programmer needs to
understand the relationship between the different tables in the data model in order to be able
to write the joins correctly.
Figure 159: Hiding Complexity with Core Data Services
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150
Lesson: Preview: Core Data Services in SAP S/4HANA
In ABAP, you can use views to create reusable join definitions in the ABAP Dictionary.
Dictionary Views have existed for a long time, but the recommended way to create views from
AS ABAP Release 7.40 onwards is to use Core Data Services, because they provide a far
greater range of functions.
The simplest kind of CDS view is one that replaces a join in a SELECT statement. In other
words, the relationships between the database tables from which you want to read are
described in the view object instead of in the SELECT statement in an ABAP program.
However, CDS views can also contain advanced functions such as case distinctions and
currency conversions.
As well as the actual view definition, CDS views can also contain metadata in the form of
annotations. This is not information that describes how to read the data from the database,
but instead what should happen with the data in a particular context. Frequently used
annotations describe whether the CDS view should be used as the basis for an SAP Gateway
service, how the fields from the view should be prioritized by the user interface, or how they
should behave in an analytical application.
You can learn more about CDS views in class S4D430.
Figure 160: Simple Core Data Service Views
In this example, there is a simple view that defines a join between two tables. It specifies that a
line from the database table spfli fits a line from the table scarr when the value of the airline
carrid matches.
A program that uses this view does not have to understand the underlying data model – it can
just consume the view and will receive the correctly arranged data.
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Unit 8: Open SQL
Figure 161: Complex Core Data Services
This example of a Core Data Service view is more complex. It does not just contain the
definition of a database view, but there are also various annotations. Annotations provide
information about a particular aspect of a view that is not directly related to reading the data
from the database. Here, there is an annotation @OData.publish: true, which indicates that
the system should generate an OData service when the view is activated. The @UI
annotations provide guidance to the system as to how it should generate a Fiori Elements
user interface based on the view.
LESSON SUMMARY
You should now be able to:
●
Understand the role of Core Data Services
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152
Unit 8
Learning Assessment
1. Which of the following statements regarding Open SQL are true?
Choose the correct answers.
X
A Open SQL is specific to SAP HANA
X
B Open SQL supports automatic client handling.
X
C Open SQL allows you to read directly from database tables, but also to use Core
Data Service views.
X
D You use Open SQL to create database tables in the SAP System.
2. You can create database tables for the SAP System either using the ABAP Dictionary or
the database management studio.
Determine whether this statement is true or false.
X
True
X
False
3. Which of the following statements regarding the key of a database table are correct?
Choose the correct answers.
X
A The key determines the order in which entries in the table will be stored.
X
B The combination of key fields identifies each row uniquely.
X
C The key must not have more than five fields.
X
D The key is mandatory.
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Unit 8: Learning Assessment
4. You use SELECT SINGLE to read a single row from a database table. What happens if
there is more than one row in the table that meets the condition that you specified?
Choose the correct answer.
X
A The statements returns all rows that correspond to your selection.
X
B The statement merely returns the first row that corresponds to your selection.
X
C An exception occurs.
X
D The system sets sy-subrc to a value unequal to zero.
5. When would you use the addition SELECT... INTO CORRESPONDING FIELDS ?
Choose the correct answer.
X
A When you only want to read one row from a table.
X
B When you want to read multiple rows from a table.
X
C When the list of fields is identical to the structure of your target variable.
X
D When the list of fields is different from the structure of your target variable.
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Unit 8
Learning Assessment - Answers
1. Which of the following statements regarding Open SQL are true?
Choose the correct answers.
X
A Open SQL is specific to SAP HANA
X
B Open SQL supports automatic client handling.
X
C Open SQL allows you to read directly from database tables, but also to use Core
Data Service views.
X
D You use Open SQL to create database tables in the SAP System.
2. You can create database tables for the SAP System either using the ABAP Dictionary or
the database management studio.
Determine whether this statement is true or false.
X
True
X
False
3. Which of the following statements regarding the key of a database table are correct?
Choose the correct answers.
X
A The key determines the order in which entries in the table will be stored.
X
B The combination of key fields identifies each row uniquely.
X
C The key must not have more than five fields.
X
D The key is mandatory.
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Unit 8: Learning Assessment - Answers
4. You use SELECT SINGLE to read a single row from a database table. What happens if
there is more than one row in the table that meets the condition that you specified?
Choose the correct answer.
X
A The statements returns all rows that correspond to your selection.
X
B The statement merely returns the first row that corresponds to your selection.
X
C An exception occurs.
X
D The system sets sy-subrc to a value unequal to zero.
5. When would you use the addition SELECT... INTO CORRESPONDING FIELDS ?
Choose the correct answer.
X
A When you only want to read one row from a table.
X
B When you want to read multiple rows from a table.
X
C When the list of fields is identical to the structure of your target variable.
X
D When the list of fields is different from the structure of your target variable.
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156
UNIT 9
Object-Orientation in
ABAP
Lesson 1
A Short History of ABAP
158
Lesson 2
Modeling a Class
161
UNIT OBJECTIVES
●
Understand the emergence of object-orientation
●
Be able to create and understand a simple UML model
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Unit 9
Lesson 1
A Short History of ABAP
LESSON OBJECTIVES
After completing this lesson, you will be able to:
●
Understand the emergence of object-orientation
The Evolution of Programming
Figure 162: The Beginning – Spaghetti Programming
In the early days of programming, no real consideration was given to how to structure the
code. Developers simply started at the top of the program and finished at the bottom. To
control the program flow, they used GOTO statements, which gave them the freedom to jump
backwards and forwards in the program to any line they needed. Unfortunately, this way of
programming made maintenance and debugging almost impossible, so while it was an
acceptable way of programming if you needed a program once to solve a problem in an
academic research project, it was not helpful as soon as product maintenance and support
became an issue.
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Lesson: A Short History of ABAP
Figure 163: Procedural Programming – Function Encapsulation
A later phase of programming was procedural programming. Driven by the conviction that
programs needed more structure than spaghetti coding, architects devised the concept of
encapsulating functions in procedures. This had two main advantages – it made the flow of a
program easier to understand, and it also meant that the procedures could be reused at
different points in the program. As time went on, the idea of reuse was extended to provide
reuse not just in one program, but across multiple programs through the concept of
procedure libraries. ABAP also had its own procedure libraries in the form of subroutine pools
(a very long time ago) and function groups (up to the present day). For many years, ABAP
supported procedural programming by providing subroutines and function modules.
The drawback to this approach was, however, that all of the procedures in the program have
unrestricted access to all of the data. While the separation of functions into procedures
makes it easier to locate errors in the code, data inconsistencies become very difficult to
solve, since it is not clear which parts of the program changed which data.
Figure 164: Object-Oriented Programming – Data Encapsulation
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Unit 9: Object-Orientation in ABAP
Moving on from procedural programming, the architects of programming languages started
to look towards representing whole, real-world entities in a program. Instead of just working
with relational models as you would find them at database level, they created representations
of these objects in their programs. Thus the object-oriented paradigm was born. An object
corresponds to a real-life object such as an employee, a vehicle, or, in our case, a flight. It has
attributes that describe it – in the case of a flight, that might be the airline, the flight number,
the number of available seats and the number that are occupied.
Now let us consider what happens when a program that is using this object wants to create a
booking. It obviously – at least to us – makes sense that the flight should not have more
bookings than available seats. Under the procedural paradigm, every program that wants to
book a flight must remember to ensure that it is not overbooked – and here is the point at
which things could go wrong!
In object-orientation, the client program cannot just create a booking by adding one to the
number of filled seats. Instead, it would call a function - called a method in object-orientation to perform this task. The method, which is part of the object itself, checks whether the flight is
overbooked. If it is, it refuses to create a new booking. Otherwise, the new booking is created.
This concept is called data encapsulation. The information about the flight is managed by the
flight object itself, and cannot be manipulated by anyone else. This ensures that no other
point in the program can bypass the check of the number of free seats – either knowingly or
unknowingly. Outside of the object itself, as you will learn later, the data is not visible at all – it
can only be changed by the object itself. This is one of the major advantages of objectorientation.
LESSON SUMMARY
You should now be able to:
●
Understand the emergence of object-orientation
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160
Unit 9
Lesson 2
Modeling a Class
LESSON OBJECTIVES
After completing this lesson, you will be able to:
●
Be able to create and understand a simple UML model
Modeling a Class
Figure 165: Classification of Objects – What Is a Vehicle?
One of the main principles of object orientation is to describe an object, but how do you do
that when there are lots of different ways of looking at the same thing? To a car manufacturer,
a car has a bill of materials stretching to around 30000 different components. To the
consumer, however, the car looks quite different and merely has a make, model, color, engine
size, top speed, and whatever other favorite features you happen to have! A car rental firm will
have yet another view of the vehicle, the licensing authorities another, and so it goes on.
When you create a description of an object in an object-oriented application, you describe the
aspects of it that are important and relevant to your application. If you are implementing an
application to manage car rentals, the licensing authority's view of the vehicle is
inappropriate. If you are a car manufacturer, the consumer's view of a car is not sufficiently
detailed. In short, different applications that deal with vehicles in different ways will describe
them differently according to their needs.
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Unit 9: Object-Orientation in ABAP
Figure 166: Classification of Objects – Describing an Entity
The first task in object-oriented programming is not programming at all – it is to identify and
describe all the entities (just a complicated word for "things") in your program. You start off
by doing this on paper or by using modeling software. The standard language that has
established itself over time is the Unified Modeling Language (UML), which was invented at
Rational Software during the 1990s. You can find the full specification of UML at http://
www.omg.org/spec/UML. There are also various Eclipse-based tools that support UML
modeling.
The figure above shows a simple description of a vehicle. The description contains two
sections – the attributes of the vehicle, which describe its state, and the methods, which
describe the functions that you can perform on a vehicle. In total, the description is called a
class.
Attributes and methods of classes are optional; theoretically, you could have a class with only
attributes. This would allow you to store information about a particular thing without there
being any special functions to manipulate it. Although unusual, it would be possible. You could
also have a class with only methods; the class is then just a container for functions instead of
managing the state of an object. Sometimes, you will see this kind of class used for helper
functions in an application as an alternative to using subroutines or function modules.
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Lesson: Modeling a Class
Figure 167: Classes and Objects
A class is a description of an object – it does not represent a real-life thing. In this respect, it is
like a data type; the data type of a structure describes the components that a structure of that
kind would have, but you need a variable in order to store real data. A class describes what
makes up a vehicle, but you need an object in order to represent an actual vehicle in your
application.
You can have any number of objects that are derived from the same class. Each represents
one particular instance of the entity that the class describes. The words object and instance
are synonymous.
All objects are simultaneously identical and independent of one another.
Identical: The elements that make up each vehicle are the same in each case – every vehicle
has a make and a model.
Independent: The state of each object is independent of that of all other objects. Each object
can be manipulated completely independently of all other instances of the same class.
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Unit 9: Object-Orientation in ABAP
Figure 168: Visibility of Components
In a class, every attribute, method, or other component must have a defined visibility. Public
components are visible both within the class itself and to all users of the class. Private
components, on the other hand, are only visible within the class, and cannot be accessed
directly by a program that is using the class.
A guiding principle of object-oriented modeling is to make as many components public as
necessary, but as few as possible. Private components allow you to ensure the consistency of
classes in the following ways:
Private attributes can only be changed within the class. This eliminates the possibility that a
program can change the value without knowing the business rules that govern the possible
values and consistency of the attribute. To allow programs to change the values of attributes,
you provide public methods to which the program can pass a value. The method then checks
that the value is acceptable, and sets the new value of the attribute accordingly.
Private methods allow you to modularize your code within a class. If two or more methods
require exactly the same coding, you can put the common code into a separate method. Since
this code is then only one part of a bigger function, it makes sense to make the method
private so that a program that is unfamiliar with the structure and implementation of the class
cannot call it by mistake.
In UML, public components are denoted with a plus sign and private components with a minus
sign.
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Lesson: Modeling a Class
Figure 169: Instance Components
Normally, each attribute and method that you define for a class exists separately in each
instance of the class. However, there are sometimes cases where this is not a good idea.
In the figure above, we have extended the vehicle class to contain a list of permitted vehicle
makes. The list is used to check that each new vehicle make is actually allowed. However,
because the attribute is an instance attribute, each instance of the class has a copy of the
vehicle list. This is ineffective, and a waste of memory; the vehicle list is actually identical for
all instances of the class and only really needs to be held once for the entire class.
Figure 170: Static Components
In this example, we have changed the attribute permitted_makes from an instance attribute
to a static attribute. Static attributes do not belong to a particular instance but instead exist
once only at class level. Every instance of the class can access static components.
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Unit 9: Object-Orientation in ABAP
As well as static attributes, you can also declare static methods. You can call these methods
using the name of the class, and they are accessible even if there are no instances of the
class.
Static methods may only access static attributes.
Static attributes are useful for: look-up data that any instance of the class might need,
counters to tell you how many instances of a particular class you have created.
Static methods are useful for: helper functions in an application that do not really apply to a
particular instance, retrieving an instance of a class when you start to work with a particular
framework.
In UML notation, static attributes are underlined.
Figure 171: Classifying Different Kinds of Vehicles
The previous figure showed the components belonging to one single class. However, most
applications consist of more than one class, and the relationships between the classes are
also important. For this reason, UML also provides ways of describing these relationships.
This figure shows one of the most common relationships (called associations in UML) – that
of generalization and specialization. On the left-hand side are three classes - vehicle, truck,
and bus - that are all independent of each other. However, in reality, it is true to say that both
trucks and buses are kinds of vehicle. In object-oriented modeling, we say that the vehicle
class is a general class. We can then say that trucks and buses are specializations of the
vehicle.
This has wide-reaching (but positive) consequences in the implementation of the application.
Everything that is common to all vehicles can be implemented in the vehicle class. These
common components are then automatically passed on to the specialized classes, which then
do not have to reimplement those features. Instead, they concentrate on the attributes and
methods that only a truck or only a bus possess.
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Lesson: Modeling a Class
Figure 172: Storing Objects Within Objects – Aggregation
An attribute of a class might be an object from a different class. So, for example, a car rental
company might keep a list of all of its vehicles, and each entry in the list is a vehicle object.
This kind of association is called aggregation. The vehicle objects are created outside the
class and then assigned to the rental class. In other words, a vehicle can survive outside the
rental class.
Figure 173: Storing Objects Within Objects – Composition
Here, we see a special kind of aggregation. The example has changed, and we are now looking
at a fictive example from the user interface world. Imagine a UI control that displays a table. In
the table are columns, each of which is represented by an instance of a class.
When we tell the table class to display the data, it creates the column objects by itself. When
the table display is finished, the column objects are no longer needed and so they are
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Unit 9: Object-Orientation in ABAP
destroyed at the same time as the table object. In other words, the entire lifecycle of the
columns objects depends on the table object. This kind of aggregation is called composition.
LESSON SUMMARY
You should now be able to:
●
Be able to create and understand a simple UML model
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168
Unit 9
Learning Assessment
1. Which of the following statements about classes is true?
Choose the correct answer.
X
A Attributes of a class are always private. Methods of a class are always public.
X
B A class must always have both private and public elements.
X
C You can create any number of independent instances of a class
X
D You use classes to make large quantities of global data available to a program.
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169
Unit 9
Learning Assessment - Answers
1. Which of the following statements about classes is true?
Choose the correct answer.
X
A Attributes of a class are always private. Methods of a class are always public.
X
B A class must always have both private and public elements.
X
C You can create any number of independent instances of a class
X
D You use classes to make large quantities of global data available to a program.
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170
UNIT 10
Creating and Using
Classes
Lesson 1
Creating Classes
172
Lesson 2
Creating Objects
179
Lesson 3
Calling Methods
183
Lesson 4
Using Constructors
189
Lesson 5
Using Factory Methods
192
UNIT OBJECTIVES
●
Create a class
●
Create objects in ABAP
●
Call methods
●
Use constructors
●
Use factory methods
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Unit 10
Lesson 1
Creating Classes
LESSON OBJECTIVES
After completing this lesson, you will be able to:
●
Create a class
Creating Classes in ABAP
Figure 174: Local And Global Classes
Classes in ABAP can be either local or global. You define local classes as part of an ABAP
program and you can only use them in the program in which they are defined. Local classes
are useful for entities or functions that you only need in a single program.
Global classes are contained in their own Repository object called a class pool. They are
stored centrally and can be used by all other ABAP programs (including other classes and
function groups).
The ABAP syntax of both local and global classes is almost identical. In this course, you will be
working with local classes.
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Lesson: Creating Classes
Figure 175: Local Classes in ABAP
The declaration of a class has two sections – the definition and the implementation.
The definition part of a class contains the declaration of all of the elements in the class, that is,
the attributes, methods, types, and constants. As you have already learned, each element in a
class has a particular visibility – public or private. You set the visibility of an element by
definining it in the relevant section. The PUBLIC SECTION statement introduces the public
section of the class. Any elements that you declare after PUBLIC SECTION and before the
introduction of the next section are public.
The public section of the class does not have an explicit closure; in our example, it ends where
the private section begins.
All elements that you define after PRIVATE SECTION are private. The private section ends at
the ENDCLASS statement.
There is also a PROTECTED SECTION, which is relevant to inheritance, and which you will
learn about in the unit covering that subject.
The declarations of the visibility sections in your class must follow the order PUBLIC SECTION
- PROTECTED SECTION- PRIVATE SECTION. However, the sections are optional; if you do not
need a particular section, you do not have to declare it.
You must not declare any elements of the class between the CLASS <class> DEFINITION
statement and the first visbility section that you declare, as this causes a syntax error.
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Unit 10: Creating and Using Classes
Figure 176: Definition Part of a Class
The figure shows the different kinds of elements you can define in a class. You can create any
of these kinds of elements in any visibility section of the class.
The TYPES statement allows you to define types within your class. You do this in exactly the
same way as you would outside a class (for example, to define a structured or internal table
type). If types are public, a program that uses the class can use the type (or types) to declare
ist own variables.
You use DATA and CLASS-DATA to declare attributes of the class. DATA declares an instance
attribute, while CLASS-DATA declares a static attribute.
Classes may also contain constants. Both constants and types are static components of the
class.
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Lesson: Creating Classes
Figure 177: Method Signatures
When you declare a method, you define its signature; that is, the set of values that the method
exchanges with its caller and the exceptions that may arise during the method. Each
parameter has a name and a type.
Importing parameters are values that the method receives from the caller. By default,
importing parameters are mandatory, but there are two ways to make them optional:
●
Using the OPTIONAL addition. The parameter is optional and its default value is the initial
value appropriate to the type of the parameter
●
Using the DEFAULT <val> addition. The parameter is optional and its default value is the
value that you specified as <val>.
The implementation of a method may not change importing parameters. If you attempt to do
so, you will cause a syntax error.
Exporting parameters are results that are returned by the method. All exporting parameters
are optional – a program only uses the values that it actually needs.
Changing parameters are values that you pass to the method. Unlike importing parameters,
the method can change the values of these parameters. They are then returned to the caller
under the same name. Changing parameters are mandatory by default; you can make them
optional in the same way as importing parameters.
A method can have any number of importing, exporting, and changing parameters. By default,
they are passed by reference.
In addition to or instead of its importing, exporting, and changing parameters, a method may
have one returning parameter. This is a single result value that can be used directly in an
expression. Returning parameters are always passed by value.
If an error occurs during a method, the method triggers an exception. The method terminates,
and the calling program can then react to the error. The signature of a method must list the
possible exceptions.
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Unit 10: Creating and Using Classes
Figure 178: Declaring Attributes Of The Vehicle Class
Let us start implementing our UML model in ABAP, and take the attributes of the vehicle class
to begin with. All of the attributes are private, so we include them after the PRIVATE SECTION
statement. To declare an attribute, use the DATA statement within the appropriate visibility
section. The prefix mv_ follows the normal prefixing rule of <scope><type>_. In this context,
m stands for member and v for an elementary variable.
The attribute gv_no_of_veh is underlined, which denotes a static attribute. You declare static
attributes using the CLASS-DATA statement. The syntax of CLASS-DATA is identical to that
of DATA.
In the DATA and CLASS-DATA statements, you can specify the type of the attribute as
follows:
Using TYPE: Use TYPE to refer to an ABAP Dictionary type, a built-in ABAP type, or a type
defined locally in the class.
Using LIKE: Use LIKE to declare an attribute with reference to another attribute that you have
already declared.
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Lesson: Creating Classes
Figure 179: Defining Methods Of The Vehicle Class
The next step in realizing the UML model is to declare the methods in the class. The
set_attributes method of the class needs two importing parameters – one for the make, and
one for the model of the vehicle.
By defining the method, you introduce a syntax error into your class; the syntax check tells
you that the implementation of the method is missing. For every method that you define, you
must also create an implementation. ADT allows you to do this using a quick fix. Position the
cursor in the statement containing the error and press Ctrl + 1. A list of possible quick fixes
appears, including the option Add implementation for set_attributes. When you choose this
option, ADT creates the corresponding implementation of the method. Your class is now
syntactically correct once again.
Figure 180: Implementing Methods
You must implement every method that you define. You do this in the implementation part of
the class between the statements METHOD <method_name> and ENDMETHOD.
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Unit 10: Creating and Using Classes
The method implementation contains ABAP statements that can access the parameters of
the method (you are not allowed to change importing parameters) and all types, attributes,
and constants that you declared in the class. Instance methods may access both instance
attributes and static attributes. Static methods may only access static components.
LESSON SUMMARY
You should now be able to:
●
Create a class
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Unit 10
Lesson 2
Creating Objects
LESSON OBJECTIVES
After completing this lesson, you will be able to:
●
Create objects in ABAP
Creating Objects
Figure 181: Reference Variables
In order to work with instances of classes in ABAP, you need a reference variable. This is a
special kind of variable that you use to create, address, and manage an object.
You declare reference variables using the DATA statement with the addition TYPE REF TO
followed by the name of a class. Using the reference variable, you will be able to create an
instance of the class that you specified. However, at the moment you declare the variable, it is
initial just like any other variable in ABAP. In other words, the object does not yet exist.
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Unit 10: Creating and Using Classes
Figure 182: Creating Instances of a Class
To create a new instance of a class, you use the NEW #( ) operator. This creates a new
instance of the class and places the address of the instance into the reference variable on the
left-hand side of the expression.
You may have noticed that the name of the class that you want to instantiate does not appear
anywhere in the expression. However, the system already knows that go_vehicle has the type
REF TO lcl_vehicle, and consequently it knows that it should create an instance of the class
lcl_vehicle. The pound sign after the NEW operator means "use the type of the variable before
the equals sign". (In more advanced scenarios, you can actually specify the name of the class
in place of the pound sign).
Note that there must be at least one space between the parentheses.
When you address a class for the first time (which could be by creating an instance of the
class), the runtime system also loads the class definition into the program memory. This
contains all of the static attributes, which only exist once in the class instead of once for each
instance.
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Lesson: Creating Objects
Figure 183: Copying Reference Variables
If you assign one reference variable to another, the system copies the address of the object to
which the first variable is pointing into the second reference variable. The result of this is that
you have two reference variables that point to the same object.
Figure 184: Overwriting Reference Variables
You can use the same reference variable to create more than one instance of a class. Each
time you use the NEW #( ) operator, the system creates a new instance of the class and
places the address of the new instance into the reference variable. However, the address of
the new instance overwrites the address of the previous instance.
In the example above, the address of vehicle 2 overwrites the address of vehicle 1.
Consequently, there is no longer a reference variable in the program pointing to vehicle 1.
When this happens to an instance, it can no longer be addressed from the program.
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Unit 10: Creating and Using Classes
To prevent the program memory from becoming filled with objects that can no longer be
addressed and eventually overflowing, the runtime system has a component called the
garbage collector. The garbage collector is a program that runs periodically to look for and
destroy objects to which no more references point. If during a program you delete the last
reference to an object by overwriting it or using the CLEAR statement, the garbage collector
will destroy the object on its next pass. Equally, at the end of a program, when all of the
reference variables are freed, the garbage collector will destroy all of the instances to which
they had pointed. You therefore do not have to worry about resource management in the
program yourself.
Figure 185: Holding Reference Variables in an Internal Table
One way in which you can keep objects alive is to place the references into an internal table.
This is a technique that you may well want to use if you are creating a whole series of objects.
It enables you to use a single reference variable to create lots of objects. However, when the
reference variable is overwritten with the address of the next object, the existing objects are
safe because the internal table contains a reference to them. You therefore never delete the
"last" reference to the objects.
LESSON SUMMARY
You should now be able to:
●
Create objects in ABAP
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182
Unit 10
Lesson 3
Calling Methods
LESSON OBJECTIVES
After completing this lesson, you will be able to:
●
Call methods
Calling Methods
Figure 186: Calling Instance Methods
You call methods of instances of a class using a reference variable. The reference variable
must already contain the address of an object. If you try to call an instance method when the
reference variable is initial, you will cause a runtime error.
To call an instance method, you use the instance component selector following the reference
variable name. This is an arrow made up of a dash and the greater than sign. Once you have
typed the component selector, you can use the code completion in ADT to display a list of
instance components that you can address. Once you have identified the component that you
want to use, you can insert the name of the method and its full signature into your code by
pressing Shift + Enter.
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Unit 10: Creating and Using Classes
Figure 187: Calling Static Methods
To call a static method, you do not need a valid object reference. Instead, you use the name of
the class, followed by the static component selector. The static component selector is an
arrow made up of the equals sign and the greater than sign.
Figure 188: Method Call Syntax
The figure shows the different syntax forms that you can use to call methods. The general
syntax contains sections for exporting, importing, and changing parameters. Remember that
the directions exporting and importing are seen from the point of view of the program, and
that the exporting parameters in the method call correspond to the importing parameters of
the method and the other way around.
As well as the general syntax, there are also various shortened forms. For example, if the
method only has importing parameters, you can call it and omit the EXPORTING addition. If
the method only has one importing parameter, you can omit the explicit assignment of the
value to the parameter and just specify the value that you want to pass.
For methods with no signature, you can just write empty parentheses. Remember that there
must be at least one space between the opening and closing parenthesis.
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Lesson: Calling Methods
If a method has, for example, importing and exporting parameters, but you do not want to
take the exporting parameters into your program, you can write the method call as though the
method had only importing parameters. Likewise, if a method has a single obligatory
importing parameter and several optional parameters, and you do not want to specify the
optional parameters, you can write the method call as though the method had only one
importing parameter.
Figure 189: Explicit Exception Handling
If a method can raise exceptions, you must take this into account when you call it. You must
assume that an error will occur, since if the method triggers an exception to which you do not
react in your program, a runtime error will occur.
There are three different ways of reacting to exceptions from a method. The first, shown
above, is a separate reaction for each exception that can occur. If the method triggers an
exception with type cx_error_1, the program will jump to the CATCH cx_error_1 statement. If
the method triggers an exception with type cx_error_2, the program will jump to the CATCH
cx_error_2 statement. In this way, you can react specifically to each error that might occur.
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Unit 10: Creating and Using Classes
Figure 190: Bundled Exception Handling
You might not always need to write a separate error handling routine for each exception that
can occur during a method. If this is the case, you can bundle exceptions in a single CATCH
statement. In this example, the program will jump to the same CATCH statement regardless
of whether cx_error_1 or cx_error_2 occurs.
Figure 191: Exception Handling with sy-subrc
When you use older classes, you may find that they do not use class-based exceptions.
Instead, they have an exception system that is based on the system field sy-subrc.
There are two ways to recognize this kind of exception; in the method definition, the
exceptions are not declared using the RAISING addition, but instead using EXCEPTIONS.
Furthermore, if you use code completion to insert the method call into your program, you will
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Lesson: Calling Methods
see that the method signature contains an EXCEPTIONS block, and there is no TRY…
CATCH… ENDTRY block.
These exceptions work as follows: If the method raises an exception, control returns to the
calling program, and the system places the corresponding numeric value from the
EXCEPTIONS section of the method call in the system field sy-subrc. You can then query the
value of sy-subrc to find out if an error occurred and which it was.
If you do not want to handle the exceptions specifically, you can use the generic exception
OTHERS. For example:
reference->do_something( EXCEPTIONS others = 1 ). This means that the system would
assign the value 1 to sy-subrc in the event of any exception of the method being raised.
Note that you can mix explicit and generic exception handling by assigning numeric values to
some named exceptions but using OTHERS as well.
Figure 192: Functional Methods
Most ABAP methods have importing, exporting, and changing parameters. If you want to
check in your program whether an exporting parameter has the value zero, you must first
assign it to a variable and then test the value of the variable in your program.
Functional methods are ABAP methods that have a returning parameter. The difference
between returning and exporting parameters is that you can use a returning parameter
directly in an ABAP expression.
You define a functional method by declaring a RETURNING parameter in the signature. There
may only be one returning parameter, and you must pass it by value. As well as the one
returning parameter, the method may also have any number of importing, exporting and
changing parameters and, of course, exceptions.
The figure above shows two examples of how you can use functional methods in expressions;
the first is a simple example in which the returning parameter is assigned directly to a
variable. The second example shows how the method call can be used in an IF expression. The
system executes the method, and then uses the value of the returning parameter to process
the logical expression.
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Unit 10: Creating and Using Classes
LESSON SUMMARY
You should now be able to:
●
Call methods
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188
Unit 10
Lesson 4
Using Constructors
LESSON OBJECTIVES
After completing this lesson, you will be able to:
●
Use constructors
Using Constructors
Figure 193: Initial State of an Object
When you create a new instance of a class, all of its attributes have the initial value
appropriate to their type. With the vehicle class, we therefore had to call the set_attributes
method in order to assign a make and model to a new vehicle. This is, of course, not ideal, as it
means that the user of the class needs to know that this is necessary, and may forget to call
the method. Furthermore, you can call a normal instance method any number of times and
this may not be suitable for some actions that you need once only to initialize a new object.
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Unit 10: Creating and Using Classes
Figure 194: Instance Constructor
To initialize a new instance of a class properly you use the instance constructor. This is a
special method with the following properties:
●
It is a public instance method with the reserved name constructor.
●
It is called automatically by the runtime system when you create a new instance of the
class, but may not be called explicitly.
●
It is guaranteed to run once and once only for each instance that you create.
●
It may have importing parameters that you use to obtain starting values for the attributes
of the new instance.
●
It may raise exceptions.
Figure 195: Instantiating Classes That Have a Constructor
When you instantiate a class that has a constructor, the system calls the method
automatically. If the constructor has importing parameters, you pass them in the NEW
expression exactly as you would pass parameters to a normal method.
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Lesson: Using Constructors
If the constructor has exceptions, you must ensure that you catch them by enclosing the
instantiation in a TRY… CATCH...ENDTRY block. If the constructor raises an exception,
control returns immediately to the program containing the NEW expression. In this case, you
do not receive a new instance of the class.
Figure 196: Defining the Static Constructor
While the instance constructor is called once when each instance of a class is created, you will
sometimes need to perform actions once only for the entire class. For example, the vehicle
class contains a static attribute gt_vehicle_types, which is an internal table containing all of
the permitted combinations of make and model of vehicle. Since the table needs to be filled
only once, and the data must be available as soon as anyone tries to instantiate the class, the
ideal place to fill the table is in the static constructor.
The static constructor is a public static method with the reserved name class_constructor.
The runtime system calls the method automatically and once only when the class is
addressed for the first time. This can be:
●
When a program instantiates the class
●
When a program calls a static method of the class
●
When a program accesses a public static attribute of the class
Note that this is not a complete list. There are other actions (related to inheritance) that can
cause a class to be addressed for the first time.
The runtime system calls the static constructor before creating the instance, calling the static
method, or addressing the static attribute.
The static constructor may not have a signature. This is because it is impossible to tell exactly
when a class will be addressed for the first time.
LESSON SUMMARY
You should now be able to:
●
Use constructors
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191
Unit 10
Lesson 5
Using Factory Methods
LESSON OBJECTIVES
After completing this lesson, you will be able to:
●
Use factory methods
Using Factory Methods
Figure 197: Not All Classes Let You Use NEW
You will sometimes come across classes that do not let you create instances directly using
NEW. You will recognize this by the syntax error shown in the figure. There are in fact, two
kinds of class that you cannot instantiate. One kind is abstract classes, which you will learn
about later in connection with inheritance. The other kind is any class that is defined with the
addition CREATE PRIVATE. These classes need to restrict who is allowed to create instances,
so they decide that only a method of the class itself may do so.
Here are some reasons why a class might restrict its instantiation:
●
The class provides a central function in the application and there must only ever be one
instance.
●
Instances of the class represent a particular business entity (flight, employee, material)
and there should only be one instance of the class per business key (and not twelve
different instances all pertaining to the same material number).
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Lesson: Using Factory Methods
●
The class may need to perform some checks before creating an instance, or some kind of
registration of the new instance after the constructor has finished.
Whatever the reason, a class with private instantiation must still provide its users with a way
to obtain instances of the class. This is most often done using a factory method.
Figure 198: Using a Factory Method
A factory method is a static method whose job it is to create and return an instance of the
class. The method will often have importing parameters that allow you to specify key
information about the object you need, and there will be an exporting or a returning
parameter with the type of the class itself by which the method gives you the instance you
requested.
If a class is designed for there only to be one instance of it, it may have a factory method.
However, it is also possible that the instance is stored in a public static attribute so that you
can retrieve it directly. In this case, the class uses its static constructor to create the single
instance.
LESSON SUMMARY
You should now be able to:
●
Use factory methods
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193
Unit 10
Learning Assessment
1. In what sequence must you declare the visibility sections of a class?
Choose the correct answer.
X
A PRIVATE SECTION. PROTECTED SECTION. PUBLIC SECTION.
X
B PROTECED SECTION. PUBLIC. SECTION. PRIVATE SECTION.
X
C PUBLIC SECTION. PROTECTED SECTION. PRIVATE SECTION.
X
D You can order the sections however you like.
2. Which of the following statements can you use in the definition part of a class?
Choose the correct answers.
X
A DATA
X
B TABLES
X
C TYPES
X
D CONSTANTS
3. When you declare a reference variable using DATA... TYPE REF TO, the newly-declared
variable already points to an instance of the class.
Determine whether this statement is true or false.
X
True
X
False
4. What happens to objects that are no longer required and to which no more reference
variables are pointing?
Choose the correct answer.
X
A Nothing. They remain in the program memory until the end of the program.
X
B They are destroyed when you release the memory explicitly.
X
C They are destroyed when you call the garbage collector explicitly.
X
D They are destroyed when the garbage collector next runs automatically.
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Unit 10: Learning Assessment
5. You have a reference variable oref that points to a valid instance of a class. You want to
call the instance method meth. You do not need to pass any parameters to the method.
Which of the following is the correct method call?
Choose the correct answer.
X
A oref=>meth( )
X
B oref=>meth()
X
C oref->meth( )
X
D oref->meth()
6. Which of the following elements may an instance constructor have?
Choose the correct answers.
X
A Importing parameters
X
B Exporting parameters
X
C Changing parameters
X
D Exceptions
7. What happens when a constructor raises an exception?
Choose the correct answer.
X
A The method terminates and the new instance is returned.
X
B The method terminates and no new instance is returned.
X
C The method continues to the end and the new instance is returned.
X
D The method continues to the end and no new instance is returned.
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195
Unit 10
Learning Assessment - Answers
1. In what sequence must you declare the visibility sections of a class?
Choose the correct answer.
X
A PRIVATE SECTION. PROTECTED SECTION. PUBLIC SECTION.
X
B PROTECED SECTION. PUBLIC. SECTION. PRIVATE SECTION.
X
C PUBLIC SECTION. PROTECTED SECTION. PRIVATE SECTION.
X
D You can order the sections however you like.
2. Which of the following statements can you use in the definition part of a class?
Choose the correct answers.
X
A DATA
X
B TABLES
X
C TYPES
X
D CONSTANTS
3. When you declare a reference variable using DATA... TYPE REF TO, the newly-declared
variable already points to an instance of the class.
Determine whether this statement is true or false.
X
True
X
False
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Unit 10: Learning Assessment - Answers
4. What happens to objects that are no longer required and to which no more reference
variables are pointing?
Choose the correct answer.
X
A Nothing. They remain in the program memory until the end of the program.
X
B They are destroyed when you release the memory explicitly.
X
C They are destroyed when you call the garbage collector explicitly.
X
D They are destroyed when the garbage collector next runs automatically.
5. You have a reference variable oref that points to a valid instance of a class. You want to
call the instance method meth. You do not need to pass any parameters to the method.
Which of the following is the correct method call?
Choose the correct answer.
X
A oref=>meth( )
X
B oref=>meth()
X
C oref->meth( )
X
D oref->meth()
6. Which of the following elements may an instance constructor have?
Choose the correct answers.
X
A Importing parameters
X
B Exporting parameters
X
C Changing parameters
X
D Exceptions
7. What happens when a constructor raises an exception?
Choose the correct answer.
X
A The method terminates and the new instance is returned.
X
B The method terminates and no new instance is returned.
X
C The method continues to the end and the new instance is returned.
X
D The method continues to the end and no new instance is returned.
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197
UNIT 11
Using Inheritance
Lesson 1
Inheritance in ABAP
199
Lesson 2
Special Features of Inheritance
216
UNIT OBJECTIVES
●
Implement inheritance
●
Use inheritance
●
Understand abstract and final classes
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198
Unit 11
Lesson 1
Inheritance in ABAP
LESSON OBJECTIVES
After completing this lesson, you will be able to:
●
Implement inheritance
●
Use inheritance
Implementing Inheritance
Figure 199: Generalization and Specialization
When we talked about modeling, we discussed the idea of having general and more
specialized classes. In the example, there is a generic vehicle class that contains all of the
elements common to all different kinds of vehicles. We then extended the model by defining
more specialized kinds of vehicle – a truck and a bus. This kind of modeling enables us to
transform a flat, unstructured list of classes into a more easily understandable hierarchy.
We must consider the role of the vehicle class in this hierarchy. In some cases, there will be
actual things in the real world with the type "vehicle". In that case, we would need instances of
the class in the application. However, it is also possible that the application only contains
trucks and buses, but for the purpose of modeling the application, we decided to invent a
vehicle class, even though the application will only need instances of the truck and bus class
and there will never be a real vehicle. This approach is allowed, and in that case, we refer to
the vehicle class as abstract. It is a help for modeling purposes, but a program may not create
any instances of it.
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Unit 11: Using Inheritance
Figure 200: Inheritance – The "Is A" Relationship
In modeling, we talk about specialization. The implementation of this relationship in a
programming language uses the concept of inheritance, in which a new class (the subclass) is
derived from an existing one (the superclass). A reliable test to see if there is a genuine
inheritance relationship between two classes is to try to say "A subclass is a superclass". If
this is clearly true, you have an inheritance relationship. If it is clearly not true, or even sounds
a bit strange, you do not have an inheritance relationship, and should not implement one – it
will probably get you into trouble later on! Testing our model, we can definitely say "A truck is
a vehicle " and "A bus is a vehicle".
A class that inherits from a superclass automatically contains all of the components of the
superclass. So, for example, the truck class has a make and a model, just as a vehicle does.
This should not be surprising, as a truck "is a" vehicle. What may at first appear surprising is
that the truck class cannot access its own make and model. This is because they are private
attributes of the superclass and, as such, they are not accessible to the subclass. Later on you
will learn a way of allowing a subclass to access components of a superclass without making
them fully public.
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200
Lesson: Inheritance in ABAP
Figure 201: Defining the Inheritance Relationship
In ABAP, you implement inheritance using the INHERITING FROM addition in the class
definition statement. In this example, the truck class is defined as a subclass of the vehicle
class. Note that the subclass knows its superclass, but the vehicle class has no idea what
subclasses it has (if any).
A subclass may only have one direct superclass. Some other object-oriented languages such
as C++ allow a class to inherit from multiple superclasses, but this is not permitted in ABAP.
Figure 202: One Superclass – Any Number of Subclasses
The vehicle class may have any number of subclasses, but it does not know about any of
them.
Each subclass has a single direct superclass. It knows its superclass, but does not know about
any sibling classes (other classes that are derived from the same superclass). In our example,
the truck class has no knowledge of the bus class.
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Figure 203: Multi-Level Hierarchies
Although a class may only have one direct superclass, it may be a part of a multi-level
hierarchy. This happens frequently in complex frameworks. Here, you can see an extract of
the class hierarchy of an ABAP framework called Run Time Type Identification. It has several
hierarchy levels with a very general class at the top. As you move down the hierarchy, the
classes become progressively more specific. If you move from the bottom of the hierarchy to
the top, you will see that every class has a maximum of one direct superclass.
When you are dealing with inheritance in a particular class, you only need to concern yourself
with the next-highest class, and not with the levels above that. The superclass of your own
class deals with its own superclass, and so on until you get to the top of the hierarchy.
Figure 204: What You Can Do with a Freshly Inherited Class
When you derive a new class from a superclass you can, in fact already use it. As long as the
superclass was not abstract, you can instantiate the subclass and call its public methods.
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When you instantiate the class, you must supply the obligatory parameters of the constructor
of the superclass. This makes perfect sense, since (in this case) a truck is a vehicle, and
instantiating a vehicle involves calling its constructor. You will learn in a moment how to add a
new constructor to the subclass.
When you call the public methods that have been inherited from the superclass, the runtime
system executes the implementation as defined in the superclass. One of the things you need
to consider when you define a subclass is whether the method implementations in the
superclass are appropriate to the subclass. If they are, you can leave things as they are. If they
are not, you will be able to supply a different implementation for an inherited method in the
subclass.
Figure 205: How to Extend a Subclass
Once you have declared a subclass, you need to add new components to it or change existing
components. This is what turns the mere copy of the vehicle class into a truck or a bus.
Thanks to inheritance, you do not need to implement the class from scratch – you merely
extend it by the new components that it needs.
There are three ways to do this:
Add new components: You can declare new attributes, types, constants, and methods in the
class. Their names must not clash with other names that have already been used to declare
components in the superclass. You must also ensure that the new components are only
relevant to the subclass, and not to all specializations of the superclass. If you declare a new
attribute or method that is generally relevant, you must move it into the superclass.
Redefine methods: When you call an inherited method, the runtime system executes the
implementation of the method from the superclass. This implementation cannot, however,
take into account any new components that you have declared in the subclass. In this case,
you may redefine the method. This means that you can assign it a new implementation that is
relevant to the subclass.
Add a new constructor: Constructors ensure that new instances of the class are properly
initialized. When you create an instance of a subclass, the runtime system always executes
the constructor of the superclass. This ensures, for example, that a truck has a make and a
model in just the same way as any other vehicle. However, the constructor of the superclass
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cannot initialize attributes of the subclass, as it does not know that they exist. For this reason,
you can define a new constructor for the subclass.
Figure 206: Declaring New Components
You can extend a subclass by declaring new components. These are visible to the class itself,
but not to the superclass, and not to any other classes that inherit from the same superclass.
You therefore need to consider whether the new components are only relevant in the current
class or if they actually need to be declared in the superclass.
It is not uncommon for you to model a class, begin to implement it, and then to realize that a
particular component is in the wrong place. ADT helps you to solve this and other problems by
providing refactoring functions. Refactoring means changing the definition of a class by
moving an element, changing its visibility, or renaming it. The danger of refactoring is that it
can invalidate other code where the element that you are changing has already been used.
The refactoring tools in ADT help by identifying where the element has already been used and
adapting the code to fit the new definition of the object.
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Figure 207: Refactoring in ADT
In this example, we had added the attribute mv_color to the truck class. However, we have
subsequently decided that color is a general attribute shared by all vehicles. To refactor the
attribute, position the cursor on the name of the attribute and press Ctrl + 1 to open the Quick
Fix suggestions.
ADT suggests what you might like to do with the attribute – you can rename it, change its
visibility, but also pull the attribute up into the superclass lcl_vehicle. This moves the attribute
into the superclass and deletes it from the current class.
Note that you can only move attributes up the inheritance hierarchy in this way; there is no
way of pushing an attribute from a superclass to a subclass.
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Unit 11: Using Inheritance
Figure 208: Protected Components
Even though a subclass contains all of the attributes of its superclass, it is not allowed to
access the private components directly by itself. Sometimes this is necessary, but there are
other times when a superclass needs to let its subclasses access particular attributes or
methods without making them fully public. It can do this by declaring them in the protected
section. Protected components are visible within the class itself but also to all subclasses.
In the class definition, you introduce the protected section using the statement PROTECTED
SECTION. You must always declare the visibility sections in the sequence PUBLIC SECTION PROTECTED SECTION - PRIVATE SECTION.
You can move an existing component of a class using a Quick Fix; to do so, place the cursor on
the name of the component in ADT and press Ctrl + 1, then choose the quick fix Make
<element> protected.
Moving a private component to the protected section is a compatible change – you have
broadened the visibility of the component. However, making a public component protected is
an incompatible change and could cause syntax errors. This is because you have restricted
the visibility of the component, which might already have been used outside the inheritance
hierarchy.
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Figure 209: Redefining Methods
Superclasses often contain methods that you want to use in a subclass, but their
implementations are unsuitable as they do not take into account the particular nature of the
subclass. For example, the get_attributes method of the vehicle class cannot return attributes
that are defined in the truck class. However, the truck class still needs a method
get_attributes.
In ABAP, you can solve this conundrum by redefining the instance method get_attributes.
When you redefine a method, you write a new implementation for it in the subclass that takes
into account things that only the subclass can know. However, the definition of the method
stays exactly the same. You must follow these rules:
●
The method retains the same name.
●
You indicate that you want to redefine the method by declaring it in the subclass with the
addition REDEFINITION.
●
The method has the same visibility as in the superclass.
●
You must not change the signature of the method.
You cannot redefine a static method.
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Figure 210: Implementing the Redefinition
When you implement the redefinition of a method, you can call the implementation in the
superclass. You do this using the implicit object reference super, which points to the
superclass within the current instance. In this way, you can reuse the method implementation
from the superclass and then add to it in your own implementation.
You do not need to declare the reference variable super. It is automatically available within the
implementation of redefined methods.
Calling the method in the superclass in this way is optional. Sometimes it will be a good idea,
sometimes there is nothing to be gained by it.
Figure 211: Defining a New Constructor
In a subclass, you may define a new constructor. In contrast to method redefinitions, a
constructor may have its own signature. The signature of a constructor will often contain the
same parameters as the constructor of the superclass, plus new parameters of its own. This
is because the new instance constructor has two tasks: firstly, to call the constructor of the
superclass, then to take care of initializing its own attributes that are specific to the subclass.
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Lesson: Inheritance in ABAP
As well as defining a new constructor, you can also define a new static constructor in a
subclass.
Figure 212: Role of the Constructor
The constructor of a subclass has two functions; it must ensure that the instance of the
superclass is properly created, and after that, it can set the initial values of its own attributes.
In our example, since a truck is a vehicle, there can be no truck instance unless the
constructor of the vehicle class has run successfully to set the make and model of the new
truck. After this, and only after this, can the truck class set its own attributes.
The constructor of the subclass must therefore be able to set not only its own attributes, but
to pass the required values to the constructor of the superclass. This is why the signature of
the constructor in the subclass often contains the entire signature of the constructor that it is
required to call.
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Unit 11: Using Inheritance
Figure 213: Implementing the New Constructor – Sequence
In the implementation of the new constructor, there is a particular sequence of operations
that you must observe. The most important event is the call to the constructor of the
superclass. This is obligatory and you call the method using the implicit reference super, just
as you can when calling the original implementation of a redefined method. Once again, you
do not need to declare super explicitly.
Before you have called the constructor of the superclass, you must not address any instance
components of the instance that is under construction. You may address static components
of the class and check the correctness of the importing parameters of the constructor. If they
are not correct, raise an exception.
Once the constructor of the superclass has run successfully, you may address the instance
components of your new instance.
Figure 214: Sequence of Constructor Calls
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Lesson: Inheritance in ABAP
The figure illustrates the sequence in which static and instance constructors are called. The
example assumes that a program wants to instantiate the truck class and that the vehicle
class has not previously been addressed.
When the program tries to create an instance of the truck class, the runtime system
automatically calls the static constructors sequentially, starting at the top of the inheritance
hierarchy. Once all of the static constructors have run, the runtime system calls the instance
constructor of the subclass, which in turn calls the instance constructor of the superclass.
Using Inheritance
Figure 215: Instantiating Subclasses
Figure 216: Using Superclass References
A reference variable with the type of a superclass – in this case lcl_vehicle – can also hold
references to objects with the type of a subclass. This makes sense, since a truck or a bus is a
vehicle.
However, a reference variable with the type of the superclass is only aware of the components
defined in the superclass. Consequently, when you use a reference with this type to manage
an instance of a subclass, you can only address the components from the superclass, even
though the actual object to which the reference points is an instance of the subclass.
Assigning object references to a reference variable of a different type is called casting.
Assigning a reference that points to a subclass to a reference variable with the type of a
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Unit 11: Using Inheritance
superclass is called an up-cast, because you are making the assignment to a type higher up in
the inheritance hierarchy.
Figure 217: Calling Methods After an Up-Cast
When you use a reference variable with type ref to lcl_Vehicle to manage a truck instance, you
cannot use it to access components that belong to the truck class, as the vehicle class does
not know that they exist and the syntax check cannot know what type of object the reference
will actually point to at runtime.
Using the reference, you can, however, access methods whose original definition is contained
in the class. This is the case with the get_attributes method in the example in the figure. The
question here is which implementation of the method will be called. In this case, the system
looks at runtime to see what type of object the reference variable is pointing to, and calls the
implementation from the corresponding class. In other words, although we are using a vehicle
reference to manage an instance of the truck class, it is still the method implementation from
the truck class that is called.
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Lesson: Inheritance in ABAP
Figure 218: Generic Types in Method Signatures
You will often use methods that return object references for you to work with. Sometimes
they always return an object of the same type, but some factory methods can return
instances of different classes depending on what you requested from them. So, for example,
there might be a factory method that returns either a truck or a bus according to your
request. The type of the returning parameter in this case must be a generic type that fits both
the truck and the bus class. This generic type is, of course, their common superclass
lcl_vehicle.
When you use this method, the object comes back to you in a reference with a generic type.
This, of course, means that you can only access the components of the object that are
actually defined in that class. In order to access specific components, you must copy the
object reference back into a reference variable with the more specific type. This operation is
called a down-cast.
Figure 219: Assigning a Generic Reference to a Specific Type
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Unit 11: Using Inheritance
You can always assign a reference variable with the type of a subclass to a reference variable
with the type of a superclass. For example, you can always assign a truck reference to a
vehicle reference. As we have seen, this is because a truck is a vehicle. Assigning reference
variables in the opposite direction is, however, more tricky. This is because a vehicle
reference can contain references to other classes than the truck class. In other words, where
every truck is a vehicle not every vehicle is a truck.
Given object references go_vehicle with type ref to lcl_vehicle and go_truck with type ref to
lcl_truck, the system will always allow you to write go_vehicle = go_truck. However, the
assignment go_truck = go_vehicle leads to a syntax error because the syntax check cannot be
sufficiently sure that the vehicle reference will actually hold a reference to a truck object at
that moment.
Figure 220: The CAST Operator
The syntax check does not let you assign a superclass reference directly to a subclass
reference. However, you can still do it using the CAST operator. Casting means looking at an
object as though it had a different type. In the example, we tell the system to treat go_vehicle
as though it were an instance of the truck class. This is syntactically correct, as we are now
pretending that go_vehichle on the right-hand side of the expression has the same type as
go_truck on the left-hand side. However, there is still not guaranteed that go_vehicle actually
contains a truck reference, and if it turns out not to contain one at runtime, the assignment
cannot be made. Left unprotected, this assignment could cause a runtime error.
In older coding, you will see the expression go_truck ?= go_vehicle. ?= is also a cast operator;
however, the introduction of the CAST expression makes the coding easier to understand.
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Lesson: Inheritance in ABAP
Figure 221: Securing a Down-Cast
If you are not certain that the object to which a reference variable is pointing is actually an
instance of the correct subclass, you can check it using the logical expression IS INSTANCE
OF. In this example, the assignment to the truck reference is only processed if go_vehicle
actually points to an instance of the class lcl_truck.
This is a new expression in ABAP. In older coding, you will find the following solution to the
same problem:
TRY.
go_truck ?= go_vehicle.
CATCH cx_sy_move_cast_error.
ENDTRY.
In this example, the unprotected assignment causes the exception cx_sy_move_cast_error if
the vehicle reference points to an instance of a different class. The CATCH block is merely
there to prevent an uncaught exception ending in a runtime error.
LESSON SUMMARY
You should now be able to:
●
Implement inheritance
●
Use inheritance
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Unit 11
Lesson 2
Special Features of Inheritance
LESSON OBJECTIVES
After completing this lesson, you will be able to:
●
Understand abstract and final classes
Understanding Abstract and Final Classes
Figure 222: Abstract Classes and Methods
When you work with inheritance, you will sometimes define classes that are useful in the
sense of software modeling because they allow you to bundle components that two or more
other classes have in common. However, at the same time, they do not represent complete
entities in your application. In other words, you need the class, but you do not want to have
instances of it in the application. In this case, you can use abstract classes.
An abstract class cannot be instantiated. It can contain all of the usual components – types,
attributes, constants, and methods – and the components can be either instance
components or static components. The methods of a static class may be fully implemented
so that the implementation can be passed on to the subclasses. On the other hand, you may
wish to define a method so as to ensure that all of the subclasses of this class contain it, but
there is no common implementation. In this case, you may define a method as abstract.
Abstract methods do not have an implementation in the class in which they are declared.
You can instantiate subclasses of an abstract class under the following conditions:
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Lesson: Special Features of Inheritance
●
The subclass must not be abstract itself
●
The subclass re-implements any abstract methods of the superclass.
Although you cannot instantiate a subclass, you can still do the following with it:
●
Use it as a type for reference variables; even though there are no instances, it can still point
to instances of any of its subclasses
●
Use its types and constants
●
Use its static attributes
●
Use its static methods
Figure 223: Final Classes and Methods
While abstract methods must have subclasses in order to be useful, there will also be times
when you do not want an inheritance hierarchy to be extended any further. Here, you can use
the FINAL addition when you define a class. Final classes may not have any subclasses.
You can also define individual methods as final. In this case, you can derive subclasses from
the class itself, but you may not redefine any of the methods that have been classified as final.
This could be particularly useful in the case of methods that perform security checks. A
subclass could bypass these checks by redefining the method and leaving the method
implementation empty. If the method is defined as final in the superclass, this is not allowed.
LESSON SUMMARY
You should now be able to:
●
Understand abstract and final classes
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Unit 11
Learning Assessment
1. In ABAP, each subclass may have only one direct superclass.
Determine whether this statement is true or false.
X
True
X
False
2. Which of the following statements apply when you redefine a method in a subclass?
Choose the correct answers.
X
A You must not change the name of the method.
X
B You must not change the signature of the method.
X
C You must not change the visibility of the method.
X
D You must not change the coding of the method.
3. You have an instance of a subclass and want to assign it to a reference variable that has
the type of the superclass. Which of the following statements is true?
Choose the correct answer.
X
A You cannot do this because the types are incompatible.
X
B You can always assign an instance of a subclass to superclass reference.
X
C You can only assign an instance of a subclass to a superclass reference after you
have checked that the types are compatible.
X
D You can only do this if the superclass provides a suitable method that performs the
assignment.
4. When you use a reference variable with the type of a superclass to call a method, the
runtime system always calls the method implementation as it is defined in the superclass.
Determine whether this statement is true or false.
X
True
X
False
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Unit 11
Learning Assessment - Answers
1. In ABAP, each subclass may have only one direct superclass.
Determine whether this statement is true or false.
X
True
X
False
2. Which of the following statements apply when you redefine a method in a subclass?
Choose the correct answers.
X
A You must not change the name of the method.
X
B You must not change the signature of the method.
X
C You must not change the visibility of the method.
X
D You must not change the coding of the method.
3. You have an instance of a subclass and want to assign it to a reference variable that has
the type of the superclass. Which of the following statements is true?
Choose the correct answer.
X
A You cannot do this because the types are incompatible.
X
B You can always assign an instance of a subclass to superclass reference.
X
C You can only assign an instance of a subclass to a superclass reference after you
have checked that the types are compatible.
X
D You can only do this if the superclass provides a suitable method that performs the
assignment.
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Unit 11: Learning Assessment - Answers
4. When you use a reference variable with the type of a superclass to call a method, the
runtime system always calls the method implementation as it is defined in the superclass.
Determine whether this statement is true or false.
X
True
X
False
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UNIT 12
Using Interfaces
Lesson 1
Using Interfaces
222
UNIT OBJECTIVES
●
Use interfaces
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Unit 12
Lesson 1
Using Interfaces
LESSON OBJECTIVES
After completing this lesson, you will be able to:
●
Use interfaces
Defining And Implementing Interfaces
Figure 224: Reasons to Use Interfaces
Interfaces allow you to define how a program will address one or more classes. In an interface,
you can define methods, attributes, types, and constants, just as you would in a class. These
components form a description of how a client program could interact with the corresponding
objects. However, an interface cannot be instantiated, and does not contain any
implementations of its methods. To make the concept work, we also need one or more
classes that implement the interface. This means that they include the interface in their own
class declaration and the components that are declared in the interface become part of the
class itself. If the interface contains method declarations, the class that implements the
interface must also provide an implementation of that method.
Interfaces have two important uses:
Providing a unified approach to different classes:
If you have classes that are not related by inheritance, but that should still provide common
services to their users, you can define the common services in an interface. This ensures that
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Lesson: Using Interfaces
each class provides a method with the same name and the same signature to perform a
particular task. Programs that use the classes then have a single way to call particular
functions regardless of which class they are actually addressing.
Providing a simplified approach to a complex class:
As you will see later on, interfaces provide a restricted view of an object. This enables you to
expose a particular set of methods to a program that will use a particular class on the basis of
the functions that program actually requires.
Figure 225: Interface Definition
The definition of an interface is very similar to the definition section of a class. However, since
an interface does not have an implementation, there is no DEFINITION addition in the
INTERFACE statement. The second major difference is that there are no visibility sections in
an interface, as all components of an interface are public. Interfaces are useful because of
their interaction with other classes, so having non-public components would make no sense.
In the interface definition, you can use types, attributes, constants, and methods, and both
instance and static components are allowed. Remember that interface methods have a
definition, but no implementation. This will be supplied by the implementing class.
In many ways, interfaces are similar to abstract superclasses. They provide a useful technique
for cases in which you need to define common components for classes without there being a
case for inheritance, in other words, there is no clear "is a" relationship.
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Unit 12: Using Interfaces
Figure 226: Declaring the Interface
To implement an interface in a class, you use the INTERFACES statement. As all components
of the interface are public and you cannot change the visibility of an existing component, you
must include the interface in the public section of the class.
As soon as you have written the INTERFACES statement, your program is syntactically
incorrect. This is because the interface contains a method that you have not yet implemented.
You can use a quick fix (Ctrl + 1) to add the method implementation to your class.
Generally, you must implement all interface methods in your class. However, some interfaces
contain optional methods. You can recognize these in the interface definition by the addition
DEFAULT IGNORE or DEFAULT FAIL. You only implement optional methods if you actually
need them in your application. If a class does not implement an optional method, but a
program calls the method anyway, the system reacts either by ignoring the call (DEFAULT
IGNORE) or by raising the exception CX_SY_DYN_CALL_ILLEGAL_METHOD (DEFAULT FAIL).
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Lesson: Using Interfaces
Figure 227: Implementing the Interface Methods
In the implementation of your class, you must implement all non-optional methods that are
declared in the interface. When you name the method in the implementation, you must use
the fully qualified name, which consists of the interface name and the method name joined by
a tilde. This is because there is no guarantee that the method name by itself is unambiguous;
remember that component names must only be unique in the class or interface in which they
are declared. Consequently, a class could declare a method called get_partner_attributes as
well as the interface lif_partner.
In an implementing class, you can declare alias names for interface components. So, for
example, in the public section of the class, after the INTERFACES statement, you could
declare:
ALIASES get_partner_attributes FOR lif_partner~get_partner_attributes.
You can then address the interface method as get_partner_attributes throughout the class.
Since the alias is public, users of the class can also do the same.
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Unit 12: Using Interfaces
Using Interface References
Figure 228: Declaring an Interface Reference
You can use interfaces as a reference type for declaring a reference variable. As you learned
earlier, you use reference variables to create and manage instances of a class. Since it is not
possible to instantiate an interface, you may be wondering what kinds of objects you can
manage using an interface reference.
Interface references may contain references to instances of any class that implements the
interface in the same way that a reference variable with the type of a superclass can contain
references to any subclasses. (If you now imagine that the superclass is abstract, you have an
identical situation of a type that cannot be instantiated holding references to other classes
that are related to it).
When you use a variable with the type of a superclass to manage an instance of one of its
subclasses, you can only access the components that are defined in the superclass. Similarly,
when you use an interface reference to manage an instance of a class that implements the
interface, you can only access the components that are defined in the interface. This means
that the developer of a particular framework of classes can ship interfaces that provide a
particular view of an object instead of exposing every single thing that the class can do.
You can call a method from an interface using a reference variable with the type of the class.
In this case, you must use the fully qualified name of the method (including the interface
name). (If you have defined an alias for the method, you can use that). When you call an
interface method using an interface reference, the name of the interface is inferred from the
type of the reference variable, and you do not need to specify the interface name before the
method name.
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Lesson: Using Interfaces
Figure 229: Casting with Interface References
Assigning object references to an interface reference is also a cast; you look at the rental or
carrier object as though it were a business partner. You can always assign an instance of the
implementing class to an interface reference. This is an up-cast, and is guaranteed by the fact
that the class implements the interface. You cannot, however, assign an object from an
interface reference to a reference with the type of an implementing class, since the syntax
check cannot tell whether the actual runtime objects will be compatible. The assignment
go_rental = go_partner would work if the object in the interface reference really is an instance
of lcl_rental, but if it is an instance of another implementing class, you would cause a runtime
error.
Here we have a down-cast in the same way that we encountered it in inheritance. The solution
is the same as well; if you need to assign an object reference from a reference variable with
the type of an interface to one with the type of a class (so that you can access specific
features of the class), you must use the CAST operator. The correct solution in this case
would be:
IF go_partner IS INSTANCE OF lcl_rental.
go_rental = CAST #( go_partner ).
ENDIF.
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Unit 12: Using Interfaces
Figure 230: Generic Types in Method Signatures
We have already seen that methods of frameworks often work with generic types to allow
them to return instances of different classes. Sometimes they do this using superclass
references, but other methods will use interface references instead.
A method that has an interface type for an exporting or returning parameter can use it to
provide an instance of any class that implements the given interface. When you use this kind
of method in your program, you can either receive the parameter into a variable with the type
of the interface or, if you need to access specific components of the class that are not
contained in the interface description, you can cast the value onto a variable of the
corresponding type. If the method has a returning parameter, you can do this directly in the
method call:
DATA go_rental TYPE REF TO lcl_rental.
go_rental = CAST #( lcl_partner_factory=>create_partner( ) ).
If the method has an exporting parameter, you must first receive the object reference into a
variable of its exact type, then perform the cast in a subsequent step.
LESSON SUMMARY
You should now be able to:
●
Use interfaces
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Unit 12
Learning Assessment
1. Which of the following statements are true of interfaces?
Choose the correct answers.
X
A They may contain method definitions.
X
B They may contain method implementations.
X
C They may contain private attributes.
X
D They may contain type definitions.
2. A class that implements an interface must implement all of the methods defined in that
interface.
Determine whether this statement is true or false.
X
True
X
False
3. What instances can you assign to an interface reference?
Choose the correct answer.
X
A Instances of the interface.
X
B Instances of any classes that implement the interface
X
C Instances of any local classes in the same program.
X
D Instances of any class.
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Unit 12
Learning Assessment - Answers
1. Which of the following statements are true of interfaces?
Choose the correct answers.
X
A They may contain method definitions.
X
B They may contain method implementations.
X
C They may contain private attributes.
X
D They may contain type definitions.
2. A class that implements an interface must implement all of the methods defined in that
interface.
Determine whether this statement is true or false.
X
True
X
False
3. What instances can you assign to an interface reference?
Choose the correct answer.
X
A Instances of the interface.
X
B Instances of any classes that implement the interface
X
C Instances of any local classes in the same program.
X
D Instances of any class.
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230