CS 5150
Software Engineering
Lecture 15
Program Design 1
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Administration
Test 3 is next Monday
The test will cover material from all classes since
the beginning of the semester, including next
Monday's class.
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Comments on Assignment 2: Schedules
The schedule must provide time for:
• User testing and revisions
• Program and system testing
• Revision and correction of problems
discovered during testing
• Deployment
• Handover and acceptance testing
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Architectural Style - Model/View/Controller
Example: Control of a unmanned model aircraft
Controller
Model
View
Controller: Receives instrument readings from the aircraft and
sends controls signals to the aircraft.
Model: Translates data received from and sent to the aircraft, and
instructions from the user into a model of flight performance.
Uses domain knowledge about the aircraft and flight.
View: Displays information about the aircraft to the user and
transmits instructions to the model.
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Model/View/Controller: Autonomous Land
Vehicle
View
Model
Controller
GPS
Steer
Sonar
Model
Laser
Control
signals
Throttle
Controls
Sensors
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Signal
processing
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Model/View/Controller for Web Applications
1
User interacts with the user interface (e.g., presses a mouse
button).
2
Controller handles input event from the user interface, (e.g.,
via a registered handler or callback) and converts the event
into appropriate user action.
3 Controller notifies Model of user action, possibly resulting
in a change in Model's state (e.g., update shopping cart.).
4
View interacts with Model to generate an appropriate user
interface response (e.g., list shopping cart's contents).
5
User interface waits for further user interactions.
from Wikipedia 10/18/2009
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Model/View/Controller for Web Applications
WebBrowser
view
functions
WebBrowser
control functions
HTTP
Controller
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Response
Input events
Model
HTML
View
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Program Design in the Modified Waterfall Model
Feasibility study
Requirements
Requirements
System design
Design
Program design
Implementation
Implementation (coding)
Testing
Acceptance & release
Operation & maintenance
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Program Design
The task of program design is to represent the software
system functions in a form that can be transformed into
one or more executable programs.
Given a system architecture, the program design specifies:
• programs, components, packages, classes and class hierarchies
• interfaces, protocols (where not part of the system architecture)
• algorithms, data structures, security mechanisms, operational
procedures
If the program design is done properly, all design decisions should
be made before implementation.
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UML Models
UML models (diagrams and specifications) can be used
for almost all aspects of program design
•
Diagram gives a general overview of the model, showing the
principal elements and how they relate to each other.
•
Specification provides details about each element of the
model. Specification for models used in program design
should have sufficient detail that they can be used to write
code from.
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Class Diagrams
Window
origin
size
open()
close()
move()
display()
name
attributes [local, instance, and
class (static) variables]
operations [methods]
responsibilities [optional text]
A class is a description of a set of objects that share the same
attributes, operations, relationships, and semantics.
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The "Hello, World!" Applet
import java.awt.Graphics;
class HelloWorld extends java.applet.Applet {
public void paint (Graphics g) {
g.drawString ("Hello, World!", 10, 10);
}
}
Example from: BJR
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The HelloWorld Example
class
name
HelloWorld
operations paint()
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Abstraction for HelloWorld
class
name
HelloWorld
annotation
operations paint()
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g.drawString
("HelloWorld", 0, 10)"
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Annotation
some text
note
A note is a symbol for rendering constraints and
comments attached to an element or a collection of
elements.
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Rational Rose: A Typical Class Diagram
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Specification Fields
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General Specification Fields
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Notation: Relationships
A dependency is a semantic relationship between two things in
which a change to one may effect the semantics of the other.
0..1
employer
*
employee
An association is a structural relationship that describes
a set of links, a link being a connection among objects.
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Relationships
ParkingLot
1
1 ... *
ParkingSpace
location
is_available()
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Notation: Relationships (continued)
child
parent
A generalization is a specialization/generalization
relationship is which objects of the specialized
element (child) are substitutable for objects of the
generalized element (parent).
A realization is a semantic relationship between
classifiers, wherein one classifier specifies a
contract that another classifier guarantees to carry
out.
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Generalization
Applet
generalization
Note that the Applet and
Graphics classes are
shown elided, i.e., just the
name is shown, not the
attributes or operations.
HelloWorld
paint()
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dependency
Graphics
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Notation: Interface
ISpelling
An interface is a collection of operations that specify a
service of a class or component, i.e., the externally
visible behavior of that element.
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Notation: Package
Business rules
A package is a general-purpose mechanism for organizing
elements into groups.
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Packaging Classes
java
HelloWorld
applet
Graphics
awt
package
lang
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Modeling Classes
Given a real-life system, how do you decide what classes to use?
• What terms do the users and implementers use to describe the
system? They are candidates for classes.
• Is each candidate class crisply defined?
• For each class, what is its set of responsibilities? Are the
responsibilities evenly balanced among the classes?
• What attributes and operations does each class need to carry
out its responsibilities?
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Candidate Classes:
Application Classes and Solution Classes
• Application classes represent application concepts.
Noun identification is an effective technique to generate
candidate application classes.
• Solution classes represent system concepts, e.g., user
interface objects, databases, etc.
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Noun Identification for Application Classes:
A Library Example
The library contains books and journals. It may have several
copies of a given book. Some of the books are reserved for
short-term loans only. All others may be borrowed by any
library member for three weeks. Members of the library can
normally borrow up to six items at a time, but members of
staff may borrow up to 12 items at one time. Only members
of staff may borrow journals.
The system must keep track of when books and journals are
borrowed and returned and enforce the rules.
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Noun Identification: A Library Example
The library contains books and journals. It may have several
copies of a given book. Some of the books are reserved for
short-term loans only. All others may be borrowed by any
library member for three weeks. Members of the library can
normally borrow up to six items at a time, but members of
staff may borrow up to 12 items at one time. Only members
of staff may borrow journals.
The system must keep track of when books and journals are
borrowed and returned and enforce the rules.
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Candidate Classes
Library
Book
Journal
Copy
ShortTermLoan
LibraryMember
Week
MemberOfLibrary
Item
Time
MemberOfStaff
System
Rule
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the name of the system
event
measure
repeat
book or journal
abstract term
general term
general term
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Relations between Classes
Book
Journal
Copy
LibraryMember
Item
MemberOfStaff
is an
is an
is a copy of a
Item
Item
Book
is a
LibraryMember
Is Item needed?
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Operations
LibraryMember
borrows
Copy
LibraryMember
returns
Copy
MemberOfStaff
borrows
Journal
MemberOfStaff
returns
Journal
Item not needed yet.
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A Possible Class Diagram
MemberOfStaff
LibraryMember
1
1
on loan
on loan
0..12
Journal
0..*
Copy
is a copy of
1..*
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Book
1
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From Candidate Classes to Completed Design
Methods used to move to final design:
Reuse: Wherever possible use existing components, or class
libraries. They may need modification.
Restructuring: Change the design to improve, understandability,
maintainability, etc. Techniques include merging similar
classes, splitting complex classes, etc.
Optimization: Ensure that the system meets anticipated
performance requirements, e.g., by changed algorithms or
restructuring.
Completion: Fill all gaps, specify interfaces, etc.
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Coupling and Cohesion
Coupling is a measure of the dependencies between two part
of a system. If two classes are strongly coupled, it is hard to
modify one without modifying the other.
Cohesion is a measure of dependencies within part of a
system. If a class contains many closely related functions its
cohesion is high.
Aim for high cohesion within classes and weak coupling
between them.
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Rough Sketch: Wholesale System
Design is empirical and iterative. The following very
artificial example, gives an idea of the process.
Example
A wholesale merchant supplies retail stores from
stocks of goods in a warehouse.
What classes would you use to model this business?
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Rough Sketch: Wholesale System
RetailStore
Order
Merchant
Product
Warehouse
Invoice
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Shipment
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Rough Sketch: Wholesale System
RetailStore
name
address
contactInfo
financialInfo
Merchant
Warehouse
Order
Product
Reversal
Invoice
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Shipment
Responsibilities
-track status of
shipped products
responsibility
(text field)
damaged()
return()
wrongItem()
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Expanding a Class:
Modeling Financial Information
RetailStore
association
1
* Transaction
Which class is
responsible for the
financial records for
a store?
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Payment
Invoice
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Modeling Invoice
Shipment
???
RetailStore
invoiceRecord
goodsShipped
Invoice
invoiceNumber
PartsList
adornments
+goodsShipped()
+ public
-sendInvoice()
- private
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Lessons Learned
Design is empirical. There is no single correct design.
During the design process:
• Eliding: Elements are hidden to simplify the diagram
• Incomplete: During the early part of the design
process, elements may be missing.
• Inconsistency: During the early part of the design
process, the model may not be consistent
The diagram is not the whole design. Diagrams must
be backed up with specifications.
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An Exam Question:
Object Oriented Design
A system generates weather maps using data collected from
unattended weather stations. Each weather station collects
meteorological data and produces summaries of the data.
On request, it sends the summary information to an area
computer. The area computer uses a database of digitized
maps to generate a set of local weather maps.
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Exam Question: Noun Identification
A system generates weather maps using data collected from
unattended weather stations. Each weather station collects
meteorological data and produces summaries of the data.
On request, it sends the summary information to an area
computer. The area computer uses a database of digitized
maps to generate a set of local weather maps.
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Exam Question: Candidate Classes
System
general term
WeatherMap
Data
same as MeteorologicalData
WeatherStation
is this a general term?
MeteorologicalData
how does this relate to WeatherStation?
DataSummary
how does this relate to MeteorologicalData?
AreaComputer
hardware
Database
general term
DigitizedMap
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Exam Question: Observations about the
Candidate Classes
WeatherMap
is a DigitizedMap
is derived from 1...* DataSummary
WeatherStation
has a set of MeteorologicalData
MeteorologicalData
DataSummary
is derived from MeteorologicalData
DigitizedMap
Can Meteorological Data be an attribute of WeatherStation?
Can DataSummary be combined with WeatherMap?
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Exam Question: Attributes and Operations
WeatherStation
location
metereologicalData
collectData()
getSummary()
DigitizedMap
location
geographicData
printMap()
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WeatherMap
location
date-time
geographicData
weather
gatherData()
printMap()
Or should
MetereologicalData be a
separate object?
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Exam Question: Class Diagram
DigitizedMap
WeatherStation
location
metereologicalData
collectData()
getSummary()
WeatherMap
1
1...*
summary
location
date-time
geographicData
weather
gatherData()
printMap()
*
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