CS 501: Software Engineering
Lecture 15
Object Oriented Design 1
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Administration
Projects
Now is the time to press ahead with final design and
implementation.
Allow plenty of time for testing, for making small changes,
and for documentation and packaging.
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The Waterfall Model
Requirements
Feasibility study
Requirements analysis
Design
System design
Implementation
Program design
Coding
Testing
Acceptance
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
• security mechanisms, operational procedures
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The Unified Modeling Language
UML is a standard language for modeling software systems
• Serves as a bridge between the requirements specification
and the implementation.
• Provides a means to specify and document the design of a
software system.
• Is process and programming language independent.
• Is particularly suited to object-oriented program
development.
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UML Models
A UML model consists of:
(a) A diagram. This gives a general overview of the model,
showing the principal elements and how they relate to each
other.
(b) A specification. This 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.
A diagram without a specification is not a complete model, but
may be very useful as a design tool.
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Diagrams in UML
A diagram is the graphical representation of a set of
elements, usually rendered as a connected graph of vertices
(things) and arcs (relationships).
• Class diagram shows a set of classes, interfaces, and
collaborations with their relationships.
• Object diagram shows a set of objects and their
relationships.
• Use case diagram shows a set of use cases and actors (a
special kind of class) and their relationships.
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Diagrams in UML (continued)
An interaction diagram shows an interaction, consisting
of a set of objects and the relationships, including the
messages that may be dispatched among them.
=> A sequence diagram emphasizes the time ordering.
=> A collaboration diagram emphasizes the structural
organization of the objects that send and receive
messages.
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Diagrams in UML (continued)
• Statechart diagram shows a state machine consisting
of states, transitions, events, and activities.
• Activity diagram is a statechart diagram that shows the
flow from activity to activity within a system.
• Component diagram shows the organization and
dependencies among a set of components.
• Deployment diagram shows the configuration of
processing nodes and the components that live on them.
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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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Class Diagrams
Window
origin
size
open()
close()
move()
display()
name
attributes
operations
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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Notation: Grouping
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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The "Hello, World!" Example
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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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
ParkingGarage
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).
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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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Class Inheritance Diagram
Object
Panel
interface
Component
ImageObserver
Applet
Container
HelloWorld
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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?
Aim for high cohesion within classes and weak coupling
between them.
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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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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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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
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Rough Sketch: Wholesale System
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
damaged()
return()
wrongItem()
Responsibilities
-track status of
shipped products
responsibility
(text field)
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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: Elements may be missing.
• Inconsistency: The model may not be consistent
The diagram is not the whole design. Diagrams must
be backed up with specifications.
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Levels of Abstraction
The complexity of a model depends on its level of abstraction:
•
High-levels of abstraction show the overall system.
•
Low-levels of abstraction are needed for implementation.
Two approaches:
•
Model entire system at same level of abstraction, but present
diagrams with different levels of detail.
•
Model parts of system at different levels of abstraction.
*
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