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1 CS 501 Spring 2007
CS 501: Software Engineering
Lecture 16
Object Oriented Design 2
2 CS 501 Spring 2007
Administration
Quiz 3: Change of date
The date of Quiz 3 has been changed to Thursday, March 29.
If this change of date causes schedule problems, please send email to [email protected].
3 CS 501 Spring 2007
Administration
Third presentation and report next week
Sign up now.
4 CS 501 Spring 2007
Modeling Dynamic Aspects of Systems
Interaction diagrams: set of objects and their relationships including messages that may be dispatched among them
• Sequence diagrams: time ordering of messages
• Collaboration diagrams: structural organization of objects that send and receive messages
Activity diagram: flow chart showing flow of control from activity to activity
Statechart diagram: models a state machine
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Notation: Statechart diagrams
Waiting
A state machine is a behavior that specifies the sequence of states an object or an interaction goes through during its lifetime in response to events.
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State Diagram: Notation
State diagram for class Book in a library system
not borrowable
returned()
returned()
borrowable
borrowed()[not last copy]
borrowed()[last copy]
guard expression
7 CS 501 Spring 2007
Notation for Classes and Objects
Classes Objects
AnyClass
attribute1attribute2
operation1()operation2()
AnyClass
or
anObject:AnyClass
:AnyClass
anObject
The names of objects are underlined.
or
or
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Notation: Active Class
EventManager
eventlist
suspend()flush()
An active class is a class whose objects own one or more processes or threads and therefore can initiate control activity.
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Interaction: Bouncing Ball Diagrams
Example: execution of http://www.cs.cornell.edu/
Client Server(s)
domain name service
TCP connection
HTTP get
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Notation: Interaction
display
An interaction is a behavior that comprises a set of messages exchanged among a set of objects within a particular context to accomplish a specific purpose.
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Actions on Objects
call
return
send
create
destroy
returnCopy(c)
okToBorrow() local
status
notifyReturn(b) asynchronous signal
<<create>>
<<destroy>>stereotypes
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Sequence Diagram: Borrow Copy of a Book
BookBorrower
libMem: LibraryMember
theCopy:Copy
theBook:Book
borrow(theCopy)okToBorrow
borrowborrow
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Sequence Diagram: Change in Cornell Program
Cornellian
:MEngStudent
1 : getName()
sequence numbers added to messages
:PhDStudent
1.1 : name
2: <<create>> PhDStudent(name)
3: <<destroy>>
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Sequence Diagram: Painting Mechanism
:Thread :Toolkit :ComponentPeer target:HelloWorld
runrun callbackLoop
handleExpose
paint
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Other Diagrams in UML
• 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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Activity Diagram: Notation
Release work order
Assign tasks
Reschedule[materials not ready]
[materials ready]guard expression
branch
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Activity Diagram: Parallel Activities
Decompress
Stream audioStream video
fork
join
start state
stop state
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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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Software Reuse
Better software at lower cost
Potential benefits of reuse:
• Reduce development time and cost
• Improved reliability of mature components
• Shared maintenance cost
Potential disadvantages of reuse:
• Difficulty in finding appropriate components
• Components may be a poor fit for application
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Software Reuse: Examples
Software developers rely heavily on software components provided by others
System software
• device drivers• file systems• exception handling• network protocols
Subsystems
• database management systems• firewalls• web servers
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Software Reuse Examples (Tools)
Standard functions• mathematical methods• formatting
User interface
• toolkits (e.g. Quickdraw)• class libraries, (e.g., Swing)
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Software Reuse (Application Packages)
Application package supports a standard application (e.g., payroll)
Functionality can be enhanced by:
=> configuration parameters (e.g., table driven)
=> extensibility at defined interfaces
=> custom written source code extensions
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Design for Reuse
The software design should anticipate possible changes in the system over its life-cycle.
New vendor or new technology
Components are replaced because its supplier goes out of business, ceases to provide adequate support, increases its price, etc., or because better software from another sources provides better functionality, support, pricing, etc.
This can apply to either open-source or vendor-supplied components.
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Design for Reuse
New implementation
The original implementation may be problematic, e.g., poor performance, inadequate back-up and recovery, difficult to trouble-shoot, or unable to support growth and new features added to the system.
Example. The portal nsdl.org was originally implemented using uPortal. This did not support important extensions that were requested and proved awkward to maintain. It was reimplemented using PHP/MySQL.
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Design for Reuse
Additions to the requirements
When a system goes into production, it is usual to reveal both weaknesses and opportunities for extra functionality and enhancement to the user interface design.
For example, in a data-intensive system it is almost certain that there will be requests for extra reports and ways of viewing the data.
Requests for enhancements are often the sign of a successful system. Clients recognize latent possibilities.
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Design for Reuse
Changes in the application domain
Most application domains change continually, e.g., because of business opportunities, external changes (such as new laws), mergers and take-overs, new groups of users, etc., etc.,
It is rarely feasible to implement a completely new system
when the application domain changes. Therefore existing systems must be modified. This may involve extensive restructuring.
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Reuse and Object Oriented Languages
Example: Java
Java is a relatively straightforward language with a very rich set of class hierarchies.
• Java programs derive much of their functionality from standard classes
• Learning and understanding the classes is difficult.
• Experienced Java programmers can write complex systems quickly
• Inexperienced Java programmers write inelegant and buggy programs
28 CS 501 Spring 2007
Design for Reuse: Inheritance and Abstract Classes
Classes can be defined in terms of other classes using inheritance. The generalization class is called the superclass and the specialization is called the subclass.
If the inheritance relationship serves only to model shared attributes and operations, i.e., the generalization is not intended to be implemented, the class is called an abstract class
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Design for Reuse: Implementation and Specification Inheritance
Implementation Inheritance
Developers reuse code quickly by subclassing an existing class and refining its behavior. Is not good for reuse.
Specification Inheritance
The classification of concepts into type hierarchies, so that an object from a specified class can be replaced by an object from one of its subclasses.
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Design for Reuse: Specification Inheritance
Liskov Substitution Principle (strict inheritance)
If an object of type S can be substituted in all the places where an object of type T is expected, then S is a subtype of T.
Interpretation
The Liskov Substitution Principle means that if all classes are subtypes of their superclasses, all inheritance relationships are specification inheritance relationships. New subclasses of T can be added without modifying the methods of T. This leads to an extensible system.
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Design for Reuse: Delegation
Delegation
A class is said to delegate to another class if it implements an operation by resending a message to another class.
Delegation is an alternative to implementation inheritance that should be used when reuse is anticipated.
The discussion of design for reuse draws from the book by Bruegge
and Dutoit in the readings.*
