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CS 501: Software Engineering

Lecture 16

Object Oriented Design 2

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Administration

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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

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State Transition Diagram: Example from Lecture 9 using UML Notation

Patients Fields Setup ReadyBeam

on

[Enter] [Enter] [Start]

[Stop]

[Select field]

[Select patient][lock on]

[lock off]

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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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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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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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Design for 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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Reuse and Object Oriented Languages: Class Hierarchies

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

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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: Specification and Implementation Inheritance

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.

In particular:

• Pre conditions cannot be strengthened in a subclass.

• Post conditions cannot be weakened in a subclass.

Implementation Inheritance

Developers reuse code quickly by subclassing an existing class and refining its behavior. Is not good for reuse.

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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.

For a discussion of design for reuse see the book by Bruegge and Dutoit in the readings.

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