5DesignPatterns2 ppt

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Lecture Notes onDesign Pat terns II

Bernd Bruegge

Technische Universitt Mnchen

Lehrstuh l fr Angewand te Softwaretechnik

June 10, 1998


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Odds and Ends

v February 2, lecture on Security


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Outline

v Review of design p attern concepts

w What is a design p attern?

w Mod ifiable designs

v Review of design patternsw Adapter: converting interfaces

w Bridge: decoup ling in terface and imp lemen tation

w Facade: simp lifying interfaces

v New p atterns:

w Proxy

w Command

w Observer


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W hat is a design pat tern? (Take 2)

A design p attern is

a template solution to a recurring d esign problem

w Look before re-inventing the wh eel just one more time

reusable design knowledge

w High er level than link lists or bin ary trees

w Lower level than app lication framework s

an example of modifiable and reusable design

w Learnin g to design starts by stud ying other d esigns


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W hy are modifiable designs important?

A modifiable design enables

an iterative and incremental development cycle

w concurrent development

w risk management

w flexib ility to change

to minimize the introd uction of new problems whenfixing old ones

to d eliver more functionality after initial delivery


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W hat makes a design modifiable?

v Low coup ling and high coherence

v Clear depend encies

v Explicit assumptions

How d o design patterns help?

v They are generalized from existing systems

v They provide a shared vocabulary to designers

v They provide examples of modifiable designs

w Abstract classes

w Delegation


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Inheritance: class v s. inheritance

v Interface inheritance(i.e., subtyp ing):

w describes when differenttypes of ob jects can b e used inp lace of each other

v Class inheritance:

w

an ob jects implemen tation isdefined in terms of theimp lementation of another. PutAt()

RemoveAt()

HashTable

Add()Remove()

Set

PutAt()RemoveAt()

HashTable

PutAt()RemoveAt()

Table

PutAt()RemoveAt()

BTree


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Interface inheritance vs. class inheritance

v Interface inheritance

w separates interface and implemen tation

w imp lementations m ay be transparently sub stituted

w decreases coup ling

v Class inheritance

w introdu ces depen den ces among an ancestor and its descend ents(inh erited state)

w mixes interface specification an d imp lemen tation

w can be achieved with delegation in stead

v Design p atterns show how to avoid class inheritance withexamples of interface inheritance and delegation


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Interface and class inheritance in Java

v An interface defines constants and abstract methods.

w An in terface can extend(i.e., inherit from ) zero, one, or m anyinterfaces.

w An interface does not define any b ehavior or attribu tes

v A class defines attributes and methods.

w A class can extend(i.e., inh erit beh avior and attribu tes from)zero or on e class.

w A class can implement(i.e., comply w ith) zero, one, or man yinterfaces in add ition to extend ing a super class.

v Abstract classes shou ld be realized by interfaces in Java.

v Concrete classes should be realized by classes in Java.


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Rev iew of last years pat terns

v Bridge

w Decouple an abstraction form its imp lemen tation so that thetwo can vary ind epend ently.

v

Adapterw Convert th e in terface of a class into an other in terface clients

expect. Adapter lets classes work together that couldntotherwise because of incomp atible in terfaces.

v Facade

w Provide a unified interface to a set of interfaces in asub system. Facade d efines a h igher-level interface that m akesthe su bsystem easier to use.


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Rev iew : Bridge pat tern

v Delegation is used to bind an Abstraction andConcreteImplementor.

v Interface inheritance is used to refine Abstraction andImplementor.

v Implementor can be implemented as a Java interfaceinstead of a class.

Client

RefinedAbstractionConcrete

ImplementorB

Concrete

ImplementorA

ImplementorAbstraction

imp


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Bridge pat tern example

v Window hierarchy implements h igh level behavior

v WindowImp hierarchy imp lements device level behavior

v Bridge patterns can be used for implementing a layereddesign.

Client

IconWindow PMWindowXWindow

WindowImpWindow

imp

TransientWindow


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JAMES Bridge Exam ple

public interface SeatImplementation {

public int GetPosition();

public void SetPosition(int newPosition);

}

public class AimSeat implements SeatImplementation {

public int GetPosition() {

// actual call to the simulator

}

}

Client

SARTSeatAIMSeat

SeatImplementationSeat

imp


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v Delegation is used tobind an Adapter and an Adaptee

v Interface inheritance is specify the interface of the

Adapter class.v Targetand Adaptee (usually) pre-exist the Adapter.

v Target may be realized as an interface in Java.

Rev iew : Adapter pat tern

Client

Target

Request()

Adaptee

ExistingRequest()

Adapter

Request()

adaptee


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Adapter pat tern example

public class ServicesEnumeration

implements Enumeration {

public boolean hasMoreElements() {

return this.currentServiceIdx


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Rev iew : facade pat tern

Compiler

compile(s)

ParseNode

create()

Lexer

getToken()

CodeGenerator

create()

Parser

generateParseTree()

Optimizer

create()

Compiler


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New pat terns

v Structural pattern

w Proxy

v Behavioral pattern

w Command

w Observer


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Proxy pat tern

v Interface inheritance is used to sp ecify the interfaceshared by Proxy and RealSubject.

v Delegation is used to catch and forward accesses to theRealSubject .

v Proxy patterns can be u sed for lazy evaluation and forremote invocation.

v Proxy patterns can be imp lemented with a Java interface.

Subject

Request()

RealSubject

Request()

Proxy

Request()

realSubject


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Proxy pat tern example

v Images are stored and loaded separately from text

v If a RealImage is not loaded a ProxyImage displays agrey rectangle in place of the image

v The client cannot tell that it is dealing with a ProxyImageinstead of a RealImage

v A proxy pattern can be easily overlayed with a Bridge

ImageboundingBox()

draw()

realSubject RealImageboundingBox()

draw()

ProxyImageboundingBox()

draw()


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Structural pat tern: summary

v Adapters, Bridges, Facades, and Proxies are variationson a single theme:

w examp les of delegation an d in terface inh eritance

w decoup le two or more classes to reduce coup ling

w introd uce an abstract class enab ling fu ture extensions

v Behavioral patterns p rovide similar benefits for controlflow.


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Command pat tern

v Encapsulate a request as an object, thereby letting you

w parameterize clients with d ifferent requests,

w qu eue or log requests, and

w sup port un doable op erations. (p . 233)

v Uses:

w Undo queues

w Datab ase transaction bu ffering


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Comm and pat tern: undo queue


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Command pat tern: classes

v Client creates a ConcreteCommand and bind s it with aReceiver.

v Client hands the ConcreteCommand over to the Invokerwhich stores it.

v The Invoker has the responsibility to execute orundo Commands.

Commandexecute()

undo()

Receiver

action()

Client

Invoker

ConcreteCommand

execute()undo()


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Command pat tern: typical sequence

rectangle anEditor moveCommand undoQueue

newCommand(info)

store(aCommand)

execute(info)move(x, y)


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Command pat tern: inst ance diagram

aRectangle

moveCommand1

status = #d one

coordObject1

x=10, y=20

undoQueue

moveCommand2

status = #d one

coordObject2

x=20, y=30

moveCommand3status = #u nd one

coordObject3x=30, y=50


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Observer pat tern

v Define a one-to-many dependency between objects sothat when one object changes state, all its dependents arenotified and updated automatically. (p. 293)

v Uses:

w Main taining consistency across redu ndan t state

w Op tmizing batch chan ges when maintaining consistency


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Observer pat tern (cont inued)

9DesignPatterns2.ppt

Observers Subject


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Observer pat tern (cont d)

Observerupdate()

Subject

attach(observer)detach(observer)

notify()

ConcreteSubjectgetState()

setState(newState)

subjectState

ConcreteObserver

update()

observerState

observers

subject

*

v The Subject represents the actual state, the Observersrepresent d ifferent views of the state.

v Observer can be imp lemented as a Java interface.

v Subject is a super class (needs to store the observersvector) notan interface.


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Observer pat tern sequence

aFile anInfoWindow aDirectoryWindow

setName(foo)

update()

getName()

update()

getName()

notify()


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Observer pat tern exam ple: Java implementat ion

// import java.util;

public class Observable extends Object {

public void addObserver(Observer o);

public void deleteObserver(Observer o);

public boolean hasChanged();

public void notifyObservers();

public void notifyObservers(Object arg);

}

public abstract interface Observer {

public abstract void update(Observable o, Object arg);

}


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Three object type archit ecture

v Interface objects

w imp lement the interaction with th e user

w constructed from UI comp onen ts

w sub ject to most m odification

v

Control objectsw imp lement the transactions w ith the u ser

w constructed w ith Comm and objects

w modified frequen tly bu t less often than in terface objects

v Entity objects

w represent the domain m odel

w often represent persistent data

w least often modified


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Three object type archit ecture (cont inued)

Menu

MoveCommand

Rectangle

DrawingView

RectangleInfo

UndoQueue

Interface objects Control objects

Ent ity objects


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Summary

v Design patterns provide solutions to comm on decoup lingproblems.

v Design patterns result into extensible models and code.

v Design patterns can be used as is or as examples ofinterface inheritance and delegation.

v

Design p atterns apply the same principles to structureand to behavior.

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