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Bibliography…

« A System of Pattern » Bushmann et All« Design Patterns » Gamma et All« Concurrent Programming in Java » D. Lea.« Distributed Objects » Orfali et All« Applying UML and Patterns » Larman

4

Patterns…

« Patterns help you build on the collectiveexperience of skilled software engineers. »« They capture existing, well-provenexperience in software development andhelp to promote good design practice »« Every pattern deals with a specific,recurring problem in the design orimplementation of a software system »« Patterns can be used to constructsoftware architectures with specificproperties… »

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Becoming a Chess Master First learn rules and physical requirements – e.g., names of pieces, legal movements, chess board

geometry and orientation, etc.Then learn principles – e.g., relative value of certain pieces, strategic value of

center squares, power of a threat, etc.However, to become a master of chess, one muststudy the games of other masters – These games contain patterns that must be understood,

memorized, and applied repeatedlyThere are hundreds of these patterns

6

Becoming a Software DesignerMaster

First learn the rules – e.g., the algorithms, data structures and languages of

softwareThen learn the principles – e.g., structured programming, modular programming,

object oriented programming, generic programming, etc.

However, to truly master software design, onemust study the designs of other masters – These designs contain patterns must be understood,

memorized, and applied repeatedlyThere are hundreds of these patterns

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Software Architecture

A software architecture is a description of thesubsystems and components of a softwaresystem and the relationships between them.Subsystems and components are typicallyspecified in different views to show therelevant functional and non-functionalproperties of a software system.

The software system is an artifact. It is theresult of the software design activity.

8

Component

A component is an encapsulated part of asoftware system. A component has aninterface.Components serve as the building blocks for

the structure of a system. At a programming-language level,components may be represented asmodules, classes, objects or as a set of related functions.

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Subsystems

A subsystem is a set of collaboratingcomponents performing a given task. Asubsystem is considered a separate entitywithin a software architecture.It performs its designated task by interactingwith other subsystems and components…

10

Architectural Patterns

An architectural Pattern expresses afundamental structural organization schemafor software systems. It provides a set ofpredefined subsystems, their responsibilities,and includes rules and guidelines fororganizing the relationships between them.

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Design patterns

A design pattern provides a scheme forrefining the subsystems or components of asoftware system, or the relation shipsbetween them. It describes a commonly-recurring structure of communicatingcomponents that solves a general designproblem within a particular context.

12

Idioms

An Idiom is a low-level pattern specific to aprogramming language. An idiom describeshow to implement particular aspects ofcomponents or the relationships betweenthem using the features of the givenlanguage.

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Framework

A framework is a partially complete software(sub-) system that is intended to beinstantiated. It defines the architecture for afamily of (sub-) systems and provides thebasic building blocks to create them. It alsodefines the places where adaptations forspecific functionality should be made.

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First Example

A Dice Game A Player rolls 10x 2 dicesIf result = 7, score=score + 10 points

At the end, score of the player is registred inthe highscore table.

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Design Stage

Manage User InterfaceManage Persistence of highscore in a file orin relational databaseRealize a layered architecture : Apply theLayer Architectural Pattern

18

Layer

Helps structure an application that can bedecomposed into groups of subtasks inwhich each group of subtasks is at aparticular level of abstraction.

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Layer: examples

20

Layer :Structure

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Layer: Structure

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Layer and components…

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Layers : Variants

Relaxed Layered System: – A layer « j » can use service of j-1, j-2… – A layer can be partially opaque

• Some service to layer j+1, others to all upper services…

Layering through inheritance: – Lower layers are implemented as base classes – Higher level can override lower level…

24

Layers : Known UsesVirtual machines: JVM and binary code format

API : Layer that encapsulates lower layersInformation System – Presentation, Application logic, Domain Layer, Database

Windows NT (relaxed for: kernel and IO and hardware) – System services, – Resource management (Object manager, security monitor, process

manager, I/O manager, VM manager, LPC), – Kernel (exception handling, interrupt, multipro synchro, threads), – HAL (Hardware Abstraction Level) – Hardware

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Layers: benefits

Reuse of layersSupport for standardization (POSIX)Dependencies are kept localExchangeabilities : – Replacement of old implementation with Adapter

Pattern – Dynamic exchange with Bridge Pattern

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Layers: Liabilities

Cascades of changing behavior Lower efficiencyUnnecessary work: functions of a layercalled many times for one service

Difficulty of establishing correct granularity oflayers: Too few layers -> less benefits, toomany layers -> complexity and overhead…

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Applying Layer Architecture

Play View High Score

Fichier ou BDD

UI

Core

Persistence

28

Package decomposition

UI

Core

Persist

Util

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Layer « core »

Contain business logic classes… Adapt analysis classes for implementationUse of singleton Idiom…

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Singleton (Idiom)

Ensure a class only has one instance, andprovide a global point of access to it.

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Singleton Structure

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Core « Layer »:First diagram

Entryname:String : type = initvalscore:int : type = initval

Entry(name:String,score:int)()

HighScore$ hs : HighScore = null

Highscore()add()load()save()

1 0..*1 0..*

Player name : Stringscore : int = 0;

Player()display()

DiefaceValue : int = 1

roll()Die()display()

DiceGame$ dg = null

DiceGame()getInstance()start()

1

-player

1-dies

22

Singleton...


play()Player()


roll()Die()

1 21 2

Rolls

DiceGame

DiceGame()start()

1

1

1

1Plays

1

1

1

1

Includes

Entryname:String : type = initvalscore:int : type = initval

Entry(name:String,score:int)()

HighScore

Highscore()add()

1

1

1

1

Scoring

0..*1 0..*1

Design Analysis

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UI

Core

Persist

Util

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Observer

One-to-many dependency betweenobjects: change of one object willautomatically notify observers

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Observer: Applicability

A change to one object requires changing anunknown set of other objectsObject should be able to notify other objectsthat may not be known at the beginning

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Observer: Structure

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Observer: Consequences

Abstract coupling between subject andobserver Support for broadcast communicationHard to maintain

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Applying Observer PatternObservable

changed : boolean = false

Observable()addObserver()deleteObserver()notifyObservers()notifyObservers()deleteObservers()setChanged()clearChanged()hasChanged()countObservers()

(from util)

DieView

DieView(die : Die)update(o : Observable, arg : Object) : void

PlayerView

PlayerView(player : Player)update(o : Observable, arg : Object) : void

Observer

update(o : Observable, arg : Object) : void

(from util)

0..*0..*

Player

name : Stringscore : int = 0;

Player()display()

(from Core)

Die

faceValue : int = 1

roll()Die()display()

(from Core)

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Observer ViewObserver


(from util)

DieView


PlayerView


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Views are graphical objects

DieView


PlayerView


Observable

changed : boolean = false

Observable()addObserver()deleteObserver()notifyObservers()notifyObservers()deleteObservers()setChanged()clearChanged()hasChanged()countObservers()

(from util)

Observer


(from util)

0..*0..*

Player

name : Stringscore : int = 0;

Player()display()

(from Core)

Die

faceValue : int = 1

roll()Die()display()setValue()

(from Core)

Panel

Panel()Panel()constructComponentName()addNotify()

(from awt)

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Setting up Observer : RollForm : Die : DieView:

Playe : PlayerView

1: display( ) 2: PlayerView(Player)

4: return component

5: display()6: DieView(Die)

8: return component

3: addObserver(Observer)

7: addObserver(Observer)

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Observer : Change Propagation: Die : Randomizer : DieView

1: getValue( )

2: setValue(int)

3: notifyObservers( )

4: update(Observable, Object)5: getState()

3

: JLabel

6: setText(3)

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Layered Architecture...


roll()Die()display()setValue()


Player()display()

Observer (from util)

Observable(from util)

0..*0..*

Displayable

PlayerView

PlayerView()update()

(from UI)

RollForm

roll_action()

cancel_action()RollForm()

(from UI)

1

+thePlayerView

1DieView

DieView()update()

(from UI) +theDieView

22

UI

Core

Decoupling classes

and interfaces

44


UI

Core

Persist

Util

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Pattern Factory Method

Intent – Define an interface for creating an object, but let

sub-classes decide which class to instantiate – let a class defer instantiation to subclasses – Also known as Virtual Constructor

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Factory Method

Applicability : Use when – a class cannot anticipate the class of objects it

must create – a class wants its subclasses to specify the

objects it creates – classes delegate responsibility to one of several

helper subclasses, and you want to localize theknowledge of which helper subclass to delegate.

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Structure

Produit

ProduitConcret

Facteur

Fabrication()UneOperation()

FacteurConcret

Fabrication()

produit=Fabrication()

return new ProduitConcret()

48

Factory method

Consequences – Provide hooks for subclasses – connects parallel class hierarchies

Known uses

– MacApp, ET++ – ClassView in smalltalk80 MVC (controller

creation) – Orbix ORB for generating PROXY object

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« Persist » Layer

Persistence technical classesEnsure independence of Core/Persist – Be able to switch « persistent engine »

For example: – Persistence via « Serialization » – Persistence via a relational database (JDBC).

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Applying FactoryHighScore

$ hs : HighScore = null

Highscore()add()load()save()

(from Core)

PersistKit

makeKit()

JdbcKit

makeKit()

SrKit

makeKit()

HighScoreJDBC

Highscore()load()save()

HighScoreSr $ filename : String = "/tmp/high.score"

Highscore()load()save()

Abstract produc

Abstract Factory

Concrete product

Concrete Factory

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: RealPlayer : S rKit : HighScoreSr : DiceGame

2: getInstance( )

3: DiceGame( )

1: SrKit( )

4: makeKit( ) 5: HighScoreSr( )

Attention!DiceGame voit SrKit commeun PersistKit et HighScoreSrcomme un HighScore

6: load( )

7: quit( ) 8: getInstance( )

9: save( )

Seul le Realplayer sait qu'ilutilise un SrKit ! DiceGamenon !

Applying Factory

52

Summary

1 Architectural pattern : Layer 2 Design Patterns : Observer, Factory1 Idiom : SingletonPb: – Combining pattern to combine their forces…

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54

Bank example… A basic bank system: – 1 bank, n Account. – Each account belong to 1 client. – Each account is credited by an amount a money.

Bank functions – Withdrawal on an account, Credit an account,

Transfer money from one account to another…

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Naive solutionBank

add Account(S tring name, int amo unt) : Accountwithdrawal(int ida, int amount) : voiddep osit(int ida, int amout) : voidtransfer(int ida1, int id a2, int amount) : voidgetAccount(int ida) : A ccount

Accountint amountint ida

withdraw(int a )

deposit(int a) Account(int ida )

Customer String name;int idc

Client(String name, int idc)ida : int

11..n 1ida : int

1..n

ida : int

1

0..n

1

ida : int

0..n

1

0..n

idc : intidc : int

1

0..n

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Naive Solution1 : A c c o u n t: c li e n t : B a n k 2 : A c c o u n t

t ra n s f e r ( 1 , 2 , 1 0 0 )

w i th d r a w a l (1 , 1 0 0 )

w i th d r a w ( 1 0 0 )

d e p o s i t (2 , 1 0 0 )

d e p o s i t ( 1 0 0 )

g e t A c c o u n t( 1 )

g e t A c c o u n t( 2 )

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Applying Command Pattern…

Encapsulate a request as an object, therebyletting you parameterize clients with differentrequests, queue or log requests, and supportundoable operations.

58

Command Example

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Command Structure

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Command Consequences

Command decouples the object thatinvokes the operation from the one thatknows how to perform it.Commands are first-class objects. They

can be manipulated and extended like anyother object.It's easy to add new Commands, becauseyou don't have to change existing classes.

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Applying Command Pattern

Ac countint amountint ida

withdraw(a : int)deposit(a : int)


Client(String name, int idc)

ida : int11..n 1

ida : int1..n

Withdrawalida : intamount : int

do() : voidundo:void()

Command b : banque

do() : voidundo() : voidCommand(Bank b)

Bank

getAccount(int ida) : AccountExecute(cmd : Command) : void

ida : int

1

0..n

1

ida : int

0..n

idc : int1

0..n

1idc : int

0..n

+receiver

Depositida : intamount : int

do() : voidundo: void()

Transfer ida1 : intida2 : intamount : int

do() : voidundo:void()opname()Transfer(ida1 : int, ida2 : int, int a, Bank b)

64

Applying Command Pattern : client t : Transfer : Bank 1 : Account 2 : Account

Transfer(1,2,100 )

Execute(t )

do( )

getAccount( 1)

withdraw( 100 )

getAccount( 2)

deposit( )

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Composite Pattern

Compose objects into tree structures torepresent part-whole hierarchies. Compositelets clients treat individual objects andcompositions of objects uniformly.

66

Composite Example

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Composite Example

68

Composite Structure

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Applying Composite on Command

70

Applying Composite




Client(String name, int idc)

ida : int11..n 1

ida : int1..n


do() : voidundo:void()Withdrawal(b : Bank, ida : int, amount : int)

Bank

getAccount(int ida) : AccountExecute(cmd : Command) : void

ida : int

1

0..n

1

ida : int

0..n

idc : int1

0..n

1idc : int

0..n


do() : voidundo: void()Deposit(b : Bank, ida : i nt, amount : int)

Macrocom

add(Command cmd)remove(Command cmd)do()undo()Macrocom(b : Bank)

Command b : banque

do() : voidundo() : voidCommand(Bank b)

+receiver

0..n0..n

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: client w : Withdrawal d : Deposit m : Macrocom : Bank 1 : Account 2 : Account

Withdrawal(b, 1,100 )

Deposit(b,2,100 )

Macrocom(b )

add( w)

add(d )

Execute(m )

do( )do( )getAccount( 1 )

withdraw( 100 )

do( )

getAccount( 2 )

deposit( 100)

72

Applying Singleton




Client(String name, int i dc)

ida : int11..n 1

ida : int1..n


do() : voidundo:void()Withdrawal(ida : int, amount : int)

Bankinstance : Bank

getAccount(int ida) : AccountExecute(cmd : Command) : voidgetInstance() : Bank

ida : int

1

0..n

1

ida : int

0..n

idc : int1

0..n

1idc : int

0..n


do() : voidundo: void()Deposit(ida : int, amount : int)

Macrocom

add(Command cmd)remove(Command cmd)do()undo()Macrocom()

Command

do() : voidundo() : void

0..n0..n

do () { Bank b=Bank.getInstance(); ...}

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And So on…

Storing state : Memento PatternObserving Account : Observer PatternVisiting all object graph : Visitor PatternRemote access : Proxy pattern…

74

Proxy Pattern

Provide a surrogate or placeholder foranother object to control access to it.

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Proxy Example

76

Proxy Structure

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Proxy benefits

remote proxy can hide the fact that anobject resides in a different address space.

A virtual proxy can perform optimizationssuch as creating an object on demand.Both protection proxies and smartreferences allow additional housekeepingtasks when an object is accessed.

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AccountCreate

sum : int

AccountCreate(arg0: String, arg1 : int)execute() : voidunexecute() : voidtoString() : String

(from remotecommand)

AccountDeposit

sum : int

AccountDeposit(arg0: String, arg1: int)execute() : voidunexecute() : void


AccountViewer

AccountViewer(arg0 : String)update(arg0: Object) : void


AccountWithdrawal

sum : int

AccountWithdrawal(arg0 : String, arg1: int)execute() : voidunexecute() : void

(from remotecommand) BankAccount

amount : int

BankAccount(arg0 : String, arg1: int)name() : Stringamount() : intamount(arg0 : int) : voidtoString() : String

(fromremotecommand)

RemoteObservable

changed: boolean

addObserver(arg0 : RemoteObserver) : voidremoveObserver(arg0 : RemoteObserver): voidsetChanged() : voidnotifyObservers() : voidnotifyObservers(arg0 : Object) : voidRemoteObservable()

(fromremotecommand)

BankBase

instance() : BankBaseaddAccount(arg0: BankAccount) : voidgetAccount(arg0 : String) : BankAccountremoveAccount(arg0 : String) : voidtoString() : StringBankBase()


$bb

BankServer

BankServer()execute(arg0 : Command) : voidmain(arg0 : String[]) : void

(fromremotecommand)

BankServerI

execute(arg0: Command) : void


RemoteObserver

update(arg0: Object) : void


RegisterObserver

RegisterObserver( arg0: String, arg1 : RemoteObserver)execute() : voidunexecute() : void

(fromremotecommand)

ro

Command

execute() : voidunexecute() : voidCommand()


MacroCommand

execute() : voidunexecute() : voidadd(arg0: Command) : voidMacroCommand()

(fromremotecommand)

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79

Adapter Pattern

Convert the interface of a class into another interface clients expect. Adapter lets classeswork together that couldn't otherwisebecause of incompatible interfaces.

80

Adapter Example

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81

Adapter Structure

82

Visitor Pattern

Represent an operation to be performed onthe elements of an object structure. Visitor lets you define a new operation withoutchanging the classes of the elements onwhich it operates.

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83

Visitor example

84

Visitor example

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Visitor applicability

many distinct and unrelated operations needto be performed on objects in an objectstructure, and you want to avoid "polluting"their classes with these operations

86

Visitor Structure

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Visitor Structure

88

Visitor Consequences

Visitor makes adding new operations easy A visitor gathers related operations and

separates unrelated ones Adding new Concrete Element classes is

hard Visiting across class hierarchies Accumulating state. Breaking encapsulation

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89

Chain of responsibility

Avoid coupling the sender of a request to itsreceiver by giving more than one object achance to handle the request. Chain thereceiving objects and pass the request alongthe chain until an object handles it.

90

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91

Chain of Responsibility

92

Chain of Responsibility

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Participants Handler (HelpHandler) – defines an interface for handling requests. – (optional) implements the successor link.

ConcreteHandler (PrintButton, PrintDialog) – handles requests it is responsible for. – can access its successor. – if the ConcreteHandler can handle the request, it does

so; otherwise it forwards the request to its successor. Client

– initiates the request to a ConcreteHandler object on thechain.

94

Example…

Awt 1.0

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95

Strategy

Define a family of algorithms, encapsulateeach one, and make them interchangeable.Strategy lets the algorithm varyindependently from clients that use it.

96

Strategy…

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97

Strategy

98

Participants Strategy (Compositor) – declares an interface common to all supported

algorithms. Context uses this interface to call thealgorithm defined by a ConcreteStrategy.

ConcreteStrategy (SimpleCompositor,TeXCompositor, ArrayCompositor) – implements the algorithm using the Strategy interface.

Context (Composition) – is configured with a ConcreteStrategy object. – maintains a reference to a Strategy object. – may define an interface that lets Strategy access its

data.

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99

Strategy…

B u t to n( fr o m a w t )

B o r d e r L a y o u t( fr o m a w t )

C o m p o n e nt( fr o m a w t )

G r i d L a y o u t( fr o m a w t )

L a y o ut M a n a g e r 2( fr o m a w t )

< < I n t e r f a c e > >

C o n ta i ne r ( fr o m a w t )

c o m p o n e n t []

L a y o u t M a n a g e r ( fr o m a w t )

< < I n t e r f a c e > >la y o u t M g r

100

State

Allow an object to alter its behavior when itsinternal state changes. The object willappear to change its class.

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101

Example

102

Structure

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103

Consequences

1. It localizes state-specific behavior and partitions behavior for different states

2. It makes state transitions explicit 3. State objects can be shared

104

Decorator

Attach additional responsibilities to an objectdynamically. Decorators provide a flexiblealternative to subclassing for extendingfunctionality.

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105

Example

106

Example

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107

Example

108

Structure

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109

Applicability

to add responsibilities to individual objectsdynamically and transparently, that is,without affecting other objects.for responsibilities that can be withdrawnwhen extension by subclassing is impractical

110

Consequences

1. More flexibility than static inheritance2. Avoids feature-laden classes high up in the

hierarchy 3. A decorator and its component aren't

identical 4. Lots of little objects

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111

FileInputStream

FileInputStream(name : String)FileInputStream(file : File)FileInputStream(fdObj : FileDescriptor)open(name : String) : voidread() : intreadBytes(b[] : byte, off : int, len : int) : intread(b[] : byte) : intread(b[] : byte, off : int, len : int) : intskip(n : long) : longavailable() : intclose() : voidgetFD() : FileDescriptor initIDs() : voidfinalize() : void

(from io)

BufferedInputStream

defaultBufferSize : int = 2048buf[] : bytecount : intpos : intmarkpos : int = - 1marklimit : int

ensureOpen()BufferedInputStream()BufferedInputStream()fill()read()read1()read()skip()available()mark()reset()markSupported()close()

(from io)PushbackInputStream

buf[] : bytepos : int

ensureOpen()PushbackInputStream()PushbackInputStream()

read()read()unread()unread()unread()available()skip()markSupported()close()

(from io)LineNumberInputStream

pushBack : int = - 1lineNumber : intmarkLineNumber : intmarkPushBack : int = - 1

LineNumberInputStream()

read()read()skip()setLineNumber()getLineNumber()available()mark()reset()

(from io)

InputStream

SKIP_BUFFER_SIZE : int = 2048skipBuffer[] : byte

read()read()read()skip()available()close()mark()reset()markSupported()

(from io)

FilterInputStream

FilterInputStream()read()read()read()skip()available()close()mark()reset()markSupported()

(from io)

#in

112

Bridge

Decouple an abstraction from itsimplementation so that the two can varyindependently.

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113

Bridge

114

Bridge

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115

Bridge Structure…

116

Bridge

1. Decoupling interface and implementation2. Improved extensibility 3. Hiding implementation details from clients

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117

ExampleUIManager

ListUI LabelUI

BasicLookAndFeel BasicLabelUI

JComponent ComponentUI

createUI()installUI()paint()

#ui

MetalLookAndFeel

LAFState

LookAndFeel

getName()installColors()

lookAndFeel

MultiLookAndFeelMultiListUI

MultiLabelUI

MetalLabelUI

BasicListUI

JList

#list

JLabel

118

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119

Builder

Separate the construction of a complexobject from its representation so that thesame construction process can createdifferent representations.

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Builder

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Builder Structure…

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Builder

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Builder Consequences

1. It lets you vary a product's internal representation

2. It isolates code for construction and representation

3. It gives you finer control over theconstruction process

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FlyWeight

Use sharing to support large numbers offine-grained objects efficiently.

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FlyWeight

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Flyweight: Structure

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Flyweight example

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Flyweight: Instances

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Flyweight: Applicability

Etat intrinsèque/extrinsèque…Les états extrinsèques peuvent êtrecalculés…

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Flyweight (il a rien compris… ☺)

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Iterator

Provide a way to access the elements of anaggregate object sequentially withoutexposing its underlying representation

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Iterator

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Iterator example:

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Example

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Memento

Without violating encapsulation, capture andexternalize an object's internal state so thatthe object can be restored to this state later.

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Memento Structure…

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Memento…

1. Preserving encapsulation boundaries2. It simplifies Originator 3. Using mementos might be expensive.4. Defining narrow and wide interfaces5. Hidden costs in caring for mementos

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Case Study

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Design problems… Document structure. The choice of internal representation

for the document affects nearly every aspect of Lexi'sdesign. All editing, formatting, displaying, and textualanalysis will require traversing the representation. Theway we organize this information will impact the design ofthe rest of the application.

Formatting. How does Lexi actually arrange text andgraphics into lines and columns? What objects areresponsible for carrying out different formatting policies?How do these policies interact with the document's

internal representation?

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Design problems… Embellishing the user interface. Lexi's user

interface includes scroll bars, borders, and dropshadows that embellish the WYSIWYG documentinterface. Such embellishments are likely tochange as Lexi's user interface evolves. Henceit's important to be able to add and removeembellishments easily without affecting the rest ofthe application.

Supporting multiple look-and-feel standards. Lexishould adapt easily to different look-and-feelstandards such as Motif and PresentationManager (PM) without major modification.

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Design problems… Embellishing the user interface. Lexi's user

interface includes scroll bars, borders, and dropshadows that embellish the WYSIWYG documentinterface. Such embellishments are likely tochange as Lexi's user interface evolves. Henceit's important to be able to add and removeembellishments easily without affecting the rest ofthe application.

Supporting multiple look-and-feel standards. Lexishould adapt easily to different look-and-feelstandards such as Motif and PresentationManager (PM) without major modification.

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Design problems…

Spelling checking and hyphenation. Howdoes Lexi support analytical operationssuch as checking for misspelled words anddetermining hyphenation points? How canwe minimize the number of classes wehave to modify to add a new analyticaloperation?

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Summary (C. Alexander)

It is possible to create building architectures bystringing together patterns in a rather looseway. A building made like this, is an assemblyof patterns. It is not dense. It is not profound.But it is also possible to put patterns together insuch way that many patterns overlap in thesame physical space: the building is verydense; it has many meanings captured in asmall space; and through this density, itbecomes profound.

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Architectural Patterns…

From MUD to Structure… – Layers, Pipe and Filters, Blackboard

Distributed Systems… – Broker, Pipe and Filters, Microkernel

Interactive Systems… – MVC, PAC

Adaptable Systems… – Microkernel, Reflection…

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Layer

helps structure application that can bedecomposed into groups of subtasks inwhich each group of subtasks is at aparticular level of abstraction.

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Layer: examples

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Layer :Structure

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Layer: Structure

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Layer and components…

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Layer and Facade DP…

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Layer and Facade DP

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Layers : Variants

Relaxed Layered System: – A layer « j » can use service of j-1, j-2… – A layer can be partially opaque

• Some service to layer j+1, others to all upper services…

Layering through inheritance: – Lower layers are implemented as base classes – Higher level can override lower level…

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Layers : Known UsesVirtual machines: JVM and binary code format

API : Layer that encapsulates lower layersInformation System – Presentation, Application logic, Domain Layer, Database

Windows NT (relaxed for: kernel and IO and hardware) – System services, – Resource management (Object manager, security monitor, process

manager, I/O manager, VM manager, LPC), – Kernel (exception handling, interrupt, multipro synchro, threads), – HAL (Hardware Abstraction Level)

– Hardware

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Layers: benefits

Reuse of layersSupport for standardization (POSIX)Dependencies are kept localExchangeabilities : – Replacement of old implementation with Adapter

Pattern – Dynamic exchange with Bridge Pattern

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Layers: Liabilities

Cascades of changing behavior Lower efficiencyUnnecessary work: functions of a layercalled many times for one serviceDifficulty of establishing correct granularity oflayers: Too few layers -> less benefits, toomany layers -> complexity and overhead…

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Pipes and Filters

Provides a structure for systems thatprocess a stream of Data. Each processingstep is encapsulated in a filter component.Data is passed through pipes betweenadjacent filters.Recombining filters allows the building of families of related systems.

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Pipes and Filters: Example

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Pipes and Filters: Structure

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Pipes and Filters

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Pipes and Filters: push pipeline

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Pipes and Filters: pull pipeline

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Pipes and Filters: push-pull pipeline

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Pipes and Filters : Threaded Filters

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Pipes and Filters: Known Uses

UnixCMS Pipelines (extension IBM mainframes)LASSPTools (Numerical Analysis) – Graphical input devices (knobs or sliders)

– Filters for numerical analysis and data extraction – Data sinks to produce animation from numerical

data streams…

Khoros : Image recognition…WEB !! Servlet !!

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Pipes and Filters Benefits

No intermediate file necessary (but possible)Flexibility by filter exchangeFlexibility by recombinationReuse of filter componentsRapid prototyping of pipelineEfficiency by parallel processing

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Pipes and Filters Liabilities

Sharing state information is expensive orinflexibleEfficiency gain by parallel processing is oftenan illusion

– Cost of data transfer, filters that consume alldata before one output, context switch on onecomputer, synchronization of filters via pipes

Data transformation overheadError Handling

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[Sun Developpers]

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Blackboard Structure

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Blackboard Structure

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Blackboard known uses

HEARSAY-II: Speech recognitionHASP/SIAP: detect enemy submarineCrysalis: infer three-dimensional structure ofprotein molecule from X-Ray diffraction Data.Tricero: Aircraft activities. Extend blackboardto distributed computing

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Blackboard benefits

Experimentation: different algo, differentcontrol heuristicsChangeability and maintainability: separationdata/control.

Reusable knowledge sourceSupport for Fault tolerance and robustness:Tolerance of noisy data…

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Blackboard Liabilities

Difficulty of testing: no deterministic algoNo good solution is guaranteed.Difficulty of establishing a good controlstrategyLow efficiency: (rejecting wrong hypothesis)High development effort : trial-and-error programmingNo support for parallelism

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Broker

Used to structure distributed softwaresystems with decoupled components thatinteract by remote service invocation.

A broker component is responsible forcoordinating communication, such asforwarding request, as well as fortransmitting result and exception.

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Broker example

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Broker structure

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Broker Structure

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Broker Structure

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Broker Structure

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Broker Structure

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Broker Variants

Direct Communication Broker System: – Direct link to server Message Passing Broker System – Focus on transmission of data. Type of the

message determine the behavior of the broker…Trader System : – service identifiers are used to access server

functionality. Request can be forwarded to morethan one server…

Callback broker system: event driven…

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Known Uses

CORBAIBM SOM/DSOMMicrosoft Ole 2.xWWW

ATM-P: Message passing broker.Telecommunication switching system basedon ATM.

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Broker benefits

Location transparencyChangeability and extensibility ofcomponentsPortability of a broker system (Layered)

Interoperability between brokers (bridge)Reusability (of services)

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Broker Liabilities

Restricted efficiency (indirection layer)Lower Fault tolerance: fault a broker or aserver… replication of components…Testability: – Of components (benefits) – Of application (liabilities)

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Model-View-Contoler (MVC)

The model contains the core functionalityand data?Views display information to the user.Controllers handle user input.

A change propagation mechanism ensureconsistency between user interface and themodel.

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MVC

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MVC Structure

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MVC Structure

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MVC Structure

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MVC Known Uses

SmalltalkMFCET++: application FrameworkJava/Swing

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MVC benefits

Multiple views of the same modelSynchronized views: change propagationPluggable views and controllersExchangeability of ‘look and feel’Framework potential

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MVC LiabilitiesIncreased complexityPotential for excessive number of updatesIntimate connection between view andcontroller Close coupling of views and controllers to amodelInefficiency of data access in viewInevitability of change to view and controller when portingDifficulty of using MVC with modern user-interface tools

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Presentation-Abstraction-Control

PAC define a hierarchy of cooperatingagents.Each agent consists of three components:presentation, abstraction, control.

Separates human computer interaction fromits functional core and its communicationwith other agents…

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PAC Example

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PAC Example

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PAC Structure

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PAC Structure

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PAC Structure

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PAC Structure

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PAC Known UsesNetwork Trafic Management (TS93) – Gathering traffic data – Threshold checking and generation exceptions – Logging and routing of network exception – Vizualisation of traffic flow and network exceptions

– Displaying various user-configurable views of the wholenetwork

– Statistical evaluation of traffic data – Access to historic traffic data – System administration and configuration

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PAC Benefits

Separation of concerns: Agent and inside anagentSupport for change and extensionSupport for multi-tasking: each PAC agentcan run its own thread on a differentcomputer…

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PAC Liabilities

Increased system complexity: Coordinationof agents…Complex control component: coordonateaction inside agent and with other agents…

Efficiency : data are propagated throught thetree… Applicability : Not a graphic editor whereeach object is a PAC agent…

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Microkernel Architecture

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Microkernel Structure

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Microkernel Structure

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Microkernel variants

Microkernel system with indirect Client-Server connections. MK establish channel ofcommunication between client and externalservers.

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Microkernel known Uses

Mach (92): Emulate other operating system(NeXTSTEP)

Amoeba (92): – Kernel: process, threads system memory,

communication, IO – Services not in the kernel are internal servers..

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Known uses

ChorusWINDOWS NT: – External servers: OS/2.1.X, posix server and

win32 server

MKDE: Microkernel Databank Engine – External server : Data model of SQL database

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Microkernel Benefits

Portability : no need to port externalservers…Flexibility and extensibilitySeparation of policy and mechanism: – Mechanism in kernel, policy in external servers

ScalabilityReliability: Distributed Microkernel… :-/Transparency : Microkernel ~ broker…

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Microkernel Liabilities

PerformanceComplexity of design and implementation. – Basic functionalities of the micro-kernel ?? – Separation mechanism/policy => deep

knowledge of domain.

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Reflection

Provides a mechanism for changingstructure and behavior of softwaredynamically.Support modification of fundamentalaspects: type structures and function callmechanismMeta-level makes the software self-aware

Base-level includes application logic. Itsimplementation builds on the meta-level.

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Reflection example

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Reflection structure

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Reflection example

Primitive

Type…

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SuperType

Pointer? Or not

Field…

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Reflection known Uses

CLOS : generic function and generic functioninvocationMIP: run-time type information system forC++Pgen: persistence component for C++ basedon MIPOle2.0, CORBA (dynamic invocation)…

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Reflection benefits

No explicit modification of source codeChanging a software is easy: no need forvisitors, factories and strategies patternsSupport for many kind of change

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Reflection Liabilities

Modification at the meta-level can causedamage.Increased number of componentLower efficiencyNot all potential changes supported (onlythose supported by the MOP)Not all languages support reflection

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Reflection example

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Reflection example

public class Main {

public static void main(String args[]) throws Exception {Point p = new Point();p.setX(3);p.setY(4);

Cercle c = new Cercle();c.setPoint(p);

c.setRadius(6);XMLEncoder e = new XMLEncoder( new BufferedOutputStream( newFileOutputStream(args[0])));

e.writeObject(c);e.close();

System.out.println(c);}

}

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Reflection example

3

4

6

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Reflection example public class Reread {

public static void main(String args[])throws Exception {XMLDecoder d = new XMLDecoder( new

BufferedInputStream( newFileInputStream(args[0])));

Cercle c = (Cercle)d.readObject();d.close();

System.out.println(c);

}}

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Summary (C. Alexander)

It is possible to build an architecture bystringing together patterns, in a rather looseway. A building made like this, is an assemblyof patterns. It is not dense. It is not profound.But it is also possible to put patterns together insuch way that many patterns overlap in thesame physical space: the building is verydense; it has many meanings captured in asmall space; and through this density, itbecomes profound.

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Drawbacks of Patterns

Patterns do not lead to direct code reuse.Individual Patterns are deceptively simple.Composition of different patterns can be verycomplex.Teams may suffer from pattern overload.Patterns are validated by experience anddiscussion rather than by automated testing.

Integrating patterns into a softwaredevelopment process is a human-intensiveactivity.

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