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IMPORTANT NOTICE TO STUDENTSThese slides are NOT to be used as a replacement for student notes.These slides are sometimes vague and incomplete on purpose to spark class discussionsGang of Four (GoF)OO Design PatternsCS 446/646 ECE452May 11th, 2011WATERLOOCHERITON SCHOOL OFCOMPUTER SCIENCE

MotivationObject Oriented Analysis (OOA) domain problem designed as (domain) objects–––addresses the functional challengeswhat a system doesprovides guidance for implementationObject Oriented Design (OOD) domain problem solved as (implementation) objects––addresses the implementation challengeshow a system realizes OOAWATERLOOCHERITON SCHOOL OFCOMPUTER SCIENCECS446/646 ECE4522

MotivationHow can we improve OOD identify common characteristics– creation, structure, behaviour & interactionsdesign patterns (design reuse)–––generic blueprints (micro architecture)language and implementation independenttwo main catalogues GoF: Gang of Four (Gamma, Helm, Johnson, Vlissides, 1995)POSA: Pattern Oriented Software Architecture (Buschmann, etal.; Wiley, 1996)WATERLOOCHERITON SCHOOL OFCOMPUTER SCIENCECS446/646 ECE4523

MotivationWhat is a Design Pattern common solution to a reoccurring problem in designAnatomy name problem/motivation solution consequences & tradeoffs which ones are important for us?WATERLOOCHERITON SCHOOL OFCOMPUTER SCIENCECS446/646 ECE4524

Design Patterns ClassificationGoF Design jectscopeWATERLOOCHERITON SCHOOL OFCOMPUTER SCIENCECS446/646 ECE4525

Design Patterns ClassificationGoF Design actory MethodAdaptor - classBehavioralInterpreterTemplate MethodAbstract FactoryAdaptor-objectChain of HERITON SCHOOL OFCOMPUTER SCIENCECS446/646 ECE4526

Design Patterns Classification“Purpose” based classification creational:– structural– concerns with creation process of objects & classescomposition of classes & objectsbehavioral–characterizes interaction & responsibility of objects &classesWATERLOOCHERITON SCHOOL OFCOMPUTER SCIENCECS446/646 ECE4527

Design Patterns Classification“Scope” based classification decided if the pattern applies to mainly classes or objectsTwo categories class scope–– relationship between classes & subclassesstatically defined at run-timeobject scope––object relationships (what type?)can be manipulated at runtime (so what?)WATERLOOCHERITON SCHOOL OFCOMPUTER SCIENCECS446/646 ECE4528

Design Patterns ClassificationCreational class Creational objectdefers object creation tosub-classes (factorymethod)Structural class inheritance to composeclasses (adapter)CHERITON SCHOOL OFCOMPUTER SCIENCE deals with objectassembly (adapter)Behavioral objectuses inheritance todescribe flow of control,algorithms (template)WATERLOOdefers object creation toother objects (abstractfactory)Structural objectBehavioral class CS446/646 ECE452group of objects workingtogether to carry out atask (iterator)9

SingletonIntent “ensure a class only has one instance, and provide aglobal point of access to it.”ConstructionWATERLOOCHERITON SCHOOL OFCOMPUTER SCIENCECS446/646 ECE45210

SingletonIntent“ensure a class only has one instance, and provide aglobal point of access to it.” Constructionpublic class Singleton {private static final Singleton INSTANCE new Singleton();// Private constructor prevents// instantiation from other classesprivate Singleton() {}}public static Singleton getInstance() {return INSTANCE;}WATERLOOCHERITON SCHOOL OFCOMPUTER SCIENCECS446/646 ECE45211

SingletonAdvantages controlled access to the class instance(s)– can dictate who, and when a client can accessrefinement of functionality–via inheritance/subclassWATERLOOCHERITON SCHOOL OFCOMPUTER SCIENCECS446/646 ECE45212

SingletonAdvantages variable number of instances––the getInstance() method needs modificationwhat else needs to change?WATERLOOCHERITON SCHOOL OFCOMPUTER SCIENCECS446/646 ECE45213

SingletonA closer look at Singleton reuse separation of concerns global presence stateful vs. stateless multiple instances life cycleWATERLOOCHERITON SCHOOL OFCOMPUTER SCIENCECS446/646 ECE45214

Singleton – A Closer LookReuse coupling–––results in tighter couplingcouples with the exact type of the singleton objectpass by reference to reduce coupling?WATERLOOCHERITON SCHOOL OFCOMPUTER SCIENCECS446/646 ECE45215

Singleton – A Closer LookReuse coupling–––results in tighter couplingcouples with the exact type of the singleton objectpass by reference to reduce coupling?public void doSomething(){Worker worker Worker.getInstance();worker.perform();}public void doSomething(Worker worker){worker.perform();}WATERLOOCHERITON SCHOOL OFCOMPUTER SCIENCECS446/646 ECE45216

Singleton – A Closer LookReuse inheritance––easy to extend functionality in subclassesnot easy to override the object instance in subclassesWATERLOOCHERITON SCHOOL OFCOMPUTER SCIENCECS446/646 ECE45217

Singleton – A Closer LookSeparation of concerns singleton class responsible for creation– acts as a builder/factorywhat if we were to separate the two concerns–example database connection as a singletonsystem 1 uses a singleton to ensure only a single databaseconnectionsystem 2 needs to connection pool of 10 databases connectionsWATERLOOCHERITON SCHOOL OFCOMPUTER SCIENCECS446/646 ECE45218

Singleton – A Closer LookGlobal presence provides a global access point to a service––aren't global variables bad?can be accessed from anywhere not part of method signature–– violation of layered accessdependency is not obviousrequires code inspectiona large system may require many singletons–use a registry/repositoryWATERLOOCHERITON SCHOOL OFCOMPUTER SCIENCECS446/646 ECE45219

Singleton – A Closer LookStateful singleton same as a global variable in principle– access concerns–– aren't global variables bad?synchronizationconcurrency – multiple threaded using a singletonmutable vs. immutable stateStateless singleton better then stateful can we have a stateless singleton?WATERLOOCHERITON SCHOOL OFCOMPUTER SCIENCECS446/646 ECE45220

Singleton – A Closer LookMultiple instances distributed systems–– is it possible to have a true singleton in a distributedsystem?global registries/repositorieslanguage (Java) specific concerns–––initialization – has to be thread safeserializationclass loadersWATERLOOCHERITON SCHOOL OFCOMPUTER SCIENCECS446/646 ECE45221

Singleton – A Closer LookLife-cycle & life span creation– singletons are long lived––– lazy initializationas long as an application's life spanregistries can outlive applicationsunit testing requires short lived statelanguage (Java) specific concern––reloading singleton class (servlets)loss of stateWATERLOOCHERITON SCHOOL OFCOMPUTER SCIENCECS446/646 ECE45222

SingletonWhen can I use a singleton considerations[1]––– will every user use this class exactly the same way?will every applications ever need only one instance?should the clients be unaware of the applicationexamples–––Java Math class (stateless – static class)top level GUI (window/frame)logging[1] e.htmlWATERLOOCHERITON SCHOOL OFCOMPUTER SCIENCECS446/646 ECE45223

AdapterIntent “convert the interface of a class into another interface.Adapter lets classes work together that couldn't otherwisebecause of incompatible interface” also known as “wrapper” boolean values can be represented by––{1,0}, {true, false}, {yes, no}does this qualify as an adapter?WATERLOOCHERITON SCHOOL OFCOMPUTER SCIENCECS446/646 ECE45224

MotivationNeed to add “Text” capability to our drawing editor.Consider an off the shelf TextView componentWATERLOOCHERITON SCHOOL OFCOMPUTER SCIENCECS446/646 ECE45225

MotivationObservations can be done in two ways––object composition (shown above)inheritance Shape provides “interface” and TextView provides animplementationLets try to draw this?WATERLOOCHERITON SCHOOL OFCOMPUTER SCIENCECS446/646 ECE45226

Adapter – ClassRequirement requires multiple inheritancewhat about implementations that do not support multipleinheritance (Java)?WATERLOOCHERITON SCHOOL OFCOMPUTER SCIENCECS446/646 ECE45227

Adapter – ObjectRequirement via object compositionWATERLOOCHERITON SCHOOL OFCOMPUTER SCIENCECS446/646 ECE45228

Adapter – Class vs. ObjectClass commitment to aconcrete adaptee class– not to its subclasses(class hierarchy) can use many adaptees– allows for specialization– Objectincluding sub-classesharder to override theadaptee behavior–why?how?static in natureWATERLOOCHERITON SCHOOL OFCOMPUTER SCIENCECS446/646 ECE45229

Adapter – Class vs. ObjectWATERLOOCHERITON SCHOOL OFCOMPUTER SCIENCECS446/646 ECE45230

Adapter & Dependency InversionDependency Inversion (DI) decouple high level layer from lower level rUtilitylayerSimple LayersWATERLOOCHERITON SCHOOL OFCOMPUTER SCIENCEAbstract LayersCS446/646 ECE45231

Dependency Inversion ExampleImplications Button implementationrelies on Lampany changes to Lamp willimpact Buttonwhat if we want to reuseButton class with adifferent component–such as a motorWATERLOOCHERITON SCHOOL OFCOMPUTER SCIENCECS446/646 ECE45232

Dependency Inversion ExampleDependency Inversion to Rescue looks good (?) still a dependency leftWATERLOOCHERITON SCHOOL OFCOMPUTER SCIENCECS446/646 ECE45233

Dependency Inversion ExampleObservation adapter enhanced the design–increased re-usability at the price of complexityWATERLOOCHERITON SCHOOL OFCOMPUTER SCIENCECS446/646 ECE45234

AdapterHow much adaptation is reasonable?WATERLOOCHERITON SCHOOL OFCOMPUTER SCIENCECS446/646 ECE45235

BridgeIntent “decouples an abstraction from its implementation so thetwo can vary independently”does this not sounds like an adapter?–will take a closer look laterWATERLOOCHERITON SCHOOL OFCOMPUTER SCIENCECS446/646 ECE45236

BridgeWATERLOOCHERITON SCHOOL OFCOMPUTER SCIENCECS446/646 ECE45237

BridgeBridgeWATERLOOCHERITON SCHOOL OFCOMPUTER SCIENCECS446/646 ECE45238

Bridge ExampleProblemSolution via inheritanceproblem1: what if we have tosupport another platform?problem2: client code is tied toan implementation.For portable code, the clientshould not refer to animplementationWATERLOOCHERITON SCHOOL OFCOMPUTER SCIENCECS446/646 ECE45239

Bridge ExampleSolution: Use bridge pattern to place abstraction andimplementation in two different hierarchiesWATERLOOCHERITON SCHOOL OFCOMPUTER SCIENCECS446/646 ECE45240

Bridge ExampleSolution: Use bridge pattern to place abstraction andimplementation in two different hierarchiesBridgeWATERLOOCHERITON SCHOOL OFCOMPUTER SCIENCECS446/646 ECE45241

BridgeFeatures flexible binding between abstraction & implementation two class hierarchies clients are decoupledWATERLOOCHERITON SCHOOL OFCOMPUTER SCIENCECS446/646 ECE45242

Adapter & BridgeCommon Elements flexibility via indirection request forwardingWATERLOOCHERITON SCHOOL OFCOMPUTER SCIENCECS446/646 ECE45243

Adapter & BridgeDifference in intent adapter–––– resolves incompatibilities between two existing interfacestwo interfaces are independent and can evolve separatelycoupling is unforeseenadapts components after they have been designedbridge–––connects an abstraction and its many implementationsevolution is in accordance with the base abstractioncoupling between the abstraction and the implementations areknownWATERLOOCHERITON SCHOOL OFCOMPUTER SCIENCECS446/646 ECE45244

CS446/646 ECE452 3 WATERLOO CHERITON SCHOOL OF COMPUTER SCIENCE Motivation How can we improve OOD identify common characteristics - creation, structure, behaviour & interactions design patterns (design reuse) - generic blueprints (micro architecture) - language and implementation independent - two main catalogues GoF: Gang of Four (Gamma, Helm, Johnson, Vlissides, 1995)