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SchoolofComputerScience

23Patternsin80Minutes:aWhirlwindJava-centricTouroftheGang-of-FourDesignPatterns

JoshBloch CharlieGarrod

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Administrivia

• Homework6checkpointdue Friday 5pm• Finalexam Tuesday,May3, 5:30-8:30pm,PH100• FinalreviewsessionSunday,May, 7-9pm,DH1112

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KeyconceptfromTuesday…MapReducewithkey/valuepairs(Googlestyle)

• Master– Assigntaskstoworkers– Pingworkerstotestforfailures

• Mapworkers– Mapforeachkey/valuepair

– Emitintermediatekey/valuepairs

the shuffle:

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• E.g.,foreachwordontheWeb,countthenumberoftimesthatwordoccurs§ ForMap:key1 isadocumentname,value isthecontentsofthatdocument

§ ForReduce:key2 isaword,values isalistofthenumberofcountsofthatword

KeyconceptfromTuesday…MapReducewithkey/valuepairs(Googlestyle)

f1(String key1, String value): for each word w in value:

EmitIntermediate(w, 1);

f2(String key2, Iterator values):int result = 0;for each v in values:

result += v;Emit(key2, result);

Map: (key1, v1) à (key2, v2)* Reduce: (key2, v2*) à (key3, v3)*

MapReduce: (key1, v1)* à (key3, v3)*

MapReduce: (docName, docText)* à (word, wordCount)*

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Outline

I. CreationalPatternsII. StructuralPatternsIII. BehavioralPatterns

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PatternName

• Intent– theaimofthispattern• Usecase– amotivatingexample• Keytypes– theinterfacesthatdefinepattern• JDK– example(s)ofthispatternintheJDK

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Illustration

• Codesample,diagram,ordrawing– Timeconstraintsmakeitimpossibletoincludeillustrationsfromsomepatterns

• SomepatternslackanillustrationL

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I.CreationalPatterns

1. Abstractfactory2. Builder3. Factorymethod4. Prototype5. Singleton

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AbstractFactory

• Intent– Allowcreationoffamiliesofrelatedobjects independentofimplementation

• Usecase– look-and-feelinaGUItoolkit• Keytype– Factory withmethodstocreateeachfamilymember

• JDK– Notcommon

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Builder

• Intent– Separateconstructionofcomplexobjectfromrepresentationsosamecreationprocesscancreatedifferentrepresentations

• Usecase– convertingrichtexttovariousformats• Keytypes– (Abstract)Builder– GoF hasextralayerofindirection(“Director”)

• JDK– StringBuilder,StringBuffer*– ButbothproduceString– AndmostbuildersintheJDKareconcrete

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MytakeonBuilder

• Emulatesnamedparametersinlanguagesthatdon’tsupportthem

• ReducesexponentialO(2n)creationalmethodstoO(n)byallowingthemtobecombinedfreely,atthecostofanintermediate(Builder)object

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BuilderIllustrationNutritionFacts twoLiterDietCoke = new NutritionFacts.Builder(

"Diet Coke", 240, 8).sodium(1).build();

public class NutritionFacts {public static class Builder {

public Builder(String name, int servingSize, int servingsPerContainer) { ... }

public Builder totalFat(int val) { totalFat = val; }public Builder saturatedFat(int val) { satFat = val; }public Builder transFat(int val) { transFat = val; }public Builder cholesterol(int val) { cholesterol = val; }... // 15 more setters

public NutritionFacts build() {return new NutritionFacts(this);

}}private NutritionFacts(Builder builder) { ... }

}

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FactoryMethod

• Intent– abstractcreationalmethodthatletssubclassesdecidewhichclasstoinstantiate

• Usecase– creatingdocumentsinaframework• Keytypes– Creator,whichcontainsabstractmethodtocreateaninstance

• JDK– notcommon.Iterable.iterator()• RelatedStaticFactorypatternisverycommon– TechnicallynotaGoF pattern,butcloseenough

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FactoryMethodIllustrationpublic interface Iterable<E> {

public abstract Iterator<E> iterator();}

public class ArrayList<E> implements List<E> {public Iterator<E> iterator() { ... }...

}

public class HashSet<E> implements Set<E> {public Iterator<E> iterator() { ... }...

}

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Prototype

• Intent– Createanobjectbycloninganotherandtweakingasnecessary

• Usecase– writingamusicscoreeditorinagraphicaleditorframework

• Keytypes– Prototype(AKACloneable)• JDK– clone,butdon’tuseit(exceptonarrays)– JavaandPrototypepatternareapoorfit

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Singleton

• Intent– ensuringaclasshasonlyoneinstance• Usecase– GoF sayprintqueue,filesystem,companyinanaccountingsystem– Compellingusesarerarebuttheydoexist

• Keytypes– Singleton• JDK– java.lang.Runtime

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SingletonIllustrationpublic enum Elvis {

ELVIS;

public sing(Song song) { ... }

public playGuitar(Riff riff) { ... }

public eat(Food food) { ... }

public take(Drug drug) { ... }}

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Mytakeonsingleton

• It’saninstance-controlled class;othersinclude– Staticutilityclass(non-instantiable)– Enum – oneinstancepervalue,allvaluesknownatcompiletime

– Internedclass– onecanonicalinstancepervalue,newvaluescreatedatruntime

• Thereisadualitybetweensingletonandstaticutilityclass

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II.StructuralPatterns

1. Adapter2. Bridge3. Composite4. Decorator5. Façade6. Flyweight7. Proxy

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Adapter

• Intent– convertinterfaceofaclassintoonethatanotherclassrequires,allowinginteroperability

• Usecase– numerous,e.g.,arraysvs.collections• Keytypes– Target,Adaptee,Adapter• JDK– Arrays.asList(T[])

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AdapterIllustration

Havethisandthis?Usethis!

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Bridge

• Intent– Decoupleanabstractionfromitsimplementationsotheycanvaryindependently

• Usecase– portablewindowingtoolkit• Keytypes– Abstraction,Implementor• JDK– JDBC,JavaCryptographyExtension(JCE)– BothareServiceProviderInterface(SPI)frameworks– SPIis BridgeImplementor!

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BridgeIllustration

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Composite

• Intent– Composeobjectsintotreestructures.Letclientstreatprimitives&compositionsuniformly.

• Usecase– GUItoolkit(widgetsandcontainers)• Keytype– Componentthatrepresentsbothprimitivesandtheircontainers

• JDK– javax.swing.JComponent

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CompositeIllustrationpublic interface Expression {

double eval(); // Returns valueString toString(); // Returns infix expression string

}

public class UnaryOperationExpression implements Expression {public UnaryOperationExpression(

UnaryOperator operator, Expression operand);}public class BinaryOperationExpression implements Expression {

public BinaryOperationExpression(BinaryOperator operator,Expression operand1, Expression operand2);

}public class NumberExpression implements Expression {

public NumberExpression(double number);}

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Decorator

• Intent– attachfeaturestoanobjectdynamically• Usecase– attachingbordersinaGUItoolkit• Keytypes– Component, implementbydecoratoranddecorated

• JDK– Collections(e.g.,Synchronizedwrappers),java.io streams,Swingcomponents

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DecoratorIllustration

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Façade

• Intent– Provideasimpleunifiedinterfacetoasetofinterfacesinasubsystem– GoF allowforvariantswherethecomplexunderpinningsareexposedandhidden

• Usecase– anycomplexsystem;GoF usecompiler• Keytypes– Façade(thesimpleunifiedinterface)• JDK– java.util.concurrent.Executors

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FacadeIllustration

Facade

√√

√

√

√

√ √

Subsystem classes

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Flyweight

• Intent– usesharingtosupportlargenumbersoffine-grainedobjectsefficiently

• Usecase– charactersinadocument• Keytypes– theFlyweight(instance-controlled!)– Statecanbemadeextrinsic tokeepFlyweightsharable

• JDK– Pervasisve!Allenums,manyothers.j.u.c.TimeUnithas#unitsasextrinsic state.

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FlyweightIllustration

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Proxy

• Intent– surrogateforanotherobject• Usecase– delayloadingofimagestillneeded• Keytypes– Subject,Proxy,RealSubject• Gof mentionseveralflavors– virtualproxy– stand-inthatinstantiateslazily– remoteproxy– localrepresentativeforremoteobj– protectionproxy– deniessomeopstosomeusers– smartreference– doeslockingorref.counting,e.g.

• JDK– RMI,collectionswrappers

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ProxyIllustrationsVirtual Proxy

Smart Reference Remote Proxy

SynchronizedList ArrayList

aTextDocumentimage anImage

data

in memory on disk

anImageProxyfileName

Client

Proxy

Server

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III.BehavioralPatterns

1. ChainofResponsibility2. Command3. Interpreter4. Iterator5. Mediator6. Memento7. Observer8. State9. Strategy10. Templatemethod11. Visitor

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ChainofResponsibility

• Intent– avoidcouplingsendertoreceiverbypassingrequestalonguntilsomeonehandlesit

• Usecase– context-sensitivehelpfacility• Keytypes– RequestHandler• JDK– Classloader,Properties• ExceptionhandlingcouldbeconsideredaformofChainofResponsibilitypattern

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Command

• Intent– encapsulaterequestasobject,lettingyouparameterizeclientswithdifferentactions,queueorlogrequests,etc.

• Usecase– menutree• Keytypes– Command (anexecutemethod)• JDK– Runnable,executorframework• IsitCommandpatternifyourunitmorethanonce?Ifittakesanargument?Returnsaval?

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Interpreter

• Intent– Givenalanguage,defineclasshierarchyforparsetree,recursivemethodtointerpretit

• Usecase– regularexpressionmatching• Keytypes– Expression,NonterminalExpression,TerminalExpression

• JDK– nousesI’mawareof– Ourexpressionevaluator(HW2)isaclassicexample

• NecessarilyusesCompositepattern!

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InterpreterIllustrationpublic interface Expression {

double eval(); // Returns valueString toString(); // Returns infix expression string

}

public class UnaryOperationExpression implements Expression {public UnaryOperationExpression(

UnaryOperator operator, Expression operand);}public class BinaryOperationExpression implements Expression {

public BinaryOperationExpression(BinaryOperator operator,Expression operand1, Expression operand2);

}public class NumberExpression implements Expression {

public NumberExpression(double number);}

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Iterator

• Intent– provideawaytoaccesselementsofacollectionwithoutexposingrepresentation

• Usecase– collections• Keytypes– Iterable,Iterator– ButGoF recognizeinternaliterationtoo

• JDK– Collections,for-eachstatement,etc.

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Mediator

• Intent– Defineanobjectthatencapsulatehowasetofobjectsinteracttoreducecoupling.– O(n)couplingsinsteadofO(n!)=O(2n)

• Usecase– dialogboxwherechangeinonecomponentaffectsbehaviorofothers

• Keytypes– Mediator,components• JDK– Unclear

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MediatorIllustration

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Memento

• Intent– Withoutviolatingencapsulation,allowclienttocaptureanobject’sstate,andrestore

• Usecase– undostackforoperationsthataren’teasilyundone,e.g.,line-arteditor

• Keytype– Memento(opaquestateobject)• JDK– nonethatI’mawareof(not serialization)

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Observer

• Intent– Letobjectsobservethebehaviorofotherobjectssotheycanstayinsync

• Usecase– multipleviewsofadataobjectinaGUI• Keytypes– Subject (“observable”),Observer– GoF areagnosticonmanydetails!

• JDK– Swing,leftandright

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State

• Intent– useanobjectinternallytorepresentthestateofanotherobject;delegatemethodinvocationstothestateobject

• Usecase– TCPConnection(whichisstateful)• Keytype– State• JDK– nonethatI’mawareofbut–Worksgreat inJava– Useenums asstates– UseAtomicReference<State> tostoreit

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Strategy

• Intent– representabehaviorthatparameterizesanalgorithmforbehaviororperformance

• Usecase– line-breakingfortextcompositing• Keytypes– Strategy• JDK– Comparator

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Templatemethod

• Intent– defineskeletonofanalgorithmordatastructure,deferringsomedecisionstosubclasses

• Usecase– applicationframeworkthatletspluginsimplementalloperationsondocuments

• Keytypes– AbstractClass,ConcreteClass• JDK– Skeletalcollectionimpls (e.g.,AbstractList)• Note– templatemethodisdualtostrategy,youcanmechanicallyconvertonetotheother

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TemplateMethodIllustration// List adapter for primitive int arrayspublic static List<Integer> intArrayList(final int[] a) {

return new AbstractList<Integer>() {public Integer get(int i) {

return a[i];}

public Integer set(int i, Integer val) {Integer oldVal = a[i];a[i] = val;return oldVal;

}

public int size() {return a.length;

}};

}

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Visitor

• Intent– Representanoperationtobeperformedonelementsofanobjectstructure(e.g.,aparsetree).Visitorletsyoudefineanewoperationwithoutmodifyingthetypehierarchy.

• Usecase– type-checking,pretty-printing,etc.• Keytypes– Visitor,ConcreteVisitor,allthetypesthatgetvisited

• JDK– NonethatI’mawareof

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MoreonVisitor

• VisitorisNOTmerelytraversingagraphstructureandapplyingamethod– That’sIterator

• Theessenceofvisitorisdouble-dispatch– FirstdynamicallydispatchontheVisitor– Thenontheobjectbeingvisited

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Summary

• NowyouknowalltheGangofFourpatterns• Definitionscanbevague• Coverageisincomplete• Butthey’reextremelyvaluable– Theygaveusavocabulary– Andawayofthinkingaboutsoftware

• Lookforpatternsasyoureadandwritesoftware– GoF,non-GoF,andundiscovered