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SchoolofComputerScience
23Patternsin80Minutes:aWhirlwindJava-centricTouroftheGang-of-FourDesignPatterns
JoshBloch CharlieGarrod
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Administrivia
• Homework6checkpointdueFriday5:00pm• FinalexamFriday,Dec16th 5:30–8:30pm,GHC4401– ReviewsessionWednesday,Dec14th 7–9:30pm,DH1112
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Outline
I. CreationalPatternsII. StructuralPatternsIII. BehavioralPatterns
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PatternName
• Intent – theaimofthispattern• Usecase– amotivatingexample• Keytypes – thetypesthatdefinepattern– Italictypenameindicatesabstractclass;typicallythisisaninterfacewhenthepatternisusedinJava
• JDK – example(s)ofthispatternintheJDK
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Illustration
• Codesample,diagram,ordrawing– Timeconstraintsmakeitimpossibletoincludeillustrationsfromsomepatterns
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I.CreationalPatterns
1. Abstractfactory2. Builder3. Factorymethod4. Prototype5. Singleton
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1.AbstractFactory
• Intent– allowcreationoffamiliesofrelatedobjectsindependentofimplementation
• Usecase– look-and-feelinaGUItoolkit– EachL&Fhasitsownwindows,scrollbars,etc.
• Keytypes– Factory withmethodstocreateeachfamilymember,Products
• JDK– notcommon
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AbstractFactoryIllustration
Client
Window
PMWindow MotifWindow
PMScrollBar MotifScrollBar
ScrollBar
WidgetFactoryCreateWindow()CreateScrollBar()
MotifWidgetFactoryCreateWindow()CreateScrollBar()
PMWidgetFactoryCreateWindow()CreateScrollBar()
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2.Builder
• Intent– separateconstructionofcomplexobjectfromrepresentationso samecreationprocesscancreatedifferentrepresentations
• usecase– convertingrichtexttovariousformats• types– Builder,ConcreteBuilders,Director,Products• JDK– StringBuilder,StringBuffer*– Butthereisno(visible)abstractsupertype…– Andbothgeneratesameproductclass(String)
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Gof4BuilderIllustrationRTFReaderParseRTF()
while(t=nextToken){switchtType{CHAR:builder->AddChar(t.Char)FONT:builder->SetFont(t.Font)PARA:builder->AddParagraph()
}}
TextConverterAddChar(char)SetFont(font)AddParagraph()
ASCIIConverterAddChar(char)GetASCIIText()
TeXConverter
AddChar(char)SetFont(font)AddParagraph()GetTeXText()
GUITextConverter
AddChar(char)SetFont(font)AddParagraph()GetGUIText()
Builders
GUITextTeXTextASCIIText
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MytakeonBuilder[EJItem1]
• Emulatesnamedparametersinlanguagesthatdon’tsupportthem
• Emulates2n constructorsorfactorieswithnbuildermethods,byallowingthemtobecombinedfreely
• Costisanintermediate(Builder)object
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EJ-styleBuilderIllustrationNutritionFacts 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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3.FactoryMethod
• Intent– abstractcreationalmethodthatletssubclassesdecidewhichclasstoinstantiate
• Usecase– creatingdocumentsinaframework• Keytypes– Creator,whichcontainsabstractmethodtocreateaninstance
• JDK– 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() { ... }...
}
Collection<String> c = ...;
for (String s : c) // Creates an Iterator appropriate to cSystem.out.println(s);
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4.Prototype
• Intent– createanobjectbycloninganotherandtweakingasnecessary
• Usecase– writingamusicscoreeditorinagraphicaleditorframework
• Keytypes– Prototype• JDK– Cloneable,butavoid(exceptonarrays)– JavaandPrototypepatternareapoorfit
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5.Singleton
• Intent– ensuringaclasshasonlyoneinstance• Usecase– GoF sayprintqueue,filesystem,companyinanaccountingsystem– Compellingusesarerarebuttheydoexist
• Keytypes– Singleton• JDK– java.lang.Runtime
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SingletonIllustrationpublic enum Elvis {
ELVIS;
sing(Song song) { ... }
playGuitar(Riff riff) { ... }
eat(Food food) { ... }
take(Drug drug) { ... }}
// Alternative implementationpublic class Elvis {
public static final Elvis ELVIS = new Elvis();private Elvis() { }...
}
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MytakeonSingleton
• It’saninstance-controlledclass;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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1.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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2.Bridge
• Intent– decoupleanabstractionfromitsimplementationsotheycanvaryindependently
• Usecase– portablewindowingtoolkit• Keytypes– Abstraction,Implementor• JDK– JDBC,JavaCryptographyExtension(JCE),JavaNaming&DirectoryInterface(JNDI)
• Bridgepatternvery similartoServiceProvider– Abstraction~API,Implementer ~SPI
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BridgeIllustration
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3.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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4.Decorator
• Intent– attachfeaturestoanobjectdynamically• Usecase– attachingbordersinaGUItoolkit• Keytypes– Component,implementbydecoratorand decorated
• JDK– Collections(e.g.,Synchronizedwrappers),java.io streams,Swingcomponents
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DecoratorIllustration
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5.Façade
• Intent– provideasimpleunifiedinterfacetoasetofinterfacesinasubsystem– GoF allowforvariantswherethecomplexunderpinningsareexposedandhidden
• Usecase– anycomplexsystem;GoF usecompiler• Keytypes– Façade(thesimpleunifiedinterface)• JDK– java.util.concurrent.Executors
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FaçadeIllustration
Façade
√√
√
√
√
√ √
Subsystem classes
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6.Flyweight
• Intent– usesharingtosupportlargenumbersoffine-grainedobjectsefficiently
• Usecase– charactersinadocument• Keytypes– Flyweight(instance-controlled!)– Somestatecanbeextrinsic toreducenumberofinstances
• JDK– Common!Allenums,manyothers– j.u.c.TimeUnit hasnumberofunitsasextrinsicstate
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FlyweightIllustration
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7.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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1.ChainofResponsibility
• Intent– avoidcouplingsendertoreceiverbypassingrequestalonguntilsomeonehandlesit
• Usecase– context-sensitivehelpfacility• Keytypes– RequestHandler• JDK– ClassLoader,Properties• ExceptionhandlingcouldbeconsideredaformofChainofResponsibilitypattern
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2.Command
• Intent– encapsulatearequestasasanobject,lettingyouparameterizeoneactionwithanother,queueorlogrequests,etc.
• Usecase– menutree• Keytype– Command (Runnable)• JDK– Common!Executorframework,etc.• IsitCommandpatternifyourunitrepeatedly?Ifittakesanargument?Returnsaval?
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CommandIllustrationpublic static void main(String[] args) {
SwingUtilities.invokeLater(() -> new Demo().setVisible(true)); }
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3.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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4.Iterator
• Intent– provideawaytoaccesselementsofacollectionwithoutexposingrepresentation
• Usecase– collections• Keytypes– Iterable,Iterator– ButGoF discussinternaliteration,too
• JDK– collections,for-eachstatement,etc.
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IteratorIllustrationpublic 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() { ... }...
}
Collection<String> c = ...;
for (String s : c) // Creates an Iterator appropriate to cSystem.out.println(s);
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5.Mediator
• Intent– defineanobjectthatencapsulateshowasetofobjectsinteract,toreducecoupling.– 𝓞(n)couplingsinsteadof𝓞(n2)
• Usecase– dialogboxwherechangeinonecomponentaffectsbehaviorofothers
• Keytypes– Mediator,Components• JDK– Unclear
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MediatorIllustration
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6.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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7.Observer
• Intent– letobjectsobservethebehaviorofotherobjectssotheycanstayinsync
• Usecase– multipleviewsofadataobjectinaGUI• Keytypes– Subject (“Observable”),Observer– GoF areagnosticonmanydetails!
• JDK– Swing,leftandright
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ObserverIllustration// Implement roll button and dice type fieldJTextField diceSpecField = new JTextField(diceSpec, 5); // Field widthJButton rollButton = new JButton("Roll");rollButton.addActionListener(event -> {if (!diceSpecField.getText().equals(diceSpec)) {
diceSpec = diceSpecField.getText();dice = Die.dice(diceSpec);jDice.resetDice(dice);
}for (Die d : dice) d.roll();jDice.repaint();
});
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8.State
• Intent– allowanobjecttoalteritsbehaviorwheninternalstatechanges.“Objectwillappeartochangeclass.”
• Usecase– TCPConnection(whichisstateful)• Keytype– State(Objectdelegatestostate!)• JDK– nonethatI’mawareof,but…–Worksgreat inJava– Useenums asstates– UseAtomicReference<State> tostoreit
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9.Strategy
• Intent– representabehaviorthatparameterizesanalgorithmforbehaviororperformance
• Usecase– line-breakingfortextcompositing• Keytypes– Strategy• JDK– Comparator
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StrategyIllustrationComparator isastrategyfororderingpublic static synchronized void main(String[] args) {
Arrays.sort(args, Comparator.reverseOrder());System.out.println(Arrays.toString(args));
Arrays.sort(args, Comparator.comparingInt(String::length));System.out.println(Arrays.toString(args));
}
java Foo i eat wondrous spam[wondrous, spam, i, eat][i, eat, spam, wondrous]
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10.TemplateMethod
• Intent– defineskeletonofanalgorithmordatastructure,deferringsomedecisionstosubclasses
• Usecase– applicationframeworkthatletspluginsimplementalloperationsondocuments
• Keytypes– AbstractClass,ConcreteClass• JDK– skeletalcollectionimpls (e.g.,AbstractList)
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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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11.Visitor
• Intent– representanoperationtobeperformedonelementsofanobjectstructure(e.g.,aparsetree).Visitorletsyoudefineanewoperationwithoutmodifyingthetypehierarchy.
• Usecase– type-checking,pretty-printing,etc.• Keytypes– Visitor,ConcreteVisitors,alltheelementtypesthatgetvisited
• JDK– nonethatI’mawareof
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VisitorIllustration(1/3)public interface Expression {
public <T> T accept(Visitor<T> v); // No eval or toString! }
public class UnaryOperationExpression implements Expression {public UnaryOperationExpression(
UnaryOperator operator, Expression operand);public <T> T accept(Visitor<T> v) { return v.visitUnaryExpr(this); }
}public class BinaryOperationExpression implements Expression {
public BinaryOperationExpression(BinaryOperator operator,Expression operand1, Expression operand2);
public <T> T accept(Visitor<T> v) { return v.visitBinaryExpr(this) ; }}public class NumberExpression implements Expression {
public NumberExpression(double number);public <T> T accept(Visitor<T> v) { return v.visitNumberExpr(this); }
}
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VisitorIllustration(2/3)public interface Visitor<T> { // T is result type
public T visitUnaryExpr(UnaryExpression ue);public T visitBinaryExpr(BinaryExpression be);public T visitNumberExpr(NumberExpression ne);
}
public class EvalVisitor implements Visitor<Double> {public Double visitUnaryExpr(UnaryExpression ue) {
return ue.operator.apply(ue.operand.accept(this));}public Double visitBinaryExpr(BinaryExpression be) {
return be.operator.apply(be.operand1.accept(this),be.operand2.accept(this));
}public Double visitNumberExpr(NumberExpression ne) { return ne.number; }
}
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VisitorIllustration(3/3)public class ToStringVisitor implements Visitor<String> {
public String visitUnaryExpr(UnaryExpression ue) {return ue.operator + ue.operand.accept(this);
}public String visitBinaryExpr(BinaryExpression be) {
return String.format("(%s %s %s)", be.operand1.accept(this),be.operator, be.operand2.accept(this));
}public String visitNumberExpr(NumberExpression ne) {
return Double.toString(ne.number);}
}
// Sample use of visitorsSystem.out.println(e.accept(new ToStringVisitor()) + " = " +
e.accept(new EvalVisitor()));
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MoreonVisitor
• VisitorisNOTmerelytraversingagraphstructureandapplyingamethod– That’sIterator!
• Theessenceofvisitorisdouble-dispatch– FirstdynamicallydispatchontheVisitor– Thenontheelementbeingvisited
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Summary
• Nowyouknowall theGangofFourpatterns• Definitionscanbevague• Coverageisincomplete• Butthey’reextremelyvaluable– Theygaveusavocabulary– Andawayofthinkingaboutsoftware
• Lookforpatternsasyoureadandwritesoftware– GoF,non-GoF,andundiscovered
