introduction to software technology 5. design...
TRANSCRIPT
IntroductiontoSoftwareTechnology5.DesignPatterns
KlausOstermann
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TopicsofthisLecture
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} DesignPatterns} Template} Strategy} Bridge} Decorator
} ThesedesignpatternsarelessgeneralthantheGRASPpatterns} Theyfocusonspecificdesignproblems
} ThesearesomeofthemostcommonandmostimportantclassicaldesignpatternsinOOdesign
TemplateMethodPattern
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} Thegoalistoseparate...} policiesfromdetailedmechanisms.} invariantandvariantparts.
} Abstractclasses...} defineinterfaces.} implementhigh-levelpolicies.
} Controlsub-classextensions.} Avoidcodeduplication.} TheTemplateMethodPatternisatthecoreofthedesignofobject-orientedframeworks.
Using the Template Method Pattern for Bubble-Sort
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public abstract class BubbleSorter{ protected int length = 0; protected void sort() { if(length <= 1) return; for(int nextToLast= length-2; nextToLast>= 0; nextToLast--) for(int index = 0; index <= nextToLast; index++) if(outOfOrder(index)) swap(index); } protected abstract void swap(int index); protected abstract boolean outOfOrder(int index);
}
IntBubbleSorter
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public class IntBubbleSorter extends BubbleSorter{ private int[] array = null; public void sort(int[] theArray) { array = theArray; length = array.length; super.sort(); } protected void swap(int index) { int temp = array[index]; array[index] = array[index+ 1]; array[index+1] = temp; } protected boolean outOfOrder(int index) { return(array[index] > array[index+ 1]); }
}
Discussion
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Discussion
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} Templatemethodforcesdetailedimplementationstoextendthetemplateclass.
} Detailedimplementationdependonthetemplate.} Cannotre-usedetailedimplementations‘functionality.(E.g.,swapandout-of-orderaregenerallyuseful.)
} Ifwewanttore-usethehandlingofintegerarrayswithothersortingstrategieswemustremovethedependency} thisleadsustotheStrategyPattern.
StrategyPattern
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} Intent} Defineafamilyofalgorithms,encapsulateeachone,andmaketheminterchangeable.Strategyletsthealgorithmvaryindependentlyfromclientsthatuseit.
} ComparisonWithTemplate} Usingthestrategypattern,both-thetemplateandthedetailedimplementations-dependonabstractions.
} BasicStructure
StrategyPattern:Example
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Strategy:GeneralStructure
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Defineafamilyofalgorithms,encapsulateeachone,andmaketheminterchangeable
StrategyPattern:Discussion
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} Useifmanyrelatedclassesdifferonlyintheirbehaviorratherthanimplementingdifferentrelatedabstractions.} Strategiesallowtoconfigureaclasswithoneofmanybehaviors.
} Useifyouneeddifferentvariantsofanalgorithm.} Strategiescanbeusedwhenvariantsofalgorithmsareimplementedasaclasshierarchy.
} Useifaclassdefinesmanybehaviorsthatappearasmultipleconditionalstatementsinitsoperations.} Moverelatedconditionalbranchesintoastrategy
StrategyvsSubclassing
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} Sub-classingContextmixesalgorithm‘simplementationwiththatofContext.Contexthardertounderstand,maintain,extend.
} Whenusingsub-classingwecan'tvarythealgorithmdynamically.
} Sub-classingresultsinmanyrelatedclasses.Onlydifferinthealgorithmorbehaviortheyemploy.
} EncapsulatingthealgorithminStrategy...} letsyouvarythealgorithmindependentlyofitscontext.} makesiteasiertoswitch,understand,andextendthealgorithm.
PassingContextInformationtoStrategy
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} TheStrategyinterfaceissharedbyallconcreteStrategyclasseswhetherthealgorithmstheyimplementaretrivialorcomplex.
} Someconcretestrategieswon'tusealltheinformationpassedtothem} Simpleconcretestrategiesmayusenoneofit.} Contextcreates/initializesparametersthatnevergetused.
} IfthisisanissueuseatightercouplingbetweenStrategyandContext;letStrategyknowaboutContext
PassingContextInformationtoStrategy
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} Twopossiblestrategies:} Passtheneededinformationasaparameter.
} ContextandStrategydecoupled} Communicationoverhead} Algorithmcan’tbeadaptedtospecificneedsofcontext
} ContextpassesitselfasaparameterorStrategyhasareferencetoitsContext.} Reducedcommunicationoverhead} Contextmustdefineamoreelaborateinterfacetoitsdata} ClosercouplingofStrategyandContext.} Avoidclosercouplingbydefininganexplicitinterfaceforretrievingcontext,whichisimplementedbythecontext
BridgePattern:Motivation
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Wewanttosupportmultipleoperatingsystems...
Wewanttoprovidedifferenttypesofwindows...
BridgePattern
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} Whichalternativewouldbebetterrepresentedusinginheritance?
} Whattechniquecanweusetoprovidebothtypesofclassifications?
BridgePattern
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} Intent} Decoupleanabstractionfromitsimplementationsothatthetwocanvaryindependently.
} Structure
BridgePattern:Example
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} Byencapsulatingtheconceptthatvarieswecanavoidproblemswithinheritanceconflicts.
} ThisisverysimilartothetechniqueusedintheStrategypattern
BridgePattern:Discussion
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} Decouplinginterfaceandimplementation:} Implementationcanbeconfiguredatrun-time.} Implementationbeingusedishiddeninsideabstraction.
} ImprovedextensibilityAbstractionandImplementerhierarchiescanbeextendedindependently.
} Issues} Mostlanguagesdonotsupportparallelhierarchiesverywell
} Typesafetyproblems
DecoratorPattern
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} Intent} Weneedtoaddresponsibilitiestoexistingindividualobjects} …dynamicallyandtransparently,withoutaffectingotherobjects.} …responsibilitiescanbewithdrawndynamically.
} Problem:Extensionbysubclassingisnotpractical:} Largenumberofindependentextensionsarepossible.} Wouldproduceanexplosionofsubclassestosupportevery
combination.} Nosupportfordynamicadaptation.} Aclassdefinitionmaybehiddenorotherwiseunavailablefor
subclassing} Cannotchangeallconstructorcallstotheclasswhoseobjectareto
beextended
LimitationsofInheritance:Example
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Evolution:AddingfunctionalitytoaByteArrayInputStreamtoreadwholesentencesandnotjustsinglebytes.
LimitationsofInheritance:Example
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Evolution:WealsowanttohavethepossibilitytoreadwholesentencesusingFileInputStreams...
Afterthen-thiteration…
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} Problems:} …anewclassforeachresponsibility.} responsibilitymixfixedstatically.
(HowaboutPipedDataBufferedInputStream?)} non-reusableextensions;codeduplication;} maintenancenightmare:exponentialgrowthofnumberofclasses
MultipleInheritanceisnoSolutionEither
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} staticresponsibilitymix} namingconflicts} hardtodispatchsupercallscorrectly
“Multipleinheritanceisgood,butthereisnogoodwaytodoit.”
A.SYNDER
StructureoftheDecoratorPattern
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Intent:Weneedtoaddresponsibilitiestoexistingindividualobjectsdynamicallyandtransparently,withoutaffectingotherobjects.
Example:Decoratorinjava.io
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} java.ioabstractsvariousdatasourcesanddestinations,aswellasprocessingalgorithms:} Programsoperateonstreamobjects...} independentlyofultimatedatasource/destination/shapeofdata.
DecoratorPattern:Discussion
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} Decoratorenablesmoreflexibilitythaninheritance:} Responsibilitiescanbeadded/removedatrun-time.} DifferentDecoratorclassesforaspecificComponentclassenabletomixandmatchresponsibilities.
} Easytoaddaresponsibilitytwice;e.g.,foradoubleborder,attachtwoBorderDecorators
} Decoratoravoidsincoherentclasses:} feature-ladenclasseshighupinthehierarchypay-as-you-goapproach:don'tbloat,butextendusingfine-grainedDecoratorclasses} functionalitycanbecomposedfromsimplepieces.} anapplicationdoesnotneedtopayforfeaturesitdoesn'tuse.
Decorator:Problems
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} Lotsoflittleobjects} AdesignthatusesDecoratoroftenresultsinsystemscomposedoflotsoflittleobjectsthatalllookalike.
} Objectsdifferonlyinthewaytheyareinterconnected,notintheirclassorinthevalueoftheirvariables.Imagineaclasstodrawaborderaroundacomponent..
} Suchsystemsareeasytocustomizebythosewhounderstandthem,butcanbehardtolearnanddebug.
} Objectidentity} Adecoratoranditscomponentaren'tidentical.Fromanobjectidentitypointofview,adecoratedcomponentisnotidenticaltothecomponentitself.
} Youshouldn'trelyonobjectidentitywhenyouusedecorators
Example:Streamsinjava.io
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} AstreamisnormallyaddressedviatheoutermostDecorator.} Sometimes,areferencetooneoftheinternalobjectsismaintainedandoperatedon} operationshouldn’tincludeactualreadsorwrites} goodstyle:allread()operationsareperformedonlytothehead
decoratorinthecompositestreamobject
} Readingfromaninternalobjectbreakstheillusionofasingleobjectaccessedviaasinglereference,andmakesthecodemoredifficulttounderstand.
FileInputStream fin = new FileInputStream(“a.txt”); BufferedInputStream din = new BufferedInputStream(fin); fin.read();
Decorator:Problems
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} Nolatebinding} DELEGATIONVERSUSFORWARDSEMANTICS
Forwardingwithbindingofthistomethodholder;"ask"anobjecttodosomethingonitsown.
Bindingofthistomessagereceiver:“ask”anobjecttodosomethingonbehalfofthemessagereceiver.
NoLateBinding:Example
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Decorator:Problems
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} Needtoimplementforwardingmethodsforthosemethodsnotrelevanttothedecorator} Alotofrepetitiveprogrammingwork} Amaintenanceproblem:Whatifthedecoratedclasschanges
} Addingnewmethodsorremovingmethodsthatareirrelevanttothedecorators
} Decoratorclassesneedtochangeaswell} Thisisavariantoftheso-called“fragilebaseclassproblem”
Decorator:Issues
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} Keepthecommonclass(Component)lightweight:} itshouldfocusondefininganinterface(e.g.implementedasinterface).
} deferdefiningdatarepresentationtosubclasses.} otherwisethecomplexityofComponentmightmakethedecoratorstooheavyweighttouseinquantity.
} PuttingalotoffunctionalityintoComponentmakesitlikelythatsubclasseswillpayforfeaturestheydon'tneed.
} Theseissuesrequirepre-planning.} Difficulttoapplydecoratorpatternto3rd-partycomponentclass.
Literature
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} A.Shalloway,J.R.Trott.DesignPatternsExplained.Addison-Wesley,2005.} Chap.9,14,15,18