南京大學(xué)-軟件工程-14面向?qū)ο竽K化課件

《計算與軟件工程II》

Ch14“面向?qū)ο蟆钡哪K化ModuleApieceofcodeMethodsClassModule(package)Coupling:amongpiecesCohesion:internalapieceWhat’sthedifferencebetweenStructuralmethodsandOOmethodsCouplingCouplingisthemeasureofthestrengthofassociationestablishedbyaconnection

fromonemoduletoanotherStructuralmethodsAconnectionisareferencetosomelabeloraddressdefinedelsewhereOOmethodsInteractioncouplingComponentcouplingInheritancecouplingMainContentsCouplingofOOInteractionCouplingComponentCouplingInheritCouplingCouplingMetricofOOCohesionofOOSingleinteractioncouplingMethodInvokeorAttributeAccessbetweentwoclassMostsimilartotheclassicaldefinitionofcouplingbetweenmodulesNotincludinginheritanceParameterandAttributenotincludingClassTypeStrengthofSingleinteractioncouplingPrinciplesofinteractioncouplingPrinciplesfromModularization1:《GlobalVariablesConsiderHarmful》2:《TobeExplicit》3:《DonotRepeat》4:ProgrammingtoInterfaceMainContentsCouplingofOOInteractionCouplingComponentCouplingInheritCouplingCouplingMetricofOOCohesionofOOComponentcouplingAbstractionDefineoneplace,usingmanyplaceClass-Level,Object-LevelComponentcouplingThecomponentrelationshipbetweenclassesisdefinedbytheuseofaclassasdomainofsomeinstancevariableofanotherclassNotincludingInheritanceUsuallyClass-LevelneededAttributeType:howtoexplicitFourKindsofComponentcouplingWholevariable:AggregationParameter:MethodParameterCreator:Creatorinsomemethod’slocalHidden:GivenbyanotherobjectHiddenComponentcouplingTheworstcoupling:ImplicitThecouplingbetweentwoclassesCandC’

isratedhiddenifC’

showsupneitherinthespecificationnorintheimplementationofCalthoughanobjectofC’

isusedintheimplementationofamethodofCAsimilarproblemisencounteredifthereturnvalueofamethodinvocationisimmediatelyusedasinputparameterinanothermethodinvocationdisallowtheuseofcascadingmessagesTobeexplicit!HiddenComponentcouplingTheLawofDemeterExamplePrinciplesofComponentCouplingPrinciple5:TheLawofDemeterYoucanplaywithyourself.Youcanplaywithyourowntoys,butyoucan'ttakethemapartYoucanplaywithtoysthatweregiventoyou.Youcanplaywithtoysyou'vemadeyourself.ScatteredComponentCouplingWeratetwoclassesCandC’

asscatteredcoupledifC’

isusedasdomaininthedefinitionofsomelocalvariableorinstancevariableintheimplementationofCyetC’

isnotincludedinthespecificationofCCommonandacceptableAggregation:GlobalLocalinstance:LocalWhichisbetter?ManyConnectionsVSStrongerSingleConnectionSpecifiedComponentCouplingWeratetwoclassesCandC’

asspecifiedcoupledifC’

isincludedinthespecificationofCwheneveritisacomponentofCGetSpecified,DesignbyContract——SufferedInterfaceVSRequiredInterfaceSufferedInterface:SpecifiedRequiredInterface:UsedScatteredComponentCouplingcanbeimprovedtoSpecifiedcomponentcouplingwithcommentsPrinciplesofComponentCouplingPrinciple4:ProgrammingtoInterfaceProgrammingtoRequiredInterface,notonlySufferedInterfaceDesignbyContractContractofModule/ClassRequiredmothods/ProvidedmethodsContractofMethodsPreCondition,PostCondition,InvariantPrinciplesofComponentCoupling

Principles6:InterfaceSegregationPrinciple(ISP)ProgrammingtoSimplerInterfaceManyclient-specificinterfacesarebetter

thanonegeneralpurposeinterfaceClientsshouldnotbeforcedtodependuponinterfacesthattheydonotuse.R.Martin,1996PrinciplesofComponentCoupling

——ISPExplainedMultipurposeclassesMethodsfallindifferentgroupsNotallusersuseallmethodsCanleadtounwanteddependenciesClientsusingoneaspectofaclassalsodependindirectlyonthedependenciesoftheotheraspectsISPhelpstosolvetheproblemUseseveralclient-specificinterfacesPrinciplesofComponentCoupling

——ISPExample:UIsUIsareisolatedfromeachotherCanaddaUIwithchangesinServer→otherUIsnotaffectedTheServer"collects"interfacesNewUI→ServerinterfacechangesAllotherUIsrecompileMainContentsCouplingofOOInteractionCouplingComponentCouplingInheritCouplingCouplingMetricofOOCohesionofOOInheritanceCouplingParentinformationisspecifiedforchildrenWithaparentreference,howmanyinformationaclientneededwheninteracting?ModificationRefinementExtensionModificationInheritanceCouplingModifyingwithoutanyrulesandrestrictsWorstInheritanceCouplingIfaclientusingaparentref,theparentandchildmethodareallneededImplicitTherearetwoconnections,morecomplexHarmtopolymorphismRefinementInheritanceCouplingdefiningnewinformationtheinheritedinformationisonlychangedduetopredefinedrulesIfaclientusingaparentref,thewholeparentandrefinementofchildareneeded1+connectionsNecessary!ExtensionInheritanceCoupling

thesubclassonlyaddsmethodsandinstancevariablesbutneithermodifiesnorrefinesanyoftheinheritedonesIfaclientusingaparentref,onlytheparentisneeded1connectionHowInheritancereducecoupling?Remember:inRefinementandExtensioninheritancecoupling,theinteractioncouplingbetweensuper-classandsubclassisignoredPrinciplesofInheritCoupling

AllderivedclassesmustbesubstituteablefortheirbaseclassBarbaraLiskov,1988Functionsthatusepointersorreferencestobaseclassesmustbeabletouseobjectsofderivedclasseswithoutknowingit.R.Martin,1996Principle7:LiskovSubstitutionPrinciple(LSP)LSP:

exampleLSP:

exampleInheritanceAppearsSimple

IsaSquareaRectangle?

Rectr=newRect();setWidth=4;setHeight=5;assert(20==getArea());classSquareextendsRect{//Squareinvariant,height=widthsetWidth(x){setHeight()=x}setHeight(x){setWidth(x)}}//violateLSP?InheritanceAppearsSimpleclassBird{

//hasbeak,wings,...

public:virtualvoidfly();

//Birdcanfly};classParrot

:publicBird{//Parrotisabird

public:virtualvoidmimic();

//CanRepeat

words...};classPenguin:publicBird{

public:voidfly(){

error(“Penguinsdon’tfly!”);}};PenguinsFailtoFly!voidPlayWithBird(Bird&abird)

{

abird.fly();

//OKifParrot.

//ifbirdhappenstobePenguin...OOOPS!!}Doesnotmodel:

“Penguinscan’tfly”Itmodels

“Penguinsmayfly,butiftheytryitiserror”

Run-timeerrorifattempttoflynotdesirable

ThinkaboutSubstitutability-FailsLSPLSPSummaryLSPisaboutSemanticsandReplacementUnderstandbeforeyoudesignThemeaningandpurposeofeverymethodandclassmustbeclearlydocumentedLackofuserunderstandingwillinducedefactoviolationsofLSPReplaceabilityiscrucialWheneveranyclassisreferencedbyanycodeinanysystem, anyfutureorexistingsubclassesofthatclassmustbe100%replaceableLSPSummaryDesignbyContractAdvertisedBehaviorofanobject:advertisedRequirements(Preconditions)advertisedPromises(Postconditions)Whenredefiningamethodinaderivateclass,youmayonlyreplaceitspreconditionbyaweakerone,anditspostconditionbyastrongeroneB.Meyer,1988Derivedclassservices

shouldrequireno

moreandpromisenoless

Principle8:FavorCompositionOverInheritanceUseinheritforpolymorphismUsedelegatenotinherittoreusecode!Coad'sRulesofUsingInheritanceUseinheritanceonlywhenallofthefollowingcriteriaaresatisfied:Asubclassexpresses"isaspecialkindof"andnot"isaroleplayedbya"AninstanceofasubclassneverneedstobecomeanobjectofanotherclassAsubclassextends,ratherthanoverridesornullifies,theresponsibilitiesofitssuperclassAsubclassdoesnotextendthecapabilitiesofwhatismerelyanutilityclassInheritance/CompositionExample1Inheritance/CompositionExample1(Continued)Inheritance/CompositionExample2classObject{ public:virtualvoidupdate(){}; virtualvoiddraw(){}; virtualvoidcollide(Objectobjects[]){};};classVisible:publicObject{public:virtualvoiddraw(){ /*drawmodelatpositionofthisobject*/};private:Model*model;};classSolid:publicObject{public:virtualvoidcollide(Objectobjects[]){ /*checkandreacttocollisionswithobjects*/};};classMovable:publicObject{ public:virtualvoidupdate(){ /*updateposition*/};};MainContentsCouplingofOOInteractionCouplingComponentCouplingInheritCouplingCouplingMetricofOOCohesionofOOCouplingMetricsbetweenclassesCouplingbetweenobjectclasses(CBO)Acountofthenumberofotherclasses:whichaccessamethodorvariableinthisclass,orcontainamethodorvariableaccessedbythisclassNotincludingInheritanceWanttokeepthislowCouplingMetricsbetweenclassesDataabstractioncoupling(DAC)ThenumberofattributehavinganADTtypedependentonthedefinitionsofotherclassesWanttokeepthislowCouplingMetricsbetweenclassesCeand&D(efferentandafferentcoupling)Ca:Thenumberofclassesoutsidethiscategorythatdependuponclasseswithinthiscategory.Ce:ThenumberofclassesinsidethiscategorythatdependuponclassesoutsidethiscategoryWanttokeeptheselowCouplingMetricsbetweenclassesDepthoftheInheritancetree(DIT)themaximumlengthfromthenodetotherootofthetreeasDITgrows,itbecomesdifficulttopredictbehaviorofaclassbecauseofthehighdegreeofinheritancePositively,largeDITvaluesimplythatmanymethodsmaybereusedCouplingMetricsbetweenclassesNumberofchildren(NOC)countofthesubclassesimmediatelysubordinatetoaclassasNOCgrows,reuseincreasesasNOCgrows,abstractioncanbecomedilutedincreaseinNOCmeanstheamountoftestingwillincreaseMainContentsCouplingofOOInteractionCouplingComponentCouplingInheritCouplingCouplingMetricofOOCohesionofOOCohesionofAttributesSeparablerepresentmultipleunrelateddataabstractionscombinedinoneobjectmultifacetedrepresentmultiplerelateddataabstractionsNon-delegatedSomeattributerepresentapartofanotherclass:notinthirdnormalformConcealedsomeattributeandreferencingmethodswhichmayberegardedasaclassofitsownModel:informationalstrengththeclassrepresentsasinglesemanticallymeaningfulconceptualCohesionofmethodsMethodsofaClassareCommoncouplingAllmethodsserveOneResponsibilityInformationalCohesionRelativefunctions(functionalCohesion)Principle9:SingleResponsibilityPrincipleSingl