南京大学-软件工程-14面向对象模块化课件

《计算与软件工程II》

Ch14“面向对象”的模块化《计算与软件工程II》

Ch14“面向对象”的模块化1ModuleApieceofcodeMethodsClassModule(package)Coupling:amongpiecesCohesion:internalapieceModuleApieceofcode2What’sthedifferencebetweenStructuralmethodsandOOmethodsCouplingCouplingisthemeasureofthestrengthofassociationestablishedbyaconnection

fromonemoduletoanotherStructuralmethodsAconnectionisareferencetosomelabeloraddressdefinedelsewhereOOmethodsInteractioncouplingComponentcouplingInheritancecouplingWhat’sthedifferencebetween3MainContentsCouplingofOOInteractionCouplingComponentCouplingInheritCouplingCouplingMetricofOOCohesionofOOMainContentsCouplingofOO4SingleinteractioncouplingMethodInvokeorAttributeAccessbetweentwoclassMostsimilartotheclassicaldefinitionofcouplingbetweenmodulesNotincludinginheritanceParameterandAttributenotincludingClassTypeSingleinteractioncouplingMet5StrengthofSingleinteractioncouplingStrengthofSingleinteraction6PrinciplesofinteractioncouplingPrinciplesfromModularization1:《GlobalVariablesConsiderHarmful》2:《TobeExplicit》3:《DonotRepeat》4:ProgrammingtoInterfacePrinciplesofinteractioncoup7MainContentsCouplingofOOInteractionCouplingComponentCouplingInheritCouplingCouplingMetricofOOCohesionofOOMainContentsCouplingofOO8ComponentcouplingAbstractionDefineoneplace,usingmanyplaceClass-Level,Object-LevelComponentcouplingThecomponentrelationshipbetweenclassesisdefinedbytheuseofaclassasdomainofsomeinstancevariableofanotherclassNotincludingInheritanceUsuallyClass-LevelneededAttributeType:howtoexplicitComponentcouplingAbstraction9FourKindsofComponentcouplingWholevariable:AggregationParameter:MethodParameterCreator:Creatorinsomemethod’slocalHidden:GivenbyanotherobjectFourKindsofComponentcoupli10HiddenComponentcouplingTheworstcoupling:ImplicitThecouplingbetweentwoclassesCandC’

isratedhiddenifC’

showsupneitherinthespecificationnorintheimplementationofCalthoughanobjectofC’

isusedintheimplementationofamethodofCAsimilarproblemisencounteredifthereturnvalueofamethodinvocationisimmediatelyusedasinputparameterinanothermethodinvocationdisallowtheuseofcascadingmessagesTobeexplicit!HiddenComponentcouplingThew11HiddenComponentcouplingHiddenComponentcoupling12TheLawofDemeterExampleTheLawofDemeterExample13PrinciplesofComponentCouplingPrinciple5:TheLawofDemeterYoucanplaywithyourself.Youcanplaywithyourowntoys,butyoucan'ttakethemapartYoucanplaywithtoysthatweregiventoyou.Youcanplaywithtoysyou'vemadeyourself.PrinciplesofComponentCoupli14南京大学-软件工程-14面向对象模块化课件15ScatteredComponentCouplingWeratetwoclassesCandC’

asscatteredcoupledifC’

isusedasdomaininthedefinitionofsomelocalvariableorinstancevariableintheimplementationofCyetC’

isnotincludedinthespecificationofCCommonandacceptableAggregation:GlobalLocalinstance:LocalWhichisbetter?ManyConnectionsVSStrongerSingleConnectionScatteredComponentCouplingWe16SpecifiedComponentCouplingWeratetwoclassesCandC’

asspecifiedcoupledifC’

isincludedinthespecificationofCwheneveritisacomponentofCGetSpecified,DesignbyContract——SufferedInterfaceVSRequiredInterfaceSufferedInterface:SpecifiedRequiredInterface:UsedScatteredComponentCouplingcanbeimprovedtoSpecifiedcomponentcouplingwithcommentsSpecifiedComponentCouplingWe17PrinciplesofComponentCouplingPrinciple4:ProgrammingtoInterfaceProgrammingtoRequiredInterface,notonlySufferedInterfaceDesignbyContractContractofModule/ClassRequiredmothods/ProvidedmethodsContractofMethodsPreCondition,PostCondition,InvariantPrinciplesofComponentCoupli18PrinciplesofComponentCoupling

Principles6:InterfaceSegregationPrinciple(ISP)ProgrammingtoSimplerInterfaceManyclient-specificinterfacesarebetter

thanonegeneralpurposeinterfaceClientsshouldnotbeforcedtodependuponinterfacesthattheydonotuse.R.Martin,1996PrinciplesofComponentCoupli19PrinciplesofComponentCoupling

——ISPExplainedMultipurposeclassesMethodsfallindifferentgroupsNotallusersuseallmethodsCanleadtounwanteddependenciesClientsusingoneaspectofaclassalsodependindirectlyonthedependenciesoftheotheraspectsISPhelpstosolvetheproblemUseseveralclient-specificinterfacesPrinciplesofComponentCoupli20PrinciplesofComponentCoupling

——ISPExample:UIsUIsareisolatedfromeachotherCanaddaUIwithchangesinServer→otherUIsnotaffectedTheServer"collects"interfacesNewUI→ServerinterfacechangesAllotherUIsrecompilePrinciplesofComponentCoupli21南京大学-软件工程-14面向对象模块化课件22MainContentsCouplingofOOInteractionCouplingComponentCouplingInheritCouplingCouplingMetricofOOCohesionofOOMainContentsCouplingofOO23InheritanceCouplingParentinformationisspecifiedforchildrenWithaparentreference,howmanyinformationaclientneededwheninteracting?ModificationRefinementExtensionInheritanceCouplingParentinf24ModificationInheritanceCouplingModifyingwithoutanyrulesandrestrictsWorstInheritanceCouplingIfaclientusingaparentref,theparentandchildmethodareallneededImplicitTherearetwoconnections,morecomplexHarmtopolymorphismModificationInheritanceCoupl25RefinementInheritanceCouplingdefiningnewinformationtheinheritedinformationisonlychangedduetopredefinedrulesIfaclientusingaparentref,thewholeparentandrefinementofchildareneeded1+connectionsNecessary!RefinementInheritanceCouplin26ExtensionInheritanceCoupling

thesubclassonlyaddsmethodsandinstancevariablesbutneithermodifiesnorrefinesanyoftheinheritedonesIfaclientusingaparentref,onlytheparentisneeded1connectionExtensionInheritanceCoupling27HowInheritancereducecoupling?Remember:inRefinementandExtensioninheritancecoupling,theinteractioncouplingbetweensuper-classandsubclassisignoredHowInheritancereducecouplin28PrinciplesofInheritCoupling

AllderivedclassesmustbesubstituteablefortheirbaseclassBarbaraLiskov,1988Functionsthatusepointersorreferencestobaseclassesmustbeabletouseobjectsofderivedclasseswithoutknowingit.R.Martin,1996Principle7:LiskovSubstitutionPrinciple(LSP)PrinciplesofInheritCoupling29LSP:

exampleLSP:example30LSP:

exampleLSP:example31InheritanceAppearsSimple

IsaSquareaRectangle?

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

Is32InheritanceAppearsSimpleclassBird{

//hasbeak,wings,...

public:virtualvoidfly();

//Birdcanfly};classParrot

:publicBird{//Parrotisabird

public:virtualvoidmimic();

//CanRepeat

words...};classPenguin:publicBird{

public:voidfly(){

error(“Penguinsdon’tfly!”);}};InheritanceAppearsSimpleclas33PenguinsFailtoFly!voidPlayWithBird(Bird&abird)

{

abird.fly();

//OKifParrot.

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

“Penguinscan’tfly”Itmodels

“Penguinsmayfly,butiftheytryitiserror”

Run-timeerrorifattempttofly

notdesirable

ThinkaboutSubstitutability-FailsLSPPenguinsFailtoFly!voidPlay34LSPSummaryLSPisaboutSemanticsandReplacementUnderstandbeforeyoudesignThemeaningandpurposeofeverymethodandclassmustbeclearlydocumentedLackofuserunderstandingwillinducedefactoviolationsofLSPReplaceabilityiscrucialWheneveranyclassisreferencedbyanycodeinanysystem, anyfutureorexistingsubclassesofthatclassmustbe100%replaceableLSPSummaryLSPisaboutSemant35LSPSummaryDesignbyContractAdvertisedBehaviorofanobject:advertisedRequirements(Preconditions)advertisedPromises(Postconditions)Whenredefiningamethodinaderivateclass,youmayonlyreplaceitspreconditionbyaweakerone,anditspostconditionbyastrongeroneB.Meyer,1988Derivedclassservices

shouldrequireno

moreandpromisenoless

LSPSummaryDesignbyContractW36南京大学-软件工程-14面向对象模块化课件37Principle8:FavorCompositionOverInheritanceUseinheritforpolymorphismUsedelegatenotinherittoreusecode!Principle8:FavorCompositio38Coad'sRulesofUsingInheritanceUseinheritanceonlywhenallofthefollowingcriteriaaresatisfied:Asubclassexpresses"isaspecialkindof"andnot"isaroleplayedbya"AninstanceofasubclassneverneedstobecomeanobjectofanotherclassAsubclassextends,ratherthanoverridesornullifies,theresponsibilitiesofitssuperclassAsubclassdoesnotextendthecapabilitiesofwhatismerelyanutilityclassCoad'sRulesofUsingInherita39Inheritance/CompositionExample1Inheritance/CompositionExampl40Inheritance/CompositionExample1(Continued)Inheritance/CompositionExampl41Inheritance/CompositionExample2Inheritance/CompositionExampl42classObject{ public:virtualvoidupdate(){}; virtualvoiddraw(){}; virtualvoidcollide(Objectobjects[]){};};classVisible:publicObject{public:virtualvoiddraw(){ /*drawmodelatpositionofthisobject*/};private:Model*model;};classSolid:publicObject{public:virtualvoidcollide(Objectobjects[]){ /*checkandreacttocollisionswithobjects*/};};classMovable:publicObject{ public:virtualvoidupdate(){ /*updateposition*/};};classObject{43MainContentsCouplingofOOInteractionCouplingComponentCouplingInheritCouplingCouplingMetricofOOCohesionofOOMainContentsCouplingofOO44CouplingMetricsbetweenclassesCouplingbetweenobjectclasses(CBO)Acountofthenumberofotherclasses:whichaccessamethodorvariableinthisclass,orcontainamethodorvariableaccessedbythisclassNotincludingInheritanceWanttokeepthislowCouplingMetricsbetweenclass45CouplingMetricsbetweenclassesDataabstractioncoupling(DAC)ThenumberofattributehavinganADTtypedependentonthedefinitionsofotherclassesWanttokeepthislowCouplingMetricsbetweenclass46CouplingMetricsbetweenclassesCeand&D(efferentandafferentcoupling)Ca:Thenumberofclassesoutsidethiscategorythatdependuponclasseswithinthiscategory.Ce:ThenumberofclassesinsidethiscategorythatdependuponclassesoutsidethiscategoryWanttokeeptheselowCouplingMetricsbetweenclass47CouplingMetricsbetweenclassesDepthoftheInheritancetree(DIT)themaximumlengthfromthenodetotherootofthetreeasDITgrows,itbecomesdifficulttopredictbehaviorofaclassbecauseofthehighdegreeofinheritancePositively,largeDITvaluesimplythatmanymethodsmaybereusedCouplingMetricsbetweenclass48CouplingMetricsbetweenclassesNumberofchildren(NOC)countofthesubclassesimmediatelysubordinatetoaclassasNOCgrows,reuseincreasesasNOCgrows,abstractioncanbecomedilutedincreaseinNOCmeanstheamountoftestingwillincreaseCouplingMetricsbetweenclass49MainContentsCouplingofOOInteractionCouplingComponentCouplingInheritCouplingCouplingMetricofOOCohesionofOOMainContentsCouplingofOO50CohesionofAttributesSeparablerepresentmultipleunrelateddataabstractionscombinedinoneobjectmultifacetedrepresentmultiplerelateddataabstractionsNon-delegatedSomeattributerepresentapartofanotherclass:notinthirdnormalformConcealedsomeattributeandreferencingmethodswhichmayberegardedasaclassofitsownModel:informationalstrengththeclassrepresentsasinglesemanticallymeaningfulconceptualCohesionofAttributesSeparabl51CohesionofmethodsMethodsofaClassareCommoncouplingAllmethodsserveOneResponsibilityInformationalCohesionRelativefunctions(functionalCohesion)Principle9:SingleResponsibilityPrincipleCohesionofmethodsMethodsof52SingleResponsibilityPrinciple(SRP)Aclassshouldhaveonlyonereason