南京大学-软件工程-15 面向对象信息隐藏

《计算与软件工程II》

Ch15“面向对象”的信息隐藏丁二玉南京大学软件学院InformationHidingEachmodulehidestheimplementationofanimportantdesigndecision(secrets)sothatonlytheconstituentsofthatmoduleknowthedetails3DesignSecretsneedtohide…PrimarySecret:ResponsibilityChangeHiddeninformationthatwasspecifiedtothesoftwaredesignerFromSRSSecondarySecret:ImplementationChangeTheimplementationdecisionsmadebythedesignerwhenimplementingthemoduledesignedtohidetheprimarysecret变化；MainContentsEncapsulationWhatshouldbehiding?Howtohidingchange?Encapsulation(1)EncapsulationallowstheprogrammertogroupdataandthesubroutinesthatoperateonthemtogetherinoneplaceResponsibilityEncapsulation(2)hideirrelevantdetailsfromtheuseraclasscanbedividedintotwoparts,theuserinterfaceandtheimplementation.Theinterfaceisthevisiblesurfaceofthecapsule.describestheessentialcharacteristicsofobjectsoftheclasswhicharevisibletotheexteriorworldTheimplementationishiddeninthecapsule.Theimplementationhidingmeansthatdatacanonlybemanipulated,thatisupdated,withintheclass,butitdoesnotmeanhidinginterfacedata. NewViewofEncapsulationOld/Beginner/Implementationviewofencapsulation:

hidingdatainsideanobjectNewview:hidinganything,including:Data(implementation)Structure(implementation)Otherobject(implementation)Type(derivedclasses)Change/vary(designdetails)…EncapsulationCorrectly——ADTADT=AbstractDataTypeAconcept,notanimplementationAsetof(homogeneous)objectstogetherwithasetofoperationsonthoseobjectsNomentionofhowtheoperationsareimplementedEncapsulation=dataabstraction+typedataabstraction:groupdataandoperationType:hidingimplementation,makeusagecorrectlyWhytype?Atypemaybeviewedasasetofclothes(orasuitofarmor)thatprotectsanunderlyinguntypedrepresentationfromarbitraryorunintendeduse.Itprovidesaprotectivecoveringthathidestheunderlyingrepresentationandconstrainsthewayobjectsmayinteractwithotherobjects.Inanuntypedsystemuntypedobjectsarenakedinthattheunderlyingrepresentationisexposedforalltosee.EncapsulateDataIfneeded,useAccessorsandMutators,NotPublicMembers

AccessorsandMutatorsismeaningfulbehaviorConstraints,transformation,format…publicvoidsetSpeed(doublenewSpeed){ if(newSpeed<0){ sendErrorMessage(...); newSpeed=Math.abs(newSpeed); } speed=newSpeed;}EncapsulatestructuresSeechapter16IteratorPatternReferencesandCollectionData-Type!Encapsulateotherobjects

CollaborationDesignComposition;delegationEncapsulatetype(subclass)LSPpointerstosuperclassesorinterfaces;AllderivedclassesmustbesubstituteablefortheirbaseclassEncapsulateChange(orvary)

Identifytheaspectsofyourapplicationthatmaychange(orvary)andseparatethemfromwhatstaysthesame.Takethepartsthatchange(vary)andencapsulatethem,sothatlateryoucanalterorextendthepartsthatvarywithoutaffectingthepartsthatdon’t.SeeDIPandOCPLaterEncapsulateImplementationDetailDataStructureOtherobjectTypeChange/vary…Principle#1:MinimizeTheAccessibilityofClassesandMembersAbstractionAnabstractionfocusesontheoutsideviewofanobjectandseparatesanobject’sbehaviorfromitsimplementationEncapsulationClassesshouldnotexposetheirinternalimplementationdetailsMainContentsEncapsulationHowtohidingchange?ExampleofResponsibility

ChangeCopyReadKeyboardWritePrintervoidCopy(ReadKeyboard&r,WritePrinter&w){

intc;

while((c=r.read())!=EOF)

w.write(c);}WriteDiskvoidCopy(ReadKeyboard&r,WritePrinter&wp,WriteDisk&wd,OutputDevicedev){

intc;

while((c=r.read())!=EOF)

if(dev==printer)wp.write(c);elsewd.write(c);}Howto…AbstractionisKey...usingpolymorphicdependencies(calls)ExampleofResponsibility

ChangeDiskWriter::Write(c){ WriteDisk(c);}CopyKeyboardReaderPrinterWriterDiskWritervoidCopy(ReadKeyboard&r,WritePrinter&w){

intc;

while((c=r.read())!=EOF)

w.write(c);}Principle10:

Open/ClosedPrinciple(OCP)Softwareentitiesshouldbeopenforextension,butclosedformodificationB.Meyer,1988/quotedbyR.Martin,1996Beopenforextensionmodule'sbehaviorcanbeextendedBeclosedformodificationsourcecodeforthemodulemustnotbechanges统计数据表明，修正bug最为频繁，但是影响很小；新增需求数量一般，但造成了绝大多数影响Modulesshouldbe

writtensotheycanbeextended

without

requiringthemtobemodifiedOCPRTTIisuglyanddangerousRTTI=Run-TimeTypeInformationIfamoduletriestodynamicallycastabaseclasspointer

toseveralderivedclasses,anytimeyouextendthe

inheritancehierarchy,youneedtochangethemodulerecognizethembytypeswitchorif-else-ifstructuresRTTIisUglyandDangerous!//

RTTIviolating

the

//open-closed

principleandLSP

class

Shape

{}

class

Square

extends

Shape

{

void

drawSquare()

{

//

draw

}

}

class

Circle

extends

Shape

{

void

drawCircle()

{

//

draw

}

}

void

drawShapes(List<Shape>

shapes)

{

for

(Shape

shape

:

shapes)

{

if

(shapes

instanceof

Square)

{

((Square)

shapes).drawSquare();

}

else

if

(shape

instanceof

Circle)

{

((Circle)

shape).drawCircle();

}

}

}

//

AbstractionandPolymorphismthat

does

//not

violate

the

open-closed

principleandLSP

interface

Shape

{

void

draw();

}

class

Square

implements

Shape

{

void

draw()

{

//

draw

implementation

}

}

class

Circle

implements

Shape

{

void

draw()

{

//

draw

implementation

}

}

void

drawShapes(List<Shape>

shapes)

{

for

(Shape

shape

:

shapes)

{

shape.draw();

}

}

OCPSummaryUseabstractiontogainexplicitclosurePlanyourclassesbasedonwhatislikelytochange.minimizesfuturechangelocationsOCPneedsDIP&&LSPNosignificantprogramcanbe100%closed

R.Martin,“TheOpen-ClosedPrinciple,”1996Principle11:DependencyInversionPrinciple(DIP)I.High-levelmodulesshouldnot

dependonlow-levelmodules.Bothshoulddependonabstractions.II.Abstractionsshouldnotdependondetails.DetailsshoulddependonabstractionsR.Martin,1996DIP:separateinterfacefromimplementation——abstractUseinheritancetoavoiddirectbindingstoclasses:

Designtoaninterface,notanimplementation!ClientInterface

(abstractclass)Implementation

(concreteclass)DIPExampleCopyReaderWriterKeyboardReaderPrinterWriterDiskWriterclassReader{public:virtualintread()=0;};classWriter{public:virtualvoidwrite(int)=0;};voidCopy(Reader&r,Writer&w){intc;while((c=r.read())!=EOF)w.write(c);}DIPProceduralvs.OOArchitectureProceduralArchitectureObject-OrientedArchitectureDIPsummaryAbstractclasses/interfaces:tendtochangelessfrequentlyabstractionsare‘hingepoints’whereitis

easiertoextend/modifyshouldn’thavetomodifyclasses/interfaces

thatrepresenttheabstraction(OCP)ExceptionsSomeclassesareveryunlikelytochange;

thereforelittlebenefittoinserting

abstractionlayerExample:StringclassIncaseslikethiscanuseconcreteclass

directlyasinJavaorC++HowtodealwithchangeOCPstatesthegoal;DIPstatesthemechanism;LSPistheinsuranceforDIPExampleofImplementationChangeDuckquack()swim()display()//otherduck-likemethods…MallardDuckdisplay(){//lookslikeamallard}RedHeadDuckdisplay(){//lookslikea