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ZYBS网上书店英文文献.doc

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ZYBS网上书店英文文献.doc

WhatIsanObjectObjectsarekeytounderstandingobjectorientedtechnology.Youcanlookaroundyounowandseemanyexamplesofrealworldobjectsyourdog,yourdesk,yourtelevisionset,yourbicycle.RealworldobjectssharetwocharacteristicsTheyallhavestateandbehavior.Forexample,dogshavestatename,color,breed,hungryandbehaviorbarking,fetching,waggingtail.Bicycleshavestatecurrentgear,currentpedalcadence,twowheels,numberofgearsandbehaviorbraking,accelerating,slowingdown,changinggears.Softwareobjectsaremodeledafterrealworldobjectsinthattheytoohavestateandbehavior.Asoftwareobjectmaintainsitsstateinoneormorevariables.Avariableisanitemofdatanamedbyanidentifier.Asoftwareobjectimplementsitsbehaviorwithmethods.Amethodisafunctionsubroutineassociatedwithanobject.DefinitionAnobjectisasoftwarebundleofvariablesandrelatedmethods.Youcanrepresentrealworldobjectsbyusingsoftwareobjects.Youmightwanttorepresentrealworlddogsassoftwareobjectsinananimationprogramorarealworldbicycleasasoftwareobjectintheprogramthatcontrolsanelectronicexercisebike.Youcanalsousesoftwareobjectstomodelabstractconcepts.Forexample,aneventisacommonobjectusedinwindowsystemstorepresenttheactionofauserpressingamousebuttonorakeyonthekeyboard.Thefollowingillustrationisacommonvisualrepresentationofasoftwareobject.Asoftwareobject.Everythingthesoftwareobjectknowsstateandcandobehaviorisexpressedbythevariablesandthemethodswithinthatobject.AsoftwareobjectthatmodelsyourrealworldbicyclewouldhavevariablesthatindicatethebicyclescurrentstateItsspeedis18mph,itspedalcadenceis90rpm,anditscurrentgearis5th.Thesevariablesareformallyknownasinstancevariablesbecausetheycontainthestateforaparticularbicycleobjectinobjectorientedterminology,aparticularobjectiscalledaninstance.Thefollowingfigureillustratesabicyclemodeledasasoftwareobject.Abicyclemodeledasasoftwareobject.Inadditiontoitsvariables,thesoftwarebicyclewouldalsohavemethodstobrake,changethepedalcadence,andchangegears.Itwouldnothaveamethodforchangingitsspeedbecausethebikesspeedisjustasideeffectofwhichgearitsinandhowfasttheriderispedaling.Thesemethodsareknownformallyasinstancemethodsbecausetheyinspectorchangethestateofaparticularbicycleinstance.Objectdiagramsshowthatanobjectsvariablesmakeupthecenter,ornucleus,oftheobject.Methodssurroundandhidetheobjectsnucleusfromotherobjectsintheprogram.Packaginganobjectsvariableswithintheprotectivecustodyofitsmethodsiscalledencapsulation.Thisconceptualpictureofanobjectanucleusofvariablespackagedwithinaprotectivemembraneofmethodsisanidealrepresentationofanobjectandistheidealthatdesignersofobjectorientedsystemsstrivefor.However,itsnotthewholestory.Often,forpracticalreasons,anobjectmayexposesomeofitsvariablesorhidesomeofitsmethods.IntheJavaprogramminglanguage,anobjectcanspecifyoneoffouraccesslevelsforeachofitsvariablesandmethods.Theaccessleveldetermineswhichotherobjectsandclassescanaccessthatvariableormethod.RefertotheControllingAccesstoMembersofaClasssectionfordetails.EncapsulatingrelatedvariablesandmethodsintoaneatsoftwarebundleisasimpleyetpowerfulideathatprovidestwoprimarybenefitstosoftwaredevelopersModularityThesourcecodeforanobjectcanbewrittenandmaintainedindependentlyofthesourcecodeforotherobjects.Also,anobjectcanbeeasilypassedaroundinthesystem.Youcangiveyourbicycletosomeoneelse,anditwillstillwork.InformationhidingAnobjecthasapublicinterfacethatotherobjectscanusetocommunicatewithit.Theobjectcanmaintainprivateinformationandmethodsthatcanbechangedatanytimewithoutaffectingotherobjectsthatdependonit.Youdontneedtounderstandabikesgearmechanismtouseit.WhatIsaMessageAsingleobjectalonegenerallyisnotveryuseful.Instead,anobjectusuallyappearsasacomponentofalargerprogramorapplicationthatcontainsmanyotherobjects.Throughtheinteractionoftheseobjects,programmersachievehigherorderfunctionalityandmorecomplexbehavior.Yourbicyclehangingfromahookinthegarageisjustabunchofmetalandrubberbyitself,itisincapableofanyactivitythebicycleisusefulonlywhenanotherobjectyouinteractswithitbypedaling.Softwareobjectsinteractandcommunicatewitheachotherbysendingmessagestoeachother.WhenobjectAwantsobjectBtoperformoneofBsmethods,objectAsendsamessagetoobjectBseethefollowingfigure.Objectsinteractbysendingeachothermessages.Sometimes,thereceivingobjectneedsmoreinformationsothatitknowsexactlywhattodoforexample,whenyouwanttochangegearsonyourbicycle,youhavetoindicatewhichgearyouwant.Thisinformationispassedalongwiththemessageasparameters.Messagesuseparameterstopassalongextrainformationthattheobjectneedsinthiscase,whichgearthebicycleshouldbein.Thesethreepartsareenoughinformationforthereceivingobjecttoperformthedesiredmethod.Nootherinformationorcontextisrequired.MessagesprovidetwoimportantbenefitsAnobjectsbehaviorisexpressedthroughitsmethods,soasidefromdirectvariableaccessmessagepassingsupportsallpossibleinteractionsbetweenobjects.

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