ZYBS網上書店英文文獻.doc

WhatIsanObject?Objectsarekeytounderstandingobject-orientedtechnology.Youcanlookaroundyounowandseemanyexamplesofreal-worldobjects:yourdog,yourdesk,yourtelevisionset,yourbicycle.Real-worldobjectssharetwocharacteristics:Theyallhavestateandbehavior.Forexample,dogshavestate(name,color,breed,hungry)andbehavior(barking,fetching,waggingtail).Bicycleshavestate(currentgear,currentpedalcadence,twowheels,numberofgears)andbehavior(braking,accelerating,slowingdown,changinggears).Softwareobjectsaremodeledafterreal-worldobjectsinthattheytoohavestateandbehavior.Asoftwareobjectmaintainsitsstateinoneormorevariables.Avariableisanitemofdatanamedbyanidentifier.Asoftwareobjectimplementsitsbehaviorwithmethods.Amethodisafunction(subroutine)associatedwithanobject.Definition:Anobjectisasoftwarebundleofvariablesandrelatedmethods.Youcanrepresentreal-worldobjectsbyusingsoftwareobjects.Youmightwanttorepresentreal-worlddogsassoftwareobjectsinananimationprogramorareal-worldbicycleasasoftwareobjectintheprogramthatcontrolsanelectronicexercisebike.Youcanalsousesoftwareobjectstomodelabstractconcepts.Forexample,aneventisacommonobjectusedinwindowsystemstorepresenttheactionofauserpressingamousebuttonorakeyonthekeyboard.Thefollowingillustrationisacommonvisualrepresentationofasoftwareobject.Asoftwareobject.Everythingthesoftwareobjectknows(state)andcando(behavior)isexpressedbythevariablesandthemethodswithinthatobject.Asoftwareobjectthatmodelsyourreal-worldbicyclewouldhavevariablesthatindicatethebicyclescurrentstate:Itsspeedis18mph,itspedalcadenceis90rpm,anditscurrentgearis5th.Thesevariablesareformallyknownasinstancevariablesbecausetheycontainthestateforaparticularbicycleobject；inobject-orientedterminology,aparticularobjectiscalledaninstance.Thefollowingfigureillustratesabicyclemodeledasasoftwareobject.Abicyclemodeledasasoftwareobject.Inadditiontoitsvariables,thesoftwarebicyclewouldalsohavemethodstobrake,changethepedalcadence,andchangegears.(Itwouldnothaveamethodforchangingitsspeedbecausethebikesspeedisjustasideeffectofwhichgearitsinandhowfasttheriderispedaling.)Thesemethodsareknownformallyasinstancemethodsbecausetheyinspectorchangethestateofaparticularbicycleinstance.Objectdiagramsshowthatanobjectsvariablesmakeupthecenter,ornucleus,oftheobject.Methodssurroundandhidetheobjectsnucleusfromotherobjectsintheprogram.Packaginganobjectsvariableswithintheprotectivecustodyofitsmethodsiscalledencapsulation.Thisconceptualpictureofanobjectanucleusofvariablespackagedwithinaprotectivemembraneofmethodsisanidealrepresentationofanobjectandistheidealthatdesignersofobject-orientedsystemsstrivefor.However,itsnotthewholestory.Often,forpracticalreasons,anobjectmayexposesomeofitsvariablesorhidesomeofitsmethods.IntheJavaprogramminglanguage,anobjectcanspecifyoneoffouraccesslevelsforeachofitsvariablesandmethods.Theaccessleveldetermineswhichotherobjectsandclassescanaccessthatvariableormethod.RefertotheControllingAccesstoMembersofaClasssectionfordetails.Encapsulatingrelatedvariablesandmethodsintoaneatsoftwarebundleisasimpleyetpowerfulideathatprovidestwoprimarybenefitstosoftwaredevelopers:Modularity:Thesourcecodeforanobjectcanbewrittenandmaintainedindependentlyofthesourcecodeforotherobjects.Also,anobjectcanbeeasilypassedaroundinthesystem.Youcangiveyourbicycletosomeoneelse,anditwillstillwork.Information-hiding:Anobjecthasapublicinterfacethatotherobjectscanusetocommunicatewithit.Theobjectcanmaintainprivateinformationandmethodsthatcanbechangedatanytimewithoutaffectingotherobjectsthatdependonit.Youdontneedtounderstandabikesgearmechanismtouseit.WhatIsaMessage?Asingleobjectalonegenerallyisnotveryuseful.Instead,anobjectusuallyappearsasacomponentofalargerprogramorapplicationthatcontainsmanyotherobjects.Throughtheinteractionoftheseobjects,programmersachievehigher-orderfunctionalityandmorecomplexbehavior.Yourbicyclehangingfromahookinthegarageisjustabunchofmetalandrubber；byitself,itisincapableofanyactivity；thebicycleisusefulonlywhenanotherobject(you)interactswithit(bypedaling).Softwareobjectsinteractandcommunicatewitheachotherbysendingmessagestoeachother.WhenobjectAwantsobjectBtoperformoneofBsmethods,objectAsendsamessagetoobjectB(seethefollowingfigure).Objectsinteractbysendingeachothermessages.Sometimes,thereceivingobjectneedsmoreinformationsothatitknowsexactlywhattodo；forexample,whenyouwanttochangegearsonyourbicycle,youhavetoindicatewhichgearyouwant.Thisinformationispassedalongwiththemessageasparameters.Messagesuseparameterstopassalongextrainformationthattheobjectneedsinthiscase,whichgearthebicycleshouldbein.Thesethreepartsareenoughinformationforthereceivingobjec