ZYBS网上书店英文文献Word格式文档下载.docx

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Definition:

Anobjectisasoftwarebundleofvariablesandrelatedmethods.Youcanrepresentreal-worldobjectsbyusingsoftwareobjects.Youmightwanttorepresentreal-worlddogsassoftwareobjectsinananimationprogramorareal-worldbicycleasasoftwareobjectintheprogramthatcontrolsanelectronicexercisebike.Youcanalsousesoftwareobjectstomodelabstractconcepts.Forexample,aneventisacommonobjectusedinwindowsystemstorepresenttheactionofauserpressingamousebuttonorakeyonthekeyboard.Thefollowingillustrationisacommonvisualrepresentationofasoftwareobject.

Asoftwareobject.

Everythingthesoftwareobjectknows（state）andcando（behavior）isexpressedbythevariablesandthemethodswithinthatobject.Asoftwareobjectthatmodelsyourreal-worldbicyclewouldhavevariablesthatindicatethebicycle'

scurrentstate:

Itsspeedis18mph,itspedalcadenceis90rpm,anditscurrentgearis5th.Thesevariablesareformallyknownasinstancevariablesbecausetheycontainthestateforaparticularbicycleobject;

inobject-orientedterminology,aparticularobjectiscalledaninstance.Thefollowingfigureillustratesa

bicyclemodeledasasoftwareobject.

Abicyclemodeledasasoftware

object.

Inadditiontoitsvariables,thesoftwarebicyclewouldalsohavemethodstobrake,changethepedalcadence,andchangegears.（Itwouldnothaveamethodforchangingitsspeedbecausethebike'

sspeedisjustasideeffectofwhichgearit'

sinandhowfasttheriderispedaling.）Thesemethodsareknownformallyasinstancemethodsbecausetheyinspectorchangethestateofaparticularbicycleinstance.

Objectdiagramsshowthatanobject'

svariablesmakeupthecenter,ornucleus,oftheobject.Methodssurroundandhidetheobject'

snucleusfromotherobjectsintheprogram.Packaginganobject'

svariableswithintheprotectivecustodyofitsmethodsiscalledencapsulation.Thisconceptualpictureofanobject—

anucleusofvariablespackagedwithinaprotectivemembraneofmethods—is

anidealrepresentationofanobjectandistheidealthatdesignersofobject-orientedsystemsstrivefor.However,it'

snotthewholestory.Often,forpracticalreasons,anobjectmayexposesomeofitsvariablesorhidesomeofitsmethods.IntheJavaprogramminglanguage,anobjectcanspecifyoneoffouraccesslevelsforeachofitsvariablesandmethods.Theaccesslevel

determineswhichotherobjectsandclassescanaccessthatvariableormethod.RefertotheControllingAccesstoMembersofaClasssectionfordetails.Encapsulatingrelatedvariablesandmethodsintoaneatsoftwarebundleisasimpleyetpowerfulideathatprovidestwoprimarybenefitstosoftwaredevelopers:

Modularity:

Thesourcecodeforanobjectcanbewrittenandmaintained

independentlyofthesourcecodeforotherobjects.Also,anobject

canbeeasilypassedaroundinthesystem.Youcangiveyourbicycle

tosomeoneelse,anditwillstillwork.

Information-hiding:

Anobjecthasapublicinterfacethatother

objectscanusetocommunicatewithit.Theobjectcanmaintainprivate

informationandmethodsthatcanbechangedatanytimewithout

affectingotherobjectsthatdependonit.Youdon'

tneedtounderstand

abike'

sgearmechanismtouseit.

WhatIsaMessage?

Asingleobjectalonegenerallyisnotveryuseful.Instead,anobjectusuallyappearsasacomponentofalargerprogramorapplicationthatcontainsmanyotherobjects.Throughtheinteractionoftheseobjects,programmersachievehigher-orderfunctionalityandmorecomplexbehavior.Yourbicyclehangingfromahookinthegarageisjustabunchofmetalandrubber;

byitself,itisincapableofanyactivity;

thebicycleisusefulonlywhenanotherobject（you）interactswithit（bypedaling）.

Softwareobjectsinteractandcommunicatewitheachotherbysendingmessages

toeachother.WhenobjectAwantsobjectBtoperformoneofB'

smethods,objectAsendsamessagetoobjectB（seethefollowingfigure）.

Objectsinteractbysendingeachothermessages.

Sometimes,thereceivingobjectneedsmoreinformationsothatitknowsexactlywhattodo;

forexample,whenyouwanttochangegearsonyourbicycle,youhavetoindicatewhichgearyouwant.Thisinformationispassedalongwiththemessageasparameters.

Messagesuseparameterstopassalong

extrainformationthattheobject

needs—inthiscase,whichgearthe

bicycleshouldbein.

Thesethreepartsareenoughinformationforthereceivingobjecttoperformthedesiredmethod.Nootherinformationorcontextisrequired.

Messagesprovidetwoimportantbenefits:

Anobject'

sbehaviorisexpressedthroughitsmethods,so（asidefrom

directvariableaccess）messagepassingsupportsallpossible

interactionsbetweenobjects.

Objectsdon'

tneedtobeinthesameprocessorevenonthesamemachine

tosendmessagesbackandforthandreceivemessagesfromeachother.WhatIsaClass?

Intherealworld,youoftenhavemanyobjectsofthesamekind.Forexample,yourbicycleisjustoneofmanybicyclesintheworld.Usingobject-orientedterminology,wesaythatyourbicycleobjectisaninstanceoftheclassofobjectsknownasbicycles.Bicycleshavesomestate（currentgear,currentcadence,twowheels）andbehavior（changegears,brake）incommon.However,eachbicycle'

sstateisindependentofandcanbedifferentfromthatofotherbicycles.Whenbuildingthem,manufacturerstakeadvantageofthefactthatbicyclessharecharacteristics,buildingmanybicyclesfromthesameblueprint.Itwouldbeveryinefficienttoproduceanewblueprintforeverybicyclemanufactured.Inobject-orientedsoftware,it'

salsopossibletohavemanyobjectsofthesamekindthatsharecharacteristics:

rectangles,employeerecords,videoclips,andsoon.Likebicyclemanufacturers,youcantakeadvantageofthefactthatobjectsofthesamekindaresimilarandyoucancreateablueprintforthoseobjects.Asoftwareblueprintforobjectsiscalledaclass（seethefollowingfigure）.

Avisualrepresentationofaclass.

Aclassisablueprintthatdefinesthevariablesandthemethodscommontoallobjectsofacertainkind.

Theclassforourbicycleexamplewoulddeclaretheinstancevariablesnecessarytocontainthecurrentgear,thecurrentcadence,andsoonforeachbicycleobject.Theclasswouldalsodeclareandprovideimplementationsfortheinstancemethodsthatallowtheridertochangegears,brake,andchangethepedalingcadence,asshowninthenextfigure.

Thebicycleclass.

Afteryou'

vecreatedthebicycleclass,youcancreateanynumberofbicycleobjectsfromthatclass.Whenyoucreateaninstanceofaclass,thesystemallocatesenoughmemoryfortheobjectandallitsinstancevariables.Eachinstancegetsitsowncopyofalltheinstancevariablesdefinedintheclass,asthenextfigureshows.

MyBikeandYourBikearetwodifferentinstancesoftheBikeclass.Eachinstance

hasitsowncopyoftheinstancevariablesdefinedintheBikeclassbuthas

differentvaluesforthesevariables.

Inadditiontoinstancevariables,classescandefineclassvariables.Aclasswariablecontainsinformationthatissharedbyallinstancesoftheclass.Forexample,supposethatallbicycleshadthesamenumberofgears.Inthiscase,defininganinstancevariabletoholdthenumberofgearsisinefficient;

eachinstancewouldhaveitsowncopyofthevariable,butthevaluewouldbethesameforeveryinstance.Insuchsituations,youcandefineaclassvariablethatcontainsthenumberofgears（seethefollowingfigure）;

allinstancessharethisvariable.Ifoneobjectchangesthevariable,itchangesforallotherobjectsofthattype.

YourBike,aninstanceofBike,hasaccesstothenumberOfGearsvariableinthe

Bikeclass;

however,theYourBikeinstancedoesnothaveacopyofthisclass

variable.

AclasscanalsodeclareclassmethodsYoucaninvokeaclassmethoddirectlyfromtheclass,whereasyoumustinvokeinstancemethodsonaparticularinstance.TheUnderstandingInstanceandClassMemberssectiondiscussesinstancevariablesandmethodsandclassvariablesandmethodsindetail.Objectsprovidethebenefitofmodularityandinformation-hiding.Classesprovidethebenefitofreusability.Bicyclemanufacturersusethesameblueprintoverandoveragaintobuildlotsofbicycles.Softwareprogrammersusethesameclass,andthusthesamecode,overandoveragaintocreatemanyobjects.ObjectsversusClasses

You'

veprobablynoticedthattheillustrationsofobjectsandclasseslookverysimilar.Andindeed,thedifferencebetweenclassesandobjectsisoftenthesourceofsomeconfusion.Intherealworld,it'

sobviousthatclassesarenotthemselvestheobjectstheydescribe;

thatis,ablueprintofabicycleisnota