ZYBS网上书店英文文献.docx

ZYBS网上书店英文文献.docx

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ZYBS网上书店英文文献

ZYBS网上书店英文文献

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-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.

Definition:

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