ZYBS網(wǎng)上書店英文文獻(xiàn)

ZYBS網(wǎng)上書店英文文獻(xiàn)

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,ablueprintofabicycleisnotabicycle.However,it'salittlemoredifficulttodifferentiateclassesandobjectsinsoftware.Thisispartiallybecausesoftwareobjectsaremerely

electronicmodelsofreal-worldobjectsorabstractconceptsinthefirstplace.Butit'salsobecausethetermobjectissometimesusedtorefertobothclasses

andinstances.

Inillustrationssuchasthetoppartoftheprecedingfigure,theclassisnotshadedbecauseitrepresentsablueprintofanobjectratherthantheobjectitself.Incomparison,anobjectisshaded,indicatingthattheobjectexistsandthatyoucanuseit.

WhatIsInheritance?

Generallyspeaking,objectsaredefinedintermsofclasses.Youknowalotaboutanobjectbyknowingitsclass.Evenifyoudon'tknowwhatapenny-farthingis,ifItoldyouitwasabicycle,youwouldknowthatithadtwowheels,handlebars,andpedals.

Object-orientedsystemstakethisastepfurtherandallowclassestobedefinedintermsofotherclasses.Forexample,mountainbikes,roadbikes,andtandemsarealltypesofbicycles.Inobject-orientedterminology,mountainbikes,roadbikes,andtandemsareallsubclassesofthebicycleclass.Similarly,thebicycleclassisthesupclassesofmountainbikes,roadbikes,andtandems.Thisrelationshipisshowninthefollowingfigure.

Thehierarchyofbicycleclasses.

Eachsubclassinheritsstate(intheformofvariabledeclarations)fromthesuperclass.Mountainbikes,roadbikes,andtandemssharesomestates:cadence,speed,andthelike.Also,eachsubclassinheritsmethodsfromthesuperclass.Mountainbikes,roadbikes,andtandemssharesomebehaviors—brakingand

changingpedalingspeed,forexample.

However,subclassesarenotlimitedtothestatesandbehaviorsprovidedtothembytheirsuperclass.Subclassescanaddvariablesandmethodstotheonestheyinheritfromthesuperclass.Tandembicycleshavetwoseatsandtwosetsofhandlebars;somemountainbikeshaveanadditionalchainring,givingthemalowergearratio.

Subclassescanalsooverrideinheritedmethodsandprovidespecializedimplementationsforthosemethods.Forexample,ifyouhadamountainbikewithanadditionalchainring,youcouldoverridethe"changegears"methodsothattheridercouldshiftintothoselowergears.

Youarenotlimitedtojustonelayerofinheritance.Theinheritancetree,orclasshierardry,canbeasdeepasneeded.Methodsandvariablesareinheriteddownthroughthelevels.Ingeneral,thefartherdowninthehierarchyaclassappears,themorespecializeditsbehavior.

Note:Classhierarchiesshouldreflectwhattheclassesare,nothowthey're

implemented.Whenimplementingatricycleclass,itmightbeconvenienttomakeitasubclassofthebicycleclass—afterall,bothtricyclesandbicycleshave

acurrentspeedandcadence.However,becauseatricycleisnotabicycle,it'sunwisetopubliclytiethetwoclassestogether.Itcouldconfuseusers,makethetricycleclasshavemethods(forexample,tochangegears)thatitdoesn'tneed,andmakeupdatingorimprovingthetricycleclassdifficult.

TheObjectclassisatthetopofclasshierarchy,andeachclassisitsdescendant(directlyorindirectly).AvariableoftypeObjectcanholdareferencetoanyobject,suchasaninstanceofaclassoranarray.ObjectprovidesbehaviorsthataresharedbyallobjectsrunningintheJavaVirtualMachine.Forexample,allclassesinheritObject'stoStringmethod,whichreturnsastringrepresentationoftheobject.TheManagingInheritancesectioncoverstheObjectclassindetail.

Inheritanceoffersthefollowingbenefits:

Subclassesprovidespecializedbehaviorsfromthebasisofcommon

elementsprovidedbythesuperclass.Throughtheuseofinheritance,

programmerscanreusethecodeinthesuperclassmanytimes.

Programmerscanimplementsuperclassescalledabstractclassesthat

definecommonbehaviors.Theabstractsuperclassdefinesandmay

partiallyimplementthebehavior,butmuchoftheclassisundefined

andunimplemented.Otherprogrammersfillinthedetailswith

specializedsubclasses.

WhatIsanInterface?

isadeviceorasystemthatunrelatedentitiesusetoIngeneral,aninterface

interact.Accordingtothisdefinition,aremotecontrolisaninterfacebetweenyouandatelevisionset,theEnglishlanguageisaninterfacebetweentwopeople,andtheprotocolofbehaviorenforcedinthemilitaryistheinterfacebetweenindividualsofdifferentranks.

WithintheJavaprogramminglanguage,aninterfaceisatype,justasaclassisatype.Likeaclass,aninterfacedefinesmethods.Unlikeaclass,aninterfaceneverimplementsmethods;instead,classesthatimplementtheinterfaceimplementthemethodsdefinedbytheinterface.Aclasscanimplementmultipleinterfaces.

Thebicycl