计算机专业论文译文对象传递.docx

计算机专业论文译文对象传递.docx

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计算机专业论文译文对象传递

专业译文

原文出处：

http:

//www.faqs.org/docs/think_java/TIJ319.htm

Passing&ReturningObjects

Bynowyoushouldbereasonablycomfortablewiththeideathatwhenyou’re“passing”anobject,you’reactuallypassingareference.

Inmanyprogramminglanguagesyoucanusethatlanguage’s“regular”waytopassobjectsaround,andmostofthetimeeverythingworksfine.Butitalwaysseemsthattherecomesapointatwhichyoumustdosomethingirregular,andsuddenlythingsgetabitmorecomplicated（orinthecaseofC++,quitecomplicated）.Javaisnoexception,andit’simportantthatyouunderstandexactlywhat’shappeningasyoupassobjectsaroundandmanipulatethem.Thisappendixwillprovidethatinsight.

Anotherwaytoposethequestionofthisappendix,ifyou’recomingfromaprogramminglanguagesoequipped,is“DoesJavahavepointers?

”Somehaveclaimedthatpointersarehardanddangerousandthereforebad,andsinceJavaisallgoodnessandlightandwillliftyourearthlyprogrammingburdens,itcannotpossiblycontainsuchthings.However,it’smoreaccuratetosaythatJavahaspointers;indeed,everyobjectidentifierinJava（exceptforprimitives）isoneofthesepointers,buttheiruseisrestrictedandguardednotonlybythecompilerbutbytherun-timesystem.Ortoputitanotherway,Javahaspointers,butnopointerarithmetic.ThesearewhatI’vebeencalling“references,”andyoucanthinkofthemas“safetypointers,”notunlikethesafetyscissorsofelementaryschool—theyaren’tsharp,soyoucannothurtyourselfwithoutgreateffort,buttheycansometimesbeslowandtedious.

Passingreferencesaround

Whenyoupassareferenceintoamethod,you’restillpointingtothesameobject.Asimpleexperimentdemonstratesthis:

//:

appendixa:

PassReferences.java

//Passingreferencesaround.

importcom.bruceeckel.simpletest.*;

publicclassPassReferences{

privatestaticTestmonitor=newTest（）;

publicstaticvoidf（PassReferencesh）{

System.out.println（"hinsidef（）:

"+h）;

}

publicstaticvoidmain（String[]args）{

PassReferencesp=newPassReferences（）;

System.out.println（"pinsidemain（）:

"+p）;

f（p）;

monitor.expect（newString[]{

"%%pinsidemain\\（\\）:

PassReferences@[a-z0-9]+",

"%%hinsidef\\（\\）:

PassReferences@[a-z0-9]+"

}）;

}

}///:

~

ThemethodtoString（ ）isautomaticallyinvokedintheprintstatements,andPassReferencesinheritsdirectlyfromObjectwithnoredefinitionoftoString（ ）.Thus,Object’sversionoftoString（ ）isused,whichprintsouttheclassoftheobjectfollowedbytheaddresswherethatobjectislocated（notthereference,buttheactualobjectstorage）.Theoutputlookslikethis:

pinsidemain（）:

PassReferences@ad3ba4

hinsidef（）:

PassReferences@ad3ba4

Youcanseethatbothpandhrefertothesameobject.ThisisfarmoreefficientthanduplicatinganewPassReferencesobjectjustsothatyoucansendanargumenttoamethod.Butitbringsupanimportantissue.

Aliasing

Aliasingmeansthatmorethanonereferenceistiedtothesameobject,asintheprecedingexample.Theproblemwithaliasingoccurswhensomeonewritestothatobject.Iftheownersoftheotherreferencesaren’texpectingthatobjecttochange,they’llbesurprised.Thiscanbedemonstratedwithasimpleexample:

//:

appendixa:

Alias1.java

//Aliasingtworeferencestooneobject.

importcom.bruceeckel.simpletest.*;

publicclassAlias1{

privatestaticTestmonitor=newTest（）;

privateinti;

publicAlias1（intii）{i=ii;}

publicstaticvoidmain（String[]args）{

Alias1x=newAlias1（7）;

Alias1y=x;//Assignthereference

System.out.println（"x:

"+x.i）;

System.out.println（"y:

"+y.i）;

System.out.println（"Incrementingx"）;

x.i++;

System.out.println（"x:

"+x.i）;

System.out.println（"y:

"+y.i）;

monitor.expect（newString[]{

"x:

7",

"y:

7",

"Incrementingx",

"x:

8",

"y:

8"

}）;

}

}///

Intheline:

Alias1y=x;//Assignthereference

anewAlias1referenceiscreated,butinsteadofbeingassignedtoafreshobjectcreatedwithnew,it’sassignedtoanexistingreference.Sothecontentsofreferencex,whichistheaddressoftheobjectxispointingto,isassignedtoy,andthusbothxandyareattachedtothesameobject.Sowhenx’siisincrementedinthestatement:

x.i++;

y’siwillbeaffectedaswell.Thiscanbeseenintheoutput:

x:

7

y:

7

Incrementingx

x:

8

y:

8

Onegoodsolutioninthiscaseissimplynottodoit;don’tconsciouslyaliasmorethanonereferencetoanobjectatthesamescope.Yourcodewillbemucheasiertounderstandanddebug.However,whenyou’repassingareferenceinasanargument—whichisthewayJavaissupposedtowork—youautomaticallyalias,becausethelocalreferencethat’screatedcanmodifythe“outsideobject”（theobjectthatwascreatedoutsidethescopeofthemethod）.Here’sanexample:

//:

appendixa:

Alias2.java

//Methodcallsimplicitlyaliastheirarguments.

importcom.bruceeckel.simpletest.*;

publicclassAlias2{

privatestaticTestmonitor=newTest（）;

privateinti;

publicAlias2（intii）{i=ii;}

publicstaticvoidf（Alias2reference）{reference.i++;}

publicstaticvoidmain（String[]args）{

Alias2x=newAlias2（7）;

System.out.println（"x:

"+x.i）;

System.out.println（"Callingf（x）"）;

f（x）;

System.out.println（"x:

"+x.i）;

monitor.expect（newString[]{

"x:

7",

"Callingf（x）",

"x:

8"

}）;

}

}///

Themethodischangingitsargument,theoutsideobject.Whenthiskindofsituationarises,youmustdecidewhetheritmakessense,whethertheuserexpectsit,andwhetherit’sgoingtocauseproblems.

Ingeneral,youcallamethodinordertoproduceareturnvalueand/orachangeofstateintheobjectthatthemethodiscalledfor.It’smuchlesscommontocallamethodinordertomanipulateitsarguments;thisisreferredtoas“callingamethodforitssideeffects.”Thus,whenyoucreateamethodthatmodifiesitsarguments,theusermustbeclearlyinstructedandwarnedabouttheuseofthatmethodanditspotentialsurprises.Becauseoftheconfusionandpitfalls,it’smuchbettertoavoidchangingtheargument.

Ifyouneedtomodifyanargumentduringamethodcallandyoudon’tintendtomodifytheoutsideargument,thenyoushouldprotectthatargumentbymakingacopyinsideyourmethod.That’sthesubjectofmuchofthisappendix.

Makinglocalcopies

Toreview:

AllargumentpassinginJavaisperformedbypassingreferences.Thatis,whenyoupass“anobject,”you’rereallypassingonlyareferencetoanobjectthatlivesoutsidethemethod,soifyouperformanymodificationswiththatreference,youmodifytheoutsideobject.Inaddition:

●Aliasinghappensautomaticallyduringargumentpassing.

●Therearenolocalobjects,onlylocalreferences.

●Referenceshavescopes,objectsdonot.

●ObjectlifetimeisneveranissueinJava.

●Thereisnolanguagesupport（e.g.,“const”）topreventobjectsfrombeingmodifiedandstopthenegativeeffectsofaliasing.Youcan’tsimplyusethefinalkeywordintheargumentlist;thatsimplypreventsyoufromrebindingthereferencetoadifferentobject.

Ifyou’reonlyreadinginformationfromanobjectandnotmodifyingit,passingareferenceisthemostefficientformofargumentpassing.Thisisnice;thedefaultwayofdoingthingsisalsothemostefficient.However,sometimesit’snecessarytobeabletotreattheobjectasifitwere“local”sothatchangesyoumakeaffectonlyalocalcopyanddonotmodifytheoutsideobject.Manyprogramminglanguagessupporttheabilitytoautomaticallymakealocalcopyoftheoutsideobject,insidethemethod.[116]Javadoesnot,butitallowsyoutoproducethiseffect.

Passbyvalue

Thisbringsuptheterminologyissue,whichalwaysseemsgoodforanargument.Thetermis“passbyvalue,”andthemeaningdependsonhowyouperceivetheoperationoftheprogram.Thegeneralmeaningisthatyougetalocalcopyofwhateveryou’repassing,buttherealquestionishowyouthinkaboutwhatyou’repassing.Whenitcomestothemeaningof“passbyvalue,”therearetwofairlydistinctcamps:

1.Javapasseseverythingbyvalue.Whenyou’repassingprimitivesintoamethod,yougetadistinctcopyoftheprimitive.Whenyou’repassingareferenceintoamethod,yougetacopyofthereference.Ergo,everythingispassbyvalue.Ofcourse,theassumptionisthatyou’realwaysthinking（andcaring）thatreferencesarebeingpassed,butitseemsliketheJavadesignhasgonealongwaytowardallowingyoutoignore（mostofthetime）thatyou’reworkingwithareference.Thatis,itseemstoallowyoutothinkofthereferenceas“theobject,”sinceitimplicitlydereferencesitwheneveryoumakeamethodcall.

2.Javapassesprimitivesbyvalue（noargumentthere）,butobjectsarepassedbyreference.Thisistheworldviewthatthereferenceisanaliasfortheobject,soyoudon’tthinkaboutpassingreferences,butinsteadsay“I’mpassingtheobject.”Sinceyoudon’tgetalocalcopyoftheobjectwhenyoupassitintoamethod,objectsareclearlynotpassedbyvalue.ThereappearstobesomesupportforthisviewwithinSun,sinceatonetime,oneofthe“reservedbutnotimplemented”keywordswasbyvalue（Thiswillprobablyneverbeimplemented）.

Havinggivenbothcampsagoodairing,andaftersaying“Itdependsonhowyouthinkofareference,”Iwillattempttosidesteptheissue.Intheend,itisn’tthatimportant—whatisimportantisthatyouunderstandthatpassingareferenceallowsthecaller’sobjecttobechangedunexpectedly.

Cloningobjects

Themostlikelyreasonformakingalocalcopyofanobjectisifyou’regoingtomodifythatobjectandyoudon’twanttomodifythecaller’sobject.Ifyoudecideth