电子商务毕业论文外文翻译范文《Thinking In Java》Word下载.docx

电子商务毕业论文外文翻译范文《Thinking In Java》Word下载.docx

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surprisingly,theywouldn’tcheckfortheerrorconditions（andsometimestheerrorconditionsweretoosillytocheckfor）.Ifyouwerethoroughenoughtocheckforanerroreverytimeyoucalledamethod,yourcodecouldturnintoanunreadablenightmare.Becauseprogrammerscouldstillcoaxsystemsoutoftheselanguages,theywereresistanttoadmittingthetruth:

thatthisapproachtohandlingerrorswasamajorlimitationtocreatinglarge,robust,maintainableprograms.

Thesolutionistotakethecasualnatureoutoferrorhandlingandtoenforceformality.Thisactuallyhasalonghistory,becauseimplementationsofexceptionhandlinggobacktooperatingsystemsinthe1960s,andeventoBASIC’s“onerrorgoto.”ButC++exceptionhandlingwasbasedonAda,andJava’sisbasedprimarilyonC++（althoughitlooksmorelikethatinObjectPascal）.

Theword“exception”ismeantinthesenseof“Itakeexceptiontothat.”Atthepointwheretheproblemoccurs,youmightnotknowwhattodowithit,butyoudoknowthatyoucan’tjustcontinueonmerrily;

youmuststop,andsomebody,somewhere,mustfigureoutwhattodo.Butyoudon’thaveenoughinformationinthecurrentcontexttofixtheproblem.Soyouhandtheproblemouttoahighercontextwheresomeoneisqualifiedtomaketheproperdecision（muchlikeachainofcommand）.

Theotherrathersignificantbenefitofexceptionsisthattheycleanup

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errorhandlingcode.Insteadofcheckingforaparticularerroranddealingwithitatmultipleplacesinyourprogram,younolongerneedtocheckatthepointofthemethodcall（sincetheexceptionwillguaranteethatsomeonecatchesit）.And,youneedtohandletheprobleminonlyoneplace,theso-calledexceptionhandler.Thissavesyoucode,anditseparatesthecodethatdescribeswhatyouwanttodofromthecodethatisexecutedwhenthingsgoawry.Ingeneral,reading,writing,anddebuggingcodebecomesmuchclearerwithexceptionsthanwhenusingtheoldwayoferrorhandling.

BecauseexceptionhandlingistheonlyofficialwaythatJavareportserrors,anditisenforcedbytheJavacompiler,thereareonlysomanyexamplesthatcanbewritteninthisbookwithoutlearningaboutexceptionhandling.Thischapterintroducesyoutothecodeyouneedtowritetoproperlyhandleexceptions,andthewayyoucangenerateyourownexceptionsifoneofyourmethodsgetsintotrouble.

1.Basicexceptions

Anexceptionalconditionisaproblemthatpreventsthecontinuationofthemethodorscopethatyou’rein.It’simportanttodistinguishanexceptionalconditionfromanormalproblem,inwhichyouhaveenoughinformationinthecurrentcontexttosomehowcopewiththedifficulty.Withanexceptionalcondition,youcannotcontinueprocessingbecauseyoudon’thavethe

informationnecessarytodealwiththeprobleminthecurrentcontext.Allyoucandoisjumpoutofthecurrentcontextandrelegatethatproblemtoahighercontext.Thisiswhathappenswhenyouthrowanexception.

Divisionisasimpleexample.Ifyou’reabouttodividebyzero,it’sworthcheckingforthatcondition.Butwhatdoesitmeanthatthedenominatoriszero?

Maybeyouknow,inthecontextoftheproblemyou’retryingtosolveinthat

particularmethod,howtodealwithazerodenominator.Butifit’sanunexpectedvalue,youcan’tdealwithitandsomustthrowanexceptionratherthancontinuingalongthatexecutionpath.

Whenyouthrowanexception,severalthingshappen.First,theexceptionobjectiscreatedinthesamewaythatanyJavaobjectiscreated:

ontheheap,withnew.Thenthecurrentpathofexecution（theoneyoucouldn’tcontinue）isstoppedandthereferencefortheexceptionobjectisejectedfromthecurrentcontext.Atthispointtheexceptionhandlingmechanismtakesoverandbeginstolookforanappropriateplacetocontinueexecutingtheprogram.This

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appropriateplaceistheexceptionhandler,whosejobistorecoverfromtheproblemsotheprogramcaneithertryanothertackorjustcontinue.

Asasimpleexampleofthrowinganexception,consideranobjectreferencecalledt.It’spossiblethatyoumightbepassedareferencethathasn’tbeeninitialized,soyoumightwanttocheckbeforetryingtocallamethodusingthatobjectreference.Youcansendinformationabouttheerrorintoalargercontextbycreatinganobjectrepresentingyourinformationand“throwing”itoutofyourcurrentcontext.Thisiscalledthrowinganexception.

1.1Exceptionarguments

LikeanyobjectinJava,youalwayscreateexceptionsontheheapusingnew,whichallocatesstorageandcallsaconstructor.Therearetwoconstructorsinallstandardexceptions;

Thefirstisthedefaultconstructor,andthesecondtakesastringargumentsoyoucanplacepertinentinformationintheexception:

thrownewNullPointerException（"

t=null"

）;

Thisstringcanlaterbeextractedusingvariousmethods,asyou’llsee.

Thekeywordthrowcausesanumberofrelativelymagicalthingstohappen.Typically,you’llfirstusenewtocreateanobjectthatrepresentstheerrorcondition.Yougivetheresultingreferencetothrow.Theobjectis,ineffect,“returned”fromthemethod,eventhoughthatobjecttypeisn’tnormallywhatthemethodisdesignedtoreturn.Asimplisticwaytothinkaboutexceptionhandlingisasadifferentkindofreturnmechanism,althoughyougetintotroubleifyoutakethatanalogytoofar.Youcanalsoexitfromordinaryscopesbythrowinganexception.Butavalueisreturned,andthemethodorscopeexits.

Anysimilaritytoanordinaryreturnfromamethodendshere,becausewhereyoureturnissomeplacecompletelydifferentfromwhereyoureturnforanormalmethodcall.（Youendupinanappropriateexceptionhandlerthatmightbefar—manylevelsawayonthecallstack—fromwheretheexceptionwasthrown.）

Inaddition,youcanthrowanytypeofThrowable（theexceptionrootclass）objectthatyouwant.Typically,you’llthrowadifferentclassofexceptionforeachdifferenttypeoferror.Theinformationabouttheerrorisrepresentedbothinsidetheexceptionobjectandimplicitlyinthenameoftheexceptionclass,sosomeoneinthebiggercontextcanfigureoutwhattodowithyourexception.（Often,theonlyinformationisthetypeofexception,andnothingmeaningfulisstoredwithintheexceptionobject.）

2.Catchinganexception

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Ifamethodthrowsanexception,itmustassumethatexceptionwillbe“caught”anddealtwith.Oneoftheadvantagesofexceptionhandlingisthatitallowsyoutoconcentrateontheproblemyou’retryingtosolveinoneplace,andthendealwiththeerrorsfromthatcodeinanotherplace.

Toseehowanexceptioniscaught,youmustfirstunderstandtheconceptofaguardedregion.Thisisasectionofcodethatmightproduceexceptionsandisfollowedbythecodetohandlethoseexceptions.

2.1Thetryblock

Ifyou’reinsideamethodandyouthrowanexception（oranothermethodyoucallwithinthismethodthrowsanexception）,thatmethodwillexitintheprocessofthrowing.Ifyoudon’twantathrowtoexitthemethod,youcanset

upaspecialblockwithinthatmethodtocapturetheexception.Thisiscalledthetryblockbecauseyou“try”yourvariousmethodcallsthere.Thetryblock

isanordinaryscopeprecededbythekeywordtry:

try{

//Codethatmightgenerateexceptions

}

Ifyouwerecheckingforerrorscarefullyinaprogramminglanguagethatdidn’tsupportexceptionhandling,you’dhavetosurroundeverymethodcallwithsetupanderrortestingcode,evenifyoucallthesamemethodseveraltimes.Withexceptionhandling,youputeverythinginatryblockandcapturealltheexceptionsinoneplace.Thismeansyourcodeismucheasiertowriteandreadbecausethegoalofthecodeisnotconfusedwiththeerrorchecking.

2.2Exceptionhandlers

Ofcourse,thethrownexceptionmustendupsomeplace.This“place”istheexceptionhandler,andthere’soneforeveryexceptiontypeyouwanttocatch.Exceptionhandlersimmediatelyfollowthetryblockandaredenotedbythekeywordcatch:

}catch（Type1id1）{

//HandleexceptionsofType1

}catch（Type2id2）{

//HandleexceptionsofType2

}catch（Type3id3）{

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//HandleexceptionsofType3

//etc...

Eachcatchclause（exceptionhandler）islikealittlemethodthattakesoneandonlyoneargumentofaparticulartype.Theidentifier（id1,id2,andsoon）canbeusedinsidethehandler,justlikeamethodargument.Sometimesyouneverusetheidentifierbecausethetypeoftheexceptiongivesyouenoughinformationtodealwiththeexception,buttheidentifiermuststillbethere.

Thehandlersmustappeardirectlyafter