Exceptions in JavaWord文档下载推荐.docx

Exceptions in JavaWord文档下载推荐.docx

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thandleitself,itmaythrowanexception.Throwinganexceptionislikethrowingabeeping,flashingredballtoindicatethereisaproblemthatcan'

tbehandledwhereitoccurred.Somewhere,youhope,thisballwillbecaughtandtheproblemwillbedealtwith.Exceptionsarecaughtbyhandlerspositionedalongthethread'

smethodinvocationstack.Ifthecallingmethodisn'

tpreparedtocatchtheexception,itthrowstheexceptionuptoitscallingmethod,andsoon.Ifoneofthethreadsofyourprogramthrowsanexceptionthatisn'

tcaughtbyanymethodalongthemethodinvocationstack,thatthreadwillexpire.WhenyouprograminJava,youmustpositioncatchers（theexceptionhandlers）strategically,soyourprogramwillcatchandhandleallexceptionsfromwhichyouwantyourprogramtorecover.

Exceptionclasses

InJava,exceptionsareobjects.Whenyouthrowanexception,youthrowanobject.Youcan'

tthrowjustanyobjectasanexception,however--onlythoseobjectswhoseclassesdescendfromThrowable.Throwableservesasthebaseclassforanentirefamilyofclasses,declaredinjava.lang,thatyourprogramcaninstantiateandthrow.Asmallpartofthisfamilyisshownin

图表1ApartialviewoftheThrowablefamily

AsyoucanseeinFigure1,Throwablehastwodirectsubclasses,ExceptionandError.Exceptions（membersoftheExceptionfamily）arethrowntosignalabnormalconditionsthatcanoftenbehandledbysomecatcher,thoughit'

spossibletheymaynotbecaughtandthereforecouldresultinadeadthread.Errors（membersoftheErrorfamily）areusuallythrownformoreseriousproblems,suchasOutOfMemoryError,thatmaynotbesoeasytohandle.Ingeneral,codeyouwriteshouldthrowonlyexceptions,noterrors.ErrorsareusuallythrownbythemethodsoftheJavaAPI,orbytheJavavirtualmachineitself.

Inadditiontothrowingobjectswhoseclassesaredeclaredinjava.lang,youcanthrowobjectsofyourowndesign.Tocreateyourownclassofthrowableobjects,youneedonlydeclareitasasubclassofsomememberoftheThrowablefamily.Ingeneral,however,thethrowableclassesyoudefineshouldextendclassException.Theyshouldbe"

exceptions."

Thereasoningbehindthisrulewillbeexplainedlaterinthisarticle.

Whetheryouuseanexistingexceptionclassfromjava.langorcreateoneofyourowndependsuponthesituation.Insomecases,aclassfromjava.langwilldojustfine.Forexample,ifoneofyourmethodsisinvokedwithaninvalidargument,youcouldthrowIllegalArgumentException,asubclassofRuntimeExceptioninjava.lang.

Othertimes,however,youwillwanttoconveymoreinformationabouttheabnormalconditionthanaclassfromjava.langwillallow.Usually,theclassoftheexceptionobjectitselfindicatesthetypeofabnormalconditionthatwasencountered.Forexample,ifathrownexceptionobjecthasclassIllegalArgumentException,thatindicatessomeonepassedanillegalargumenttoamethod.Sometimesyouwillwanttoindicatethatamethodencounteredanabnormalconditionthatisn'

trepresentedbyaclassintheThrowablefamilyofjava.lang.

Asanexample,imagineyouarewritingaJavaprogramthatsimulatesacustomerofavirtualcafé

drinkingacupofcoffee.Considertheexceptionalconditionsthatmightoccurwhilethecustomersips.TheclasshierarchyofexceptionsshowninFigure2representsafewpossibilities.

Figure2.Exceptionhierarchyforcoffeesipping

Ifthecustomerdiscovers,withdismay,thatthecoffeeiscold,yourprogramcouldthrowaTooColdException.Ontheotherhand,ifthecustomerdiscoversthatthecoffeeisoverlyhot,yourprogramcouldthrowaTooHotException.Theseconditionscouldbeexceptionsbecausetheyare（hopefully）notthenormalsituationinyourcafé

.（Exceptionalconditionsarenotnecessarilyrare,justoutsidethenormalflowofevents.）Thecodeforyournewexceptionclassesmightlooklikethis:

//InSourcePacketinfileexcept/ex1/TemperatureException.java

classTemperatureExceptionextendsException{

}

//InSourcePacketinfileexcept/ex1/TooColdException.java

classTooColdExceptionextendsTemperatureException{

//InSourcePacketinfileexcept/ex1/TooHotException.java

classTooHotExceptionextendsTemperatureException{

Thisfamilyofclasses,theTemperatureExceptionfamily,declaresthreenewtypesofexceptionsforyourprogramtothrow.Notethateachexceptionindicatesbyitsclassthekindofabnormalconditionthatwouldcauseittobethrown:

TemperatureExceptionindicatessomekindofproblemwithtemperature;

TooColdExceptionindicatessomethingwastoocold;

andTooHotExceptionindicatessomethingwastoohot.NotealsothatTemperatureExceptionextendsException--notThrowable,Error,oranyotherclassdeclaredinjava.lang.

Throwingexceptions

Tothrowanexception,yousimplyusethethrowkeywordwithanobjectreference,asin:

thrownewTooColdException（）;

ThetypeofthereferencemustbeThrowableoroneofitssubclasses.

Thefollowingcodeshowshowaclassthatrepresentsthecustomer,classVirtualPerson,mightthrowexceptionsifthecoffeedidn'

tmeetthecustomer'

stemperaturepreferences.NotethatJavaalsohasathrowskeywordinadditiontothethrowkeyword.Onlythrowcanbeusedtothrowanexception.Themeaningofthrowswillbeexplainedlaterinthisarticle.

//InSourcePacketinfileexcept/ex1/VirtualPerson.java

classVirtualPerson{

privatestaticfinalinttooCold=65;

privatestaticfinalinttooHot=85;

publicvoiddrinkCoffee（CoffeeCupcup）throws

TooColdException,TooHotException{

inttemperature=cup.getTemperature（）;

if（temperature<

=tooCold）{

thrownewTooColdException（）;

}

elseif（temperature>

=tooHot）{

thrownewTooHotException（）;

//...

//InSourcePacketinfileexcept/ex1/CoffeeCup.java

classCoffeeCup{

//75degreesCelsius:

thebesttemperatureforcoffee

privateinttemperature=75;

publicvoidsetTemperature（intval）{

temperature=val;

publicintgetTemperature（）{

returntemperature;

Catchingexceptions

TocatchanexceptioninJava,youwriteatryblockwithoneormorecatchclauses.Eachcatchclausespecifiesoneexceptiontypethatitispreparedtohandle.Thetryblockplacesafencearoundabitofcodethatisunderthewatchfuleyeoftheassociatedcatchers.Ifthebitofcodedelimitedbythetryblockthrowsanexception,theassociatedcatchclauseswillbeexaminedbytheJavavirtualmachine.Ifthevirtualmachinefindsacatchclausethatispreparedtohandlethethrownexception,theprogramcontinuesexecutionstartingwiththefirststatementofthatcatchclause.

Asanexample,consideraprogramthatrequiresoneargumentonthecommandline,astringthatcanbeparsedintoaninteger.WhenyouhaveaStringandwantanint,youcaninvoketheparseInt（）methodoftheIntegerclass.Ifthestringyoupassrepresentsaninteger,parseInt（）willreturnthevalue.Ifthestringdoesn'

trepresentaninteger,parseInt（）throwsNumberFormatException.Hereishowyoumightparseanintfromacommand-lineargument:

//InSourcePacketinfileexcept/ex1/Example1.java

classExample1{

publicstaticvoidmain（String[]args）{

inttemperature=0;

if（args.length>

0）{

try{

temperature=Integer.parseInt（args[0]）;

catch（NumberFormatExceptione）{

System.out.println（

"

Mustenterintegerasfirstargument."

）;

return;

else{

Mustentertemperatureasfirstargument."

//Createanewcoffeecupandsetthetemperatureof

//itscoffee.

CoffeeCupcup=newCoffeeCup（）;

cup.setTemperature（temperature）;

//Createandserveavirtualcustomer.

VirtualPersoncust=newVirtualPerson（）;

VirtualCafe.serveCustomer（cust,cup）;

Here,theinvocationofparseInt（）sitsinsideatryblock.AttachedtothetryblockisacatchclausethatcatchesNumberFormatException:

catch（NumberFormatExceptione）{

Thelowercasecharactereisareferencetothethrown（andcaught）NumberFormatExceptionobject.Thisreferencecouldhavebeenusedinsidethecatchclause,althoughinthiscaseitisn'

t.（Examplesofcatchclausesthatusethereferenceareshownlaterinthisarticle.）

IftheusertypesHarumphasthefirstargumenttotheExample1program,parseInt（）willthrowaNumberFormatExceptionexceptionandthecatchclausewillcatchit.Theprogramwillprint:

Mustenterintegerasfirstargument.

Althoughtheaboveexamplehadonlyonecatchclause,youcanhavemanycatchclausesassociatedwithasingletryblock.Here'

sanexample:

//InSourcePacketinfileexcept/ex1/VirtualCafe.java

classVirtualCafe{

publicstaticvoidserveCustomer（VirtualPersoncust,

CoffeeCupcup）{

cust.drinkCoffee（cup）;

System.out.println（"

Coffeeisjustright."