java语言程序设计基础篇课件(第13章)英文

Chapter13ExceptionHandling1MotivationsWhenaprogramrunsintoaruntimeerror,theprogramterminatesabnormally.Howcanyouhandletheruntimeerrorsothattheprogramcancontinuetorunorterminategracefully?Thisisthesubjectwewillintroduceinthischapter.2

ObjectivesTogetanoverviewofexceptionsandexceptionhandling(§13.2).Toexploretheadvantagesofusingexceptionhandling(§13.3).Todistinguishexceptiontypes:Error(fatal)vs.Exception(nonfatal),andcheckedvs.unchecked(§13.4).Todeclareexceptionsinamethodheader(§13.5.1).Tothrowexceptionsinamethod(§13.5.2).Towriteatry-catchblocktohandleexceptions(§13.5.3).Toexplainhowanexceptionispropagated(§13.5.3).Tousethefinallyclauseinatry-catchblock(§13.6).Touseexceptionsonlyforunexpectederrors(§13.7).Torethrowexceptionsinacatchblock(§13.8).Tocreatechainedexceptions(§13.9).Todefinecustomexceptionclasses(§13.10).3Exception-HandlingOverview4QuotientRunQuotientWithIfRunQuotientWithExceptionRunShowruntimeerrorFixitusinganifstatementWhatiftheruntimeerroroccursinacalledmethod?ExceptionAdvantages5QuotientWithMethodRunNowyouseetheadvantagesofusingexceptionhandling.Itenablesamethodtothrowanexceptiontoitscaller.Withoutthiscapability,amethodmusthandletheexceptionorterminatetheprogram.HandlingInputMismatchException6InputMismatchExceptionDemoRunByhandlingInputMismatchException,yourprogramwillcontinuouslyreadaninputuntilitiscorrect.ExceptionTypes7SystemErrors8SystemerrorsarethrownbyJVMandrepresentedintheErrorclass.TheErrorclassdescribesinternalsystemerrors.Sucherrorsrarelyoccur.Ifonedoes,thereislittleyoucandobeyondnotifyingtheuserandtryingtoterminatetheprogramgracefully.

Exceptions9Exceptiondescribeserrorscausedbyyourprogramandexternalcircumstances.Theseerrorscanbecaughtandhandledbyyourprogram.RuntimeExceptions10RuntimeExceptioniscausedbyprogrammingerrors,suchasbadcasting,accessinganout-of-boundsarray,andnumericerrors.CheckedExceptionsvs.UncheckedExceptions11RuntimeException,Errorandtheirsubclassesareknownasunchecked

exceptions.Allotherexceptionsareknownascheckedexceptions,meaningthatthecompilerforcestheprogrammertocheckanddealwiththeexceptions.

UncheckedExceptions12Inmostcases,uncheckedexceptionsreflectprogramminglogicerrorsthatarenotrecoverable.Forexample,aNullPointerExceptionisthrownifyouaccessanobjectthroughareferencevariablebeforeanobjectisassignedtoit;anIndexOutOfBoundsExceptionisthrownifyouaccessanelementinanarrayoutsidetheboundsofthearray.Thesearethelogicerrorsthatshouldbecorrectedintheprogram.Uncheckedexceptionscanoccuranywhereintheprogram.Toavoidcumbersomeoveruseoftry-catchblocks,Javadoesnotmandateyoutowritecodetocatchuncheckedexceptions.UncheckedExceptions13Uncheckedexception.Declaring,Throwing,andCatchingExceptions14DeclaringExceptionsEverymethodmuststatethetypesofcheckedexceptionsitmightthrow.Thisisknownasdeclaringexceptions.publicvoidmyMethod()throwsIOExceptionpublicvoidmyMethod()throwsIOException,OtherException15ThrowingExceptionsWhentheprogramdetectsanerror,theprogramcancreateaninstanceofanappropriateexceptiontypeandthrowit.Thisisknownasthrowinganexception.Hereisanexample,thrownewTheException();TheExceptionex=newTheException();

throwex;16ThrowingExceptionsExample

/**Setanewradius*/publicvoidsetRadius(doublenewRadius)

throwsIllegalArgumentException{if(newRadius>=0)radius=newRadius;else

thrownewIllegalArgumentException("Radiuscannotbenegative");}17CatchingExceptionstry{statements;//Statementsthatmaythrowexceptions}catch(Exception1exVar1){handlerforexception1;}catch(Exception2exVar2){handlerforexception2;}...catch(ExceptionNexVar3){handlerforexceptionN;}

18CatchingExceptions19CatchorDeclareCheckedExceptionsJavaforcesyoutodealwithcheckedexceptions.Ifamethoddeclaresacheckedexception(i.e.,anexceptionotherthanErrororRuntimeException),youmustinvokeitinatry-catchblockordeclaretothrowtheexceptioninthecallingmethod.Forexample,supposethatmethodp1invokesmethodp2andp2maythrowacheckedexception(e.g.,IOException),youhavetowritethecodeasshownin(a)or(b).20Example:Declaring,Throwing,andCatchingExceptionsObjective:Thisexampledemonstratesdeclaring,throwing,andcatchingexceptionsbymodifyingthesetRadiusmethodintheCircleclassdefinedinChapter8.ThenewsetRadiusmethodthrowsanexceptionifradiusisnegative.21TestCircleWithExceptionRunCircleWithExceptionRethrowingExceptionstry{statements;}catch(TheExceptionex){performoperationsbeforeexits;throwex;}22ThefinallyClausetry{statements;}catch(TheExceptionex){handlingex;}finally{finalStatements;}23TraceaProgramExecutiontry{statements;}catch(TheExceptionex){handlingex;}finally{finalStatements;}Nextstatement;24animationSupposenoexceptionsinthestatementsTraceaProgramExecutiontry{statements;}catch(TheExceptionex){handlingex;}finally{finalStatements;}Nextstatement;25animationThefinalblockisalwaysexecutedTraceaProgramExecutiontry{statements;}catch(TheExceptionex){handlingex;}finally{finalStatements;}Nextstatement;26animationNextstatementinthemethodisexecutedTraceaProgramExecutiontry{statement1;statement2;statement3;}catch(Exception1ex){handlingex;}finally{finalStatements;}Nextstatement;27animationSupposeanexceptionoftypeException1isthrowninstatement2TraceaProgramExecutiontry{statement1;statement2;statement3;}catch(Exception1ex){handlingex;}finally{finalStatements;}Nextstatement;28animationTheexceptionishandled.TraceaProgramExecutiontry{statement1;statement2;statement3;}catch(Exception1ex){handlingex;}finally{finalStatements;}Nextstatement;29animationThefinalblockisalwaysexecuted.TraceaProgramExecutiontry{statement1;statement2;statement3;}catch(Exception1ex){handlingex;}finally{finalStatements;}Nextstatement;30animationThenextstatementinthemethodisnowexecuted.TraceaProgramExecutiontry{statement1;statement2;statement3;}catch(Exception1ex){handlingex;}catch(Exception2ex){handlingex;throwex;}finally{finalStatements;}Nextstatement;31animationstatement2throwsanexceptionoftypeException2.TraceaProgramExecutiontry{statement1;statement2;statement3;}catch(Exception1ex){handlingex;}catch(Exception2ex){handlingex;throwex;}finally{finalStatements;}Nextstatement;32animationHandlingexceptionTraceaProgramExecutiontry{statement1;statement2;statement3;}catch(Exception1ex){handlingex;}catch(Exception2ex){handlingex;throwex;}finally{finalStatements;}Nextstatement;33animationExecutethefinalblockTraceaProgramExecutiontry{statement1;statement2;statement3;}catch(Exception1ex){handlingex;}catch(Exception2ex){handlingex;throwex;}finally{finalStatements;}Nextstatement;34animationRethrowtheexceptionandcontrolistransferredtothecallerCautionsWhenUsingExceptionsExceptionhandlingseparateserror-handlingcodefromnormalprogrammingtasks,thusmakingprogramseasiertoreadandtomodify.Beaware,however,thatexceptionhandlingusuallyrequiresmoretimeandresourcesbecauseitrequiresinstantiatinganewexceptionobject,rollingbackthecallstack,andpropagatingtheerrorstothecallingmethods.35WhentoThrowExceptionsAnexceptionoccursinamethod.Ifyouwanttheexceptiontobeprocessedbyitscaller,youshouldcreateanexceptionobjectandthrowit.Ifyoucanhandletheexceptioninthemethodwhereitoccurs,thereisnoneedtothrowit.36WhentoUseExceptionsWhenshouldyouusethetry-catchblockinthecode?Youshoulduseittodealwithunexpectederrorconditions.Donotuseittodealwithsimple,expectedsituations.Forexample,thefollowingcode37try{System.out.println(refVar.toString());}catch(NullPointerExceptionex){System.out.println("refVarisnull");}WhentoUseExceptionsisbettertobereplacedby38if(refVar!=null)System.out.println(refVar.toString());elseSystem.out.println("refVarisnull");DefiningCustomExceptionClasses39UsetheexceptionclassesintheAPIwheneverpossible.Definecustomexceptionclassesifthepredefinedclassesarenotsufficient.DefinecustomexceptionclassesbyextendingExceptionorasubclassofException.CustomExceptionClassExample40RunInvalidRadiusExceptionInListing13.8,thesetRadiusmethodthrowsanexceptioniftheradiusisnegative.Supposeyouwishtopasstheradiustothehandler,youhavetocreateacustomexceptionclass.CircleWithRadiusExceptionTestCircleWithRadiusExceptionAssertionsAnassertionisaJavastatementthatenablesyoutoassertanassumptionaboutyourprogram.AnassertioncontainsaBooleanexpressionthatshouldbetrueduringprogramexecution.Assertionscanbeusedtoassureprogramcorrectnessandavoidlogicerrors.

41CompanionWebsiteDeclaringAssertionsAnassertionisdeclaredusingthenewJavakeywordassertinJDK1.4asfollows:assertassertion;orassertassertion:detailMessage;whereassertionisaBooleanexpressionanddetailMessageisaprimitive-typeoranObjectvalue.42CompanionWebsiteExecutingAssertionsWhenanassertionstatementisexecuted,Javaevaluatestheassertion.Ifitisfalse,anAssertionErrorwillbethrown.TheAssertionErrorclasshasano-argconstructorandsevenoverloadedsingle-argumentconstructorsoftypeint,long,float,double,boolean,char,andObject.Forthefirstassertstatementwithnodetailmessage,theno-argconstructorofAssertionErrorisused.Forthesecondassertstatementwithadetailmessage,anappropriateAssertionErrorconstructorisusedtomatchthedatatypeofthemessage.SinceAssertionErrorisasubclassofError,whenanassertionbecomesfalse,theprogramdisplaysamessageontheconsoleandexits.43CompanionWebsiteExecutingAssertionsExamplepublicclassAssertionDemo{publicstaticvoidmain(String[]args){inti;intsum=0;for(i=0;i<10;i++){sum+=i;}asserti==10;assertsum>10&&sum<5*10:"sumis"+sum;}}44CompanionWebsiteCompilingProgramswithAssertionsSinceassertisanewJavakeywordintroducedinJDK1.4,youhavetocompiletheprogramusingaJDK1.4compiler.Furthermore,youneedtoincludetheswitch–source1.4inthecompilercommandasfollows:javac–source1.4AssertionDemo.javaNOTE:IfyouuseJDK1.5,thereisnoneedtousethe–source1.4optioninthecommand.45CompanionWebsiteRunningProgramswithAssertionsBydefault,theassertionsaredisabledatruntime.Toenableit,usetheswitch–enableassertions,or–eaforshort,asfollows:java–eaAssertionDemoAssertionscanbeselectivelyenabledordisabledatclasslevelorpackagelevel.Thedisableswitchis–disableassertionsor–daforshort.Forexample,thefollowingcommandenablesassertionsinpackagepackage1anddisablesassertionsinclassClass1.java–ea:package1–da:Class1AssertionDemo46CompanionWebsiteUsingExceptionHandlingorAssertionsAssertionshouldnotbeusedtoreplaceexceptionhandling.Exceptionhandlingdealswithunusualcircumstancesduringprogramexecution.Assertionsaretoassurethecorrectnessoftheprogram.Exceptionhandlingaddressesrobustnessand