课件Java程序设计方案教程文件

Java程序设计JavaProgrammingSpring,20131成都信息工程学院计算机系ContentsWhyExceptions?WhatareExceptions?HandlingExceptionsCreatingExceptionTypes2chapter8ExceptionsWhyExceptions(异常)?Duringexecution(执行),programscanrunintomanykindsoferrors;Whatdowedowhenanerroroccurs?Javausesexceptionstoprovidetheerror-handlingcapabilitiesforitsprograms.3chapter8ExceptionsExceptions(异常)ErrorClassCritical(严重的)

errorwhichisnotacceptableinnormalapplicationprogram.

ExceptionClassPossibleexceptioninnormalapplicationprogramexecution;

Possibletohandlebyprogrammer.

4chapter8ExceptionsExceptions(异常)Treatexceptionasanobject.AllexceptionsareinstancesofaclassextendedfromThrowable

classoritssubclass.

Generally,aprogrammermakesnewexceptionclasstoextendtheExceptionclasswhichisasubclassofThrowableclass.6chapter8ExceptionsExceptionClass继承关系ThrowableErrorExceptionRuntimeExceptionIOExceptionObject7异常类的层次结构：8Classifying(分类)JavaExceptionsUncheckedExceptions(非检查性异常)Itisnotrequiredthatthesetypesofexceptionsbecaughtordeclaredonamethod.RuntimeexceptionscanbegeneratedbymethodsorbytheJVMitself.Errors

aregeneratedfromdeepwithintheJVM,andoftenindicateatrulyfatalstate.CheckedExceptions(检查性异常)Musteitherbecaughtbyamethodordeclaredinitssignaturebyplacingexceptionsinthemethodsignature.9Exception的分类非检查性异常(uncheckedexception)：以RuntimeException为代表的一些类，编译时发现不了，只在能运行时才能发现。检查性异常(checkedexception)：一般程序中可预知的问题，其产生的异常可能会带来意想不到的结果，因此Java编译器要求Java程序必须捕获或声明所有的非运行时异常。以IOException为代表的一些类。如果代码中存在检查性异常，必须进行异常处理，否则编译不能通过。如：用户连接数据库SQLException、FileNotFoundException。10chapter8ExceptionsException的分类什么是RuntimeException：Java虚拟机在运行时生成的异常，如：被0除等系统错误、数组下标超范围等，其产生比较频繁，处理麻烦，对程序可读性和运行效率影响太大。因此由系统检测,用户可不做处理，系统将它们交给默认的异常处理程序(当然，必要时，用户可对其处理)。Java程序异常处理的原则是：对于Error和RuntimeException，可以在程序中进行捕获和处理，但不是必须的。对于IOException及其他异常，必须在程序进行捕获和处理。异常处理机制主要处理检查性异常。11chapter8ExceptionsJavaExceptionTypeHierarchy(层次)virtualmachineerrors12系统异常类的层次结构：13chapter8ExceptionsException的分类System-DefinedException(系统定义的异常)Programmer-DefinedException(程序员自定义异常)14chapter8ExceptionsSystem-DefinedException

(系统定义的异常)Raisedimplicitlybysystembecauseofillegalexecutionofprogram;CreatedbyJavaSystemautomatically;ExceptionextendedfromErrorclassandRuntimeExceptionclass.

15chapter8ExceptionsSystem-DefinedExceptionIndexOutOfBoundsException:Whenbeyondtheboundofindexintheobjectwhichuseindex,suchasarray,string,andvectorArrayStoreException:WhenassignobjectofincorrecttypetoelementofarrayNegativeArraySizeException:

WhenusinganegativesizeofarrayNullPointerException:WhenrefertoobjectasanullpointerSecurityException:Whenviolatesecurity.CausedbysecuritymanagerIllegalMonitorStateException:Whenthethreadwhichisnotownerofmonitorinvolveswaitornotifymethod16Programmer-DefinedException

(程序员自定义异常)

Exceptionsraisedbyprogrammer

SubclassofException

classCheckbycompilerwhethertheexceptionhandlerforexceptionoccurredexistsornotIfthereisnohandler,itiserror.

17chapter8ExceptionsUser-definedExceptions

(用户自定义异常)classUserErrextendsException

{ …}CreatingExceptionTypes:18Example1:publicclassInsufficientFundsException

extendsException

{ privateBankAccountexcepbank;privatedoubleexcepAmount;

InsufficientFundsException(Bankba,doubledAmount) { excepbank=ba;excepAmount=dAmount; }}19chapter8Exceptions程序运行时发生异常，系统会抛出异常，如果程序中未处理和捕获异常，异常抛出后程序运行中断。publicclassTest{publicint[]bar(){ inta[]=newint[2]; for(intx=0;x<=2;x++){

a[x]=0;//抛出异常，程序中断} returna;}publicstaticvoidmain(String[]args){ Testt=newTest(); t.bar(); //程序中断 System.out.println(“Method:foo”);//不被执行}}系统给出的错误信息：Exceptioninthread"main"java.lang.ArrayIndexOutOfBoundsException:2atexception.Test.bar(Test.java:7)atexception.Test.main(Test.java:25)20HandlingExceptions(处理异常)Throwinganexception(抛出异常)Whenanerroroccurswithinamethod.Anexceptionobjectiscreatedandhandedofftotheruntimesystem(运行时系统).Theruntimesystemmustfindthecodetohandletheerror.Catchinganexception(捕获异常)Theruntimesystem(运行时系统)searchesforcodetohandlethethrownexception.Itcanbeinthesamemethodorinsomemethodinthecall(调用)stack(堆栈).21chapter8ExceptionsKeywordsforJavaExceptionstry

Marksthestartofablockassociatedwithasetofexceptionhandlers.catch

Iftheblockenclosedbythetrygeneratesanexceptionofthistype,controlmoveshere;watchoutforimplicitsubsumption.finally

Alwayscalledwhenthetryblockconcludes,andafteranynecessarycatchhandleriscomplete.throws

Describestheexceptionswhichcanberaisedbyamethod.throw

Raisesanexceptiontothefirstavailablehandlerinthecallstack,unwindingthestackalongtheway.22抛出异常–throw,throws在一个方法的运行过程中，如果一个语句引起了错误时，含有这个语句的方法就会创建一个包含有关异常信息的异常对象，并将它传递给Java运行时系统。我们把生成异常对象并把它提交给运行时系统的过程称为抛出(throw)异常。throw

—在方法体中用throw手工抛出异常；throws—在方法头部间接抛出异常，即：申明方法中可能抛出的异常。231.在方法体中用throw手工抛出异常throw抛出异常,可以是系统定义的异常，也可以是用户自定义的异常。语法格式：或例如:下面语句就抛出了一个IOException异常：thrownewIOException()；异常类名对象名＝new异常类构造函数；throw对象名；thrownew异常类构造函数；24throw

－ExampleclassThrowStatementextendsException{publicstaticvoidexp(intptr){ try{ if(ptr==0)

thrownewNullPointerException();

} catch(NullPointerExceptione){}}

publicstaticvoidmain(String[]args){inti=0;

ThrowStatement.exp(i);}}252.throws—间接抛出异常(申明异常)一个方法不处理它产生的异常,而是沿着调用层次向上传递,由调用它的方法来处理这些异常,叫声明异常。在定义方法时用throws关键字将方法中可能产生的异常间接抛出。若一个方法可能引发一个异常，但它自己却没有处理，则应该声明异常，并让其调用者来处理这个异常，这时就需要用throws关键字来指明方法中可能引发的所有异常。类型方法名(参数列表)throws

异常列表{ //…代码}26Example1：publicclassExceptionTest{ voidProc(intsel)throwsArrayIndexOutOfBoundsException{

System.out.println(“InSituation"+sel); if(sel==1){ intiArray[]=newint[4];iArray[10]=3;//抛出异常 }

}

}27异常向上传递－ExampleclassThrowStatementextendsException{publicstaticvoidexp(intptr)throws

NullPointerException{if(ptr==0)

thrownewNullPointerException();}

publicstaticvoidmain(String[]args){inti=0;

ThrowStatement.exp(i);}}运行结果：

java.lang.NullPointerExceptionatThrowStatement.exp(ThrowStatement.java:4)atThrowStatement.main(ThrowStatement.java:8)28Exceptions-throwingmultiple(多个)

exceptionsAMethodcanthrowmultipleexceptions.Multipleexceptionsareseparatedbycommasafterthethrowskeyword:publicclassMyClass{ publicintcomputeFileSize()

throwsIOException,ArithmeticException { ...}}29HandlingExceptions在一个方法中，对于可能抛出的异常，处理方式有两种：一个方法不处理它产生的异常,只在方法头部声明使用throws抛出异常，使异常沿着调用层次向上传递,由调用它的方法来处理这些异常。用try-catch-finally语句对异常及时处理；30HandlingExceptionspublicvoidreplaceValue(Stringname,Objectvalue)

throwsNoSuchAttributeException{Attrattr=find(name);if(attr==null)

thrownewNoSuchAttributeException(name);

attr.setValue(value);}try{…

replaceValue(“att1”,“newValue”);…}catch(NoSuchAttributeExceptione){e.printStackTrace();}当replaceValue(…)方法被调用时，调用者需处理异常。31处理异常语句try-catch-finally的基本格式为：try{ //可能产生异常的代码；}//不能有其它语句分隔catch(异常类名异常对象名){ //异常处理代码；//要处理的第一种异常}catch(异常类名异常对象名){ //异常处理代码；//要处理的第二种异常}…finally{ //最终处理（缺省处理）}32Exceptions–Syntax(语法)示例try{

//Codewhichmightthrowanexception //...}catch(FileNotFoundExceptionx){

//codetohandleaFileNotFoundexception}catch(IOExceptionx){

//codetohandleanyotherI/Oexceptions}catch(Exceptionx){

//Codetocatchanyothertypeofexception}finally{

//ThiscodeisALWAYSexecutedwhetheranexceptionwasthrown //ornot.Agoodplacetoputclean-upcode.ie.close //anyopenfiles,etc...}33HandlingExceptions(处理异常)try-catch或try-catch-finally.Threestatementshelpdefinehowexceptionsarehandled:tryidentifiesablockofstatementswithinwhichanexceptionmightbethrown;Atrystatementcanhavemultiplecatchstatementsassociatedwithit.catchmustbeassociatedwithatrystatementandidentifiesablockofstatementsthatcanhandleaparticulartypeofexception.Thestatementsareexecutedifanexceptionofaparticulartypeoccurswithinthetryblock.34HandlingExceptionsfinally(可选项)mustbeassociatedwithatrystatementandidentifiesablockofstatementsthatareexecutedregardlessofwhetherornotanerroroccurswithinthetryblock.Evenifthetryandcatchblockhaveareturnstatementinthem,finallywillstillrun.35用try-catch-finally语句对异常及时处理－ExampleclassThrowStatementextendsException{publicstaticvoidexp(intptr){ try{ if(ptr==0)

thrownewNullPointerException(); } catch(NullPointerExceptione){}}

publicstaticvoidmain(String[]args){inti=0;

ThrowStatement.exp(i);}}36系统抛出异常后，捕获异常，运行finally块，程序运行继续。publicclassTest{ publicvoidfoo(){

try{ inta[]=newint[2];

a[4]=1;/*causesaruntimeexceptionduetotheindex*/System.out.println(“Method:foo”);//若异常发生，不执行}catch(ArrayIndexOutOfBoundsExceptione){System.out.println("exception:"+e.getMessage()); e.printStackTrace();}finally{System.out.println("Finallyblockalwaysexecute!!!");}}publicstaticvoidmain(String[]args){Testt=newTest();t.foo();

System.out.println(“ExcecutionafterException!!!”);

//继续执行}}37运行结果：exception:4java.lang.ArrayIndexOutOfBoundsException:4 atexception.Test.foo(Test.java:8) atexception.Test.main(Test.java:20)Finallyblockalwaysexecute!!!ExcecutionafterException!!!38Exceptions-throwingmultipleexceptionspublicvoidmethod1(){ MyClassanObject=newMyClass(); try{ inttheSize=anOputeFileSize(); }catch(ArithmeticExceptionx){ //... }catch(IOExceptionx){ //...}}39Exceptions-catchingmultipleexceptionsEachtryblockcancatchmultipleexceptions.StartwiththemostspecificexceptionsFileNotFoundExceptionisasubclassofIOExceptionItMUSTappearbeforeIOException

inthecatchlistpublicvoidmethod1(){ FileInputStreamaFile; try{ aFile=newFileInputStream(...); intaChar=aFile.read(); //... }catch(FileNotFoundExceptionx){ //... }catch(IOExceptionx){ //...}}40Exception-Thecatch-allHandlerSinceallExceptionclassesareasubclassoftheExceptionclass,acatchhandlerwhichcatches"Exception"willcatchallexceptions.ItmustbethelastinthecatchList.publicvoidmethod1(){ FileInputStreamaFile; try{ aFile=newFileInputStream(...); intaChar=aFile.read(); //... } catch(IOExceptionx){ //... } catch(Exceptionx){ //CatchAllExceptions}}41User-definedExceptions(用户自定义异常)的处理classUserErrextendsException{ …}用户自定义异常是检查性异常(checkedexception)，必须用throw手工抛出并处理。classUserClass{ …

UserErr

x=newUserErr();...if(val<1)

throw

x;}42//ThrowExample.javaclassIllegalValueException extendsException{}

classUserTrial{intval1,val2;publicUserTrial(inta,intb){val1=a;val2=b;}voidshow()throwsIllegalValueException{

if((val1<0)||(val2>0)) thrownewIllegalValueException();System.out.println(“Value1=”+val1);//不运行System.out.println("Value2="+val2);}}classThrowExample{publicstaticvoidmain(Stringargs[]){

UserTrialvalues=newUserTrial(-1,1);try{ values.show();}catch(IllegalValueExceptione){Syst