java基础教程6-1异常

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1、例一：publicclassTestFor{publicstaticvoidmain(String[]arg){for(inti=1,j=i+10;i<5;i++){j=i*2;System.out.println("i="+i+"j="+j);}}}例二、publicclassTestFor{publicstaticvoidmain(String[]arg){for(inti=1,j=i+10;i<5;i++,j=i*2)System.out.println("i="+i+"j="+j);}}例三：publicclassTestBreak{publicstaticvoidmai

2、n(String[]arg){Stringoutput="";inti;for(i=1;i<=10;i++){if(i==5)break;output+=i+"";}output+="Brokeoutofloopati="+1;System.out.println(output);}}例四：publicclassTestContinue{publicstaticvoidmain(String[]arg){Stringoutput="";inti;for(i=1;i<=10;i++){if(i==5)continue;output+=i+"";}output+="Broke

3、outofloopati="+1;System.out.println(output);}}第四章Java的异常处理4.1什么是异常异常就是在程序的运行过程中所发生的异常事件，它中断指令的正常执行。没有处理错误的程序:read-file{openTheFile;determineitssize;allocatethatmuchmemory;closeTheFile;}以常规方法处理错误openFiles;if(theFilesOpen){determinethelenthofthefile;if(gotTheFileLength){allocatethatmuchmemory;i

4、f(gotEnoughMemory){readthefileintomemory;if(readFailed)errorCode=-1;elseerrorCode=-2;}elseerrorCode=-3;}elseerrorCode=-4;}elseerrorCode=-5;观察前面的程序你会发现大部分精力花在出错处理上了.只把能够想到的错误考虑到,对以外的情况无法处理程序可读性差出错返回信息量太少用异常的形式处理错误read-File;{try{openTheFile;determineitssize;allocatethatmuchmemory;closeTheFile;}

5、catch(fileopenFailed){dosomething;}catch(sizeDetermineFailed){dosomething;}catch(memoryAllocateFailed){dosomething;}catch(readFailed){dosomething;}catch(fileCloseFailed){dosomething;}}和传统的方法比较异常的优点:1.把错误代码从常规代码中分离出来2.把错误传播给调用堆栈3.按错误类型和错误差别分组4.系统提供了对于一些无法预测的错误的捕获和处理5.克服了传统方法的错误信息有限的问题4．1．1异常示例

6、importjava.io.*;classExceptionDemo1{publicstaticvoidmain(Stringargs[]){FileInputStreamfis=newFileInputStream("text");intb;while((b=fis.read())!=-1){System.out.print(b);}fis.close();}}//编译时异常G:javaExceptionDemo1.java:5:unreportedexceptionjava.io.FileNotFoundException;mustbecaughtordeclaredto

7、bethrown{FileInputStreamfis=newFileInputStream("text");^G:javaExceptionDemo1.java:7:unreportedexceptionjava.io.IOException;mustbecaughtordeclaredtobethrownwhile((b=fis.read())!=-1)^G:javaExceptionDemo1.java:11:unreportedexceptionjava.io.I