动手动脑4

异常处理：

　　java中通过try····catch语句来实现抛出捕捉异常来实现程序的正常运行，异常包括error和exception异常错误。可以通过自定义异常类，自定义异常类继承exception类，来实现异常处理的操作。

实例代码：

import javax.swing.*;

class AboutException {
   public static void main(String[] a) 
   {
      int i=1, j=0, k;
      k=i/j;


    try
    {
        k = i/j;  
    }
    
    catch ( ArithmeticException e)
    {
        System.out.println("被0除.  "+ e.getMessage());
    }
    
    catch (Exception e)
    {
        if (e instanceof ArithmeticException)
            System.out.println("被0除");
        else
        {  
            System.out.println(e.getMessage());
            
        }
    }

    
    finally
     {
             JOptionPane.showConfirmDialog(null,"OK");
     }
        
  }
}

多层异常捕获：

　　对于多层异常捕获，是寻找跑出的异常如果外层没有，继续跳出，知道找到抛出的错误。如果找到了，则继续进行以下的代码。

源代码：

package lianxi;

public class APP{

    public static void main(String[] args) {
         try { 
             try { 
                 throw new ArrayIndexOutOfBoundsException(); 
             } 
             catch(ArithmeticException e) { 
                    System.out.println(  "ArrayIndexOutOfBoundsException" +  "/内层try-catch"); 
             }

         throw new ArithmeticException(); 
     } 
     catch(ArithmeticException e) { 
         System.out.println("发生ArithmeticException"); 
     } 
     catch(ArrayIndexOutOfBoundsException e) { 
        System.out.println(  "ArrayIndexOutOfBoundsException" + "/外层try-catch"); 
    }
    }
}
结果：
ArrayIndexOutOfBoundsException/外层try-catch

public class APP{

    public static void main(String[] args) {
         try { 
             try { 
                 throw new ArrayIndexOutOfBoundsException(); 
             } 
             catch(ArrayIndexOutOfBoundsException e) { 
                    System.out.println(  "ArrayIndexOutOfBoundsException" +  "/内层try-catch"); 
             }

         throw new ArithmeticException(); 
     } 
     catch(ArithmeticException e) { 
         System.out.println("发生ArithmeticException"); 
     } 
     catch(ArrayIndexOutOfBoundsException e) { 
        System.out.println(  "ArrayIndexOutOfBoundsException" + "/外层try-catch"); 
    }
    }
}

结果：

ArrayIndexOutOfBoundsException/内层try-catch
发生ArithmeticException

　　当多个finally嵌套时，抛出一条异常，系统会实现捕捉该条异常，之后会依次执行finally中的语句。

源代码：

public class Jiecheng {
    public static void main(String[] args) {
    int result;
        try {
                  System.out.println("in Level 1");
             try {
                System.out.println("in Level 2");   
                 try {            
                     System.out.println("in Level 3");             
                     result=100/0;  //Level 3      
                }       
                catch (Exception e) {              
                    System.out.println("Level 3:" + e.getClass().toString());              
                }       
                finally {      
                    System.out.println("In Level 3 finally");        
                }         
                }
            catch (Exception e) {          
                 System.out.println("Level 2:" + e.getClass().toString());
             }
             finally {        
                System.out.println("In Level 2 finally");
             }
        } 
        catch (Exception e) {
            System.out.println("Level 1:" + e.getClass().toString());
        }
        finally {
             System.out.println("In Level 1 finally");
        }
    }
}
运行结果：
in Level 1
in Level 2
in Level 3
Level 3:class java.lang.ArithmeticException
In Level 3 finally
In Level 2 finally
In Level 1 finally

　　finally语句不一定会执行，可以在之前实行终止程序的语句System.exit(0);

public class Jiecheng {

      
        public static void main(String[] args)
        {      
            try{           
                System.out.println("in main");       
                throw new Exception("Exception is thrown in main");        
            }     
            catch(Exception e)
                {
                System.out.println(e.getMessage());  
                System.exit(0);
            }
            finally
            {  
                System.out.println("in finally");
            }
        }
}
运行结果：
in main
Exception is thrown in main