JAVA的异常处理

一、观察代码，了解异常处理机制

import javax.swing.*;

class AboutException {
   public static void main(String[] a) 
   {
      int i=1, j=0,k;
//      k=i/j;
    try
    {
        k = i/j;    // Causes division-by-zero exception
        throw new Exception("Hello.Exception!");
    }
    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//finally会执行地代码
     {
             JOptionPane.showConfirmDialog(null,"OK");
     }        
  }
}

输出：被0除.  / by zero

从中我们可以看出，在try中一共有两个异常，但最终只执行了ArithmeticException异常，这就代表着try的运行就如同他的名字一样，尝试一下，遇到错误的话就抛出exception，而catch会根据exception的种类不同捕捉异常，最后会输出相应的错误类型，而finally是无论如何都会执行。

  有关JAVA中抛出的exception类型：

在上面的代码中我们可以看到，try遇到错误的时候会抛出exception错误，exception的类型在如下：

 

 二，浮点数的异常

public class ThrowDemo { 
    public static void main(String[] args) { 
        try {
            double data = 100 / 0.0;
            System.out.println("浮点数除以零：" + data); 
            if(String.valueOf(data).equals("Infinity"))
            { 
                System.out.println("In Here" ); 
                throw new ArithmeticException("除零异常");
            }
        } 
        catch(ArithmeticException e) { 
            System.out.println(e); 
        } 
    } 
}

输出：

浮点数除以零：Infinity

In Here
java.lang.ArithmeticException: 除零异常

为啥会输出Infinity呢？

因为浮点数在除0时，他不是把他当做0而是看做是最小值，因此得出的结果为极大值。

 三、多层异常的捕获

public class CatchWho { 
    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"); 
        } 
    } 
}

输出：

ArrayIndexOutOfBoundsException/内层try-catch
发生ArithmeticException

当内层的try抛出异常，会被最近的符合要求的catch捕获，之后又抛出一个新的异常，再被离他最近的符合要求的catch捕获，因此输出如上

再看接下来的多层异常捕获

public class CatchWho2 { 
    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"); 
        } 
    } 
}

输出：

ArrayIndexOutOfBoundsException/外层try-catch

在最内层的异常抛出后，内层没有可以接受异常的catch，就跑到外层的try结构里，发现也没有，就跳出外层的try，被外层的catch接受，输出上面的内容。

四、多层嵌套导致finaly执行顺序的差异

public class EmbededFinally {

    
    public static void main(String args[]) {
        
        int result;
        
        try {
            
            System.out.println("in Level 1");
            //result=100/0;  //Level 1
           
             try {
                
                System.out.println("in Level 2");
                //result=100/0;  //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");
                
                }
                
               
                 result=100/0;  //Level 2

            
                }
            
            catch (Exception e) {
               
                 System.out.println("Level 2:" + e.getClass().toString());
           
             }
             finally {
                
                System.out.println("In Level 2 finally");
           
             }
             
             result = 100 / 0;  //level 1
        
        } 
        
        catch (Exception e) {
            
            System.out.println("Level 1:" + e.getClass().toString());
        
        }
        
        finally {
           
             System.out.println("In Level 1 finally");
        
        }
    
    }

}

输出：

in Level 1
in Level 2
in Level 3
Level 3:class java.lang.ArithmeticException
In Level 3 finally
Level 2:class java.lang.ArithmeticException
In Level 2 finally
Level 1:class java.lang.ArithmeticException
In Level 1 finally

通过不同的异常导致有些try语句块未能执行，其原因在于发生异常的try块外有无final语句，如果有就会执行，那么凡是在发生异常的try语句块里的finaly语句将都不会执行。

话说finaly语句块一定会执行吗？

public class SystemExitAndFinally {

    
    public static void main(String[] args)
    {
        
        try{

            
            System.out.println("in main");
            
            throw new Exception("Exception is thrown in main");

                    //System.exit(0);

        
        }
        
        catch(Exception e)

            {
            
            System.out.println(e.getMessage());
            
            System.exit(0);
        
        }
        
        finally
        
        {
            
            System.out.println("in finally");
        
        }
    
    }


}

输出：

in main
Exception is thrown in main

当发生异常后，catch捕获后自动结束程序，那么之后的finaly语句块就不会被执行了。

// UsingExceptions.java
// Demonstrating the getMessage and printStackTrace
// methods inherited into all exception classes.
public class PrintExceptionStack {
   public static void main( String args[] )
   {
      try {
         method1();
      }
      catch ( Exception e ) {
         System.err.println( e.getMessage() + "\n" );
         e.printStackTrace();
      }
   }

   public static void method1() throws Exception
   {
      method2();
   }

   public static void method2() throws Exception
   {
      method3();
   }

   public static void method3() throws Exception
   {
      throw new Exception( "Exception thrown in method3" );
   }
}

输出：

Exception thrown in method3

java.lang.Exception: Exception thrown in method3
at PrintExceptionStack.method3(PrintExceptionStack.java:28)
at PrintExceptionStack.method2(PrintExceptionStack.java:23)
at PrintExceptionStack.method1(PrintExceptionStack.java:18)
at PrintExceptionStack.main(PrintExceptionStack.java:8)

通过printStackTrace()打印方法调用堆栈，我们可以跟踪到程序的出错来源和传播路径

五、自定义异常

package com.test1;
import java.util.Scanner;

//自定义的异常
class MyException extends Exception
{
    public MyException(String Message) {
        super(Message);
    }
    public MyException(String message, Throwable cause) {
        super(message, cause);
    }
     public MyException( Throwable cause) {
        super(cause);
    }
}

class Sanj
{
    private double x,y,z;

    public Sanj(double x, double y, double z) {
        super();
        this.x = x;
        this.y = y;
        this.z = z;
    }
    //调用异常的方法
    public void getArea() throws Exception 
    {
        int flag=0;
        if(this.x+this.y<=this.z||Math.abs(this.x-this.y)>=this.z)
        {
            flag=1;
        }
        if(flag==1)
        {
            throw new MyException("NotSanjiaoException");
        }
        else
        {
            double sum=(this.x+this.y+this.z)*1.0/2;
            double ans=Math.sqrt(sum*(sum-this.x)*(sum-this.y)*(sum-this.z));
            System.out.println("三角形面积为："+ans);
        }
    }
    public void showInfo() throws Exception
    {
        int flag=0;
        if(this.x+this.y<=this.z||Math.abs(this.x-this.y)>=this.z)
        {
            flag=1;
        }
        if(flag==1)
        {
            throw new MyException("NotSanjiaoException");
        }
        else
        {
            System.out.println("三边长分别是："+this.x+" "+this.y+" "+this.z);
        }
    }
}
public class Demo1 {

    public static void main(String[] args) {
        // TODO Auto-generated method stub
        double a,b,c;
        Scanner in=new Scanner(System.in);
        System.out.println("请输入三角形三个边（整数）：");
        a=in.nextDouble();
        b=in.nextDouble();
        c=in.nextDouble();
        Sanj s=new Sanj(a,b,c);
        //try-catch语句块
        try {
            s.getArea();
        } catch (Exception e) {
            // TODO Auto-generated catch block
            System.out.println(e.getMessage());
        }
        
        try {
            s.showInfo();
        } catch (Exception e) {
            // TODO Auto-generated catch block
            System.out.println(e.getMessage());
        }
        
    }

}