关于try-catch-finally-return语句的执行顺序

Posted 范二er

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直接贴代码

main函数

 public static void main(String[] args) 
        System.out.println("=============NoException==================");
        System.out.println(noException());
        System.out.println("===============================");
        System.out.println("=============NoException1==================");
        System.out.println(noException1());
        System.out.println("===============================");
        System.out.println("=============WithException==================");
        System.out.println(WithException());
        System.out.println("===============================");
        System.out.println("=============WithException1==================");
        System.out.println(WithException1());
        System.out.println("===============================");
        System.out.println("=============WithException2==================");
        System.out.println(WithException2());
        System.out.println("===============================");
        System.out.println("=============WithException3==================");
        System.out.println(WithException3());
        System.out.println("===============================");
    

1.try中没有异常,try,catch,finally中都有return语句

测试代码:

 public static int noException() 
        int i = 10;
        try 
            System.out.println("i in try block is :" + i);
            return --i;
         catch (Exception e) 
            --i;
            System.out.println("i in catch block is :" + i);
            return --i;
         finally 
            System.out.println("i in finally - from try or catch block is:" + i);
            return --i;
        
    

运行结果:

=============NoException==================
i in try block is :10
i in finally - from try or catch block is9
8
===============================

执行顺序:
执行try块,执行到return语句时,先执行return的语句,–i,但是不返回到main方法,执行finally块,
遇到finally块中的return语句,执行–i,并将值返回到main方法,这里就不会再回去返回try块中计算得到的值。
结论:
try-catch-finally都有return语句时,没有异常时,返回值是finally中的return返回的。


2.try中没有异常抛出,仅try和catch中有return

测试代码:

    public static int noException1() 
        int i = 10;
        try 
            System.out.println("i in try block is:" + i);
            return --i;
         catch (Exception e) 
            --i;
            System.out.println("i in catch - form try block is:" + i);
            return --i;
         finally 
            System.out.println("i in finally - from try or catch block is:" + i);
            --i;
            System.out.println("i in finally block is:" + i);
            //return --i;
        
    

运行结果:

=============NoException1==================
i in try block is10
i in finally - from try or catch block is9
i in finally block is8
9
===============================

执行顺序:
try中执行完return的语句后,不返回,执行finally块,finally块执行结束后,返回到try块中,返回i在try块中最后的值。

结论:
try-catch都有return语句时,没有异常时,返回值是try中的return返回的。


3.try块中抛出异常,try、catch和finally中都有return语句

测试代码:

    public static int WithException() 
        int i = 10;
        try 
            System.out.println("i in try block is:" + i);
            i = i / 0;
            return --i;
         catch (Exception e) 
            System.out.println("i in catch - form try block is:" + i);
            --i;
            System.out.println("i in catch block is:" + i);
            return --i;
         finally 
            System.out.println("i in finally - from try or catch block is--" + i);
            --i;
            System.out.println("i in finally block is--" + i);
            return --i;
        
    

运行结果:

=============WithException==================
i in try block is10
i in catch - form try block is10
i in catch block is9
i in finally - from try or catch block is--8
i in finally block is--7
6
===============================

执行顺序:
抛出异常后,执行catch块,在catch块的return的–i执行完后,并不直接返回而是执行finally,因finally中有return语句, 所以,执行,返回结果6。
结论:
try块中抛出异常,try、catch和finally中都有return语句,返回值是finally中的return。


4.try块中抛出异常,try和catch中都有return语句

测试代码:

   public static int WithException1() 
        int i = 10;
        try 
            System.out.println("i in try block is:" + i);
            i = i / 0;
            return --i;
         catch (Exception e) 
            System.out.println("i in catch - form try block is:" + i);
            return --i;
         finally 

            System.out.println("i in finally - from try or catch block is:" + i);
            --i;
            System.out.println("i in finally block is:" + i);
            //return i;
        
    

运行结果:

=============WithException1==================
i in try block is10
i in catch - form try block is10
i in finally - from try or catch block is9
i in finally block is8
9
===============================

执行顺序:
抛出异常后,执行catch块,执行完finally语句后,依旧返回catch中的执行return语句后的值,而不是finally中修改的值。
结论:
返回的catch中return值。


5.try、catch中都出现异常,在finally中有返回

测试代码:

    public static int WithException2() 
        int i = 10;
        try 
            System.out.println("i in try block is:" + i);
            i = i / 0;
            return --i;
         catch (Exception e) 
            System.out.println("i in catch - form try block is:" + i);
            int j = i / 0;
            return --i;
         finally 

            System.out.println("i in finally - from try or catch block is:" + i);
            --i;
            --i;
            System.out.println("i in finally block is:" + i);
            return --i;
        
    

运行结果:

=============WithException2==================
i in try block is10
i in catch - form try block is10
i in finally - from try or catch block is10
i in finally block is8
7
===============================

执行顺序:
try块中出现异常到catch,catch中出现异常到finally,finally中执行到return语句返回,不检查异常。

结论:
返回finally中return值。


6、只在函数最后出现return语句

测试代码:

    public static int WithException3() 
        int i = 10;
        try 
            System.out.println("i in try block is:" + i);
            i = i / 0;
            //return --i;
         catch (Exception e) 
            System.out.println("i in catch - form try block is:" + i);
            //int j = i/0;
            //return --i;
         finally 

            System.out.println("i in finally - from try or catch block is:" + i);
            --i;
            --i;
            System.out.println("i in finally block is:" + i);
            //return --i;
        
        return --i;
    

运行结果:

=============WithException3==================
i in try block is10
i in catch - form try block is10
i in finally - from try or catch block is10
i in finally block is8
7
===============================

总结:
try中执行完return的语句后,不返回,执行finally块,finally块执行结束后,返回到try块中,返回i在try块中最后的值。
结论一:
return语句并不是函数的最终出口,如果有finally语句,这在return之后还会执行finally
(return的值会暂存在栈里面,等待finally执行后再返回)
结论二:
finally里面不建议放return语句,根据需要,return语句可以放在try和catch里面和函数的最后。可行的做法有四:
(1)return语句只在函数最后出现一次。
(2)return语句仅在try和catch里面都出现。
(3)return语句仅在try和函数的最后都出现。
(4)return语句仅在catch和函数的最后都出现。
注意,除此之外的其他做法都是不可行的,编译器会报错。


2017.2.21
原来以为碰到这种类型的题目,再也不会错,但是碰到了一下的“套中套” 。。 。 防不胜防

public class ZeroTest 
    public static void main(String[] args) 
     try
       int i = 100 / 0;
       System.out.print(i);
  catch(Exception e)
       System.out.print(1);
       throw new RuntimeException();
  finally
       System.out.print(2);
  
      System.out.print(3);
 
 

这段程序的输出为12 ‘3’是不会输出的,因为在try中捕捉到异常后,程序跳进catch,会执行throw newRuntimeException(),这个时候,程序出了会执行finally中的代码,其他位置处的代码都不会执行。

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