Java学习--异常

根据课上的课件进行练习

1、阅读以下代码（CatchWho.java），写出程序运行结果：

package exception;

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");
        }
    }
}

结果：

ArrayIndexOutOfBoundsException/内层try-catch
发生ArithmeticException

2、写出CatchWho2.java程序运行的结果

package exception;

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");
        }
    }
}

结果：ArrayIndexOutOfBoundsException/外层try-catch

3、阅读 EmbedFinally.java示例，再运行它，观察其输出并进行总结

package exception;


class EmbededFinally {
    public static void main(String args[]) {

        int result;

        try {

            System.out.println("in 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");

        }

    }

}

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

try捕捉到的异常会从一到二进行抛出，finally的异常会从二到一抛出。

4、finally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");
        
        }
    
    }


}

结果：

in main
Exception is thrown in main

finally不一定会执行。

当程序执行try块，catch块时遇到return语句或者throw语句，这两个语句都会导致该方法立即结束，所以系统并不会立即执行这两个语句，而是 去寻找该异常处理流程中的finally块，如果没有finally块，程序立即执行return语句或者throw语句，方法终止。如果有 finally块，系统立即开始执行finally块，只有当finally块执行完成后，系统才会再次跳回来执行try块、catch块里的 return或throw语句，如果finally块里也使用了return或throw等导致方法终止的语句，则finally块已经终止了方法，不用 再跳回去执行try块、catch块里的任何代码了。

综上：尽量避免在finally块里使用return或throw等导致方法终止的语句，否则可能出现一些很奇怪的情况！