java - 关于多个 'catch'的问题

Question

谁能告诉我为什么这个类的输出是'xa'？

为什么另一个异常(RuntimeException 和 Exception )不会被捕获？

public class Tree {
    public static void main(String... args) {
        try
        {
            throw new NullPointerException(new Exception().toString());
        }
        catch (NullPointerException e)
        {
            System.out.print("x");
        }
        catch (RuntimeException e)
        {
            System.out.print("y");
        }
        catch (Exception e)
        {
            System.out.print("z");   
        }        
        finally{System.out.println("a");}
    }
}

Answer 1

仅仅因为异常被创建，并不意味着它被抛出

public static void main(String[] args) {
    new Exception();
    System.out.println("Yippee!!");
    // prints "Yippee!!"
}

只是因为有一个 catch 子句，并不意味着某些东西被捕获了

public static void main(String[] args) throws Exception {
    try {
        System.out.println("No math for me!");
    } catch (ArithmeticException e) {
        System.out.println("Math was wronged!");
    } // prints "No math for me!"
}

异常可以在创建另一个异常时抛出

public static void main(String[] args) {
    try {
        throw new NullPointerException(args[-1]);
    } catch (NullPointerException e) {
        System.out.println("Ooops!");
    } catch (ArrayIndexOutOfBoundsException e) {
        System.out.println("Ooh lala!!");           
    } // prints "Ooh lala!!"
}

你只能捕获从你的try所在的地方抛出的东西

public static void main(String[] args) throws Exception {
    try {
        args[-1] = null;
    } catch (ArrayIndexOutOfBoundsException e) {
        System.out.println("Gotcha!");
        args[1/0] = null;
    } catch (ArithmeticException e) {           
        System.out.println("You missed me!");
    } // prints "Gotcha!"
} // Exception in thread "main" java.lang.ArithmeticException: / by zero

实际上在“所有”情况下，总是执行finally

public static void main(String[] args) {
    try {
        throw new Exception();
    } catch (Exception e) {
        System.out.println("Oops!");
        args[-1] = null;
    } finally {
        System.out.println("Yay!");
    } // prints "Oops!", "Yay!",
} // Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: -1

finally 的突然完成优于 try/catch 的突然完成

static String greetings() {
    try {
        return "No mood!";
    } finally {
        return "Hey buddy!";
    }
}   
public static void main(String[] args) throws Exception {
    System.out.println(greetings()); // prints "Hey buddy!"
    try {
        args[-1] = null;
    } catch (ArrayIndexOutOfBoundsException e) {
        throw new Exception("Catch me if you can!");
    } finally {
        throw new Exception("Yoink!");
    }
} // Exception in thread "main" java.lang.Exception: Yoink!