prediction failed

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gk - 13 Sep 2006 17:57 GMT

class Unchecked {
    public static void main(String[] args) {
        try {
            method();
        } catch (Exception e) {

        }
    }

    static void method() {
        try {
            wrench();

            System.out.println("a");
        } catch (ArithmeticException e) {
            System.out.println("b");
        } finally {
            System.out.println("c");
        }

        System.out.println("d");
    }

    static void wrench() {
        throw new NullPointerException();
    }
}

Question:

whats the output ?

i know whats the output ...because i have run it .....but problem is my
prediction and the real output did not match.

see, them method wrench throwing a null pointer exception.

can anybody please tell me the output in steps ?

i dont want to manipulate.

here are things so far........

method() calls wrench()

but wrench() thorws null pointer

this is caught in try-catch .....but there is no matching exception so
it will go to finally ...so "c' would be printed ....and at last d
is there ....so d would be printed.

so, predicted output is : c d

real output: c

question: why d is missing in the real output ?

Reply to this Message

Arne Vajhøj - 13 Sep 2006 18:09 GMT

> so, predicted output is : c d
>
> real output: c
>
> question: why d is missing in the real output ?

Try this code:

class Unchecked {
    public static void main(String[] args) {
        try {
            method();
        } catch (Exception e) {
            System.out.println("e");
        }
    }

    static void method() {
        try {
            wrench();

            System.out.println("a");
        } catch (ArithmeticException e) {
            System.out.println("b");
        } finally {
            System.out.println("c");
        }

        System.out.println("d");
    }

    static void wrench() {
        throw new NullPointerException();
    }
}

Arne

Reply to this Message

Arne Vajhøj - 13 Sep 2006 18:09 GMT

>> so, predicted output is : c d
>>
[quoted text clipped - 3 lines]
>
> Try this code:

The point is that finally is not a "catch all" but
a "do this no matter what".

Arne

Reply to this Message

gk - 13 Sep 2006 18:54 GMT

} finally {
System.out.println("c");
}

so,it seems that finally does not catch anything ..........it just
executes at last.....but we see there is a null pointer exception which
nobody caught nd so the program got unexpected stop and passes the
control tothe methdo() without printing "d".........is this all right
?

thanks for response.

> >> so, predicted output is : c d
> >>
[quoted text clipped - 8 lines]
>
> Arne

Reply to this Message

Patricia Shanahan - 13 Sep 2006 20:03 GMT

> } finally {
> System.out.println("c");
[quoted text clipped - 5 lines]
> control tothe methdo() without printing "d".........is this all right
> ?

"finally" is called "finally" not "catch" because its job is to be
executed at the end of the try-catch-finally statement, not to catch
anything.

Of course, in the unlikely event that you really did want to go ahead
with the "d" printout after a NullPointerException, you could have
chosen to catch it:

public class UncheckedTest {
public static void main(String[] args) {
method();
}

static void method() {
try {
wrench();
System.out.println("a");
} catch (ArithmeticException e) {
System.out.println("b");
} catch (NullPointerException e) {
System.out.println("NullPointerException caught");
}
finally {
System.out.println("c");
}

System.out.println("d");
}

static void wrench() {
throw new NullPointerException();
}
}

output:

NullPointerException caught
c
d

In the original program, the NullPointerException was caught in the end,
by the try-catch in main. That was the closest try-catch with a catch
clause that could handle it.

Patricia

Reply to this Message

Patricia Shanahan - 13 Sep 2006 18:36 GMT

> class Unchecked {
> public static void main(String[] args) {
[quoted text clipped - 52 lines]
>
> question: why d is missing in the real output ?

"If the run-time type of V is not assignable to the parameter of any
catch clause of the try statement, then the finally block is executed.
Then there is a choice:

* If the finally block completes normally, then the try statement
completes abruptly because of a throw of the value V."

[JLS, 14.19.2 Execution of try-catch-finally,
http://java.sun.com/docs/books/jls/second_edition/html/statements.doc.html#236653]

This is the case you have. NullPointerException is not assignable to an
ArithmeticException parameter. The finally clause does complete
normally. The whole try-catch-finally statement completes abruptly due
to a throw of a NullPointerException, preventing execution of any
following code in method().

I have yet to see a situation in which a catch of something like
Exception with an empty catch block is a good idea. If the body of main
had just called method, without the try-catch, you would have seen main
terminate due to the NullPointerExceptionn from wrench, and had a much
better chance of working out for yourself what was going on.

Patricia

Reply to this Message

prediction failed

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