a quizz question about java bit operation???

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tyshanchn - 10 May 2006 04:03 GMT

hi ,

what is the value of

-8>>-1
-8<<-1

it will be like

public class BitOperation {

    /**
    * @param args
    */
    public static void main(String[] args) {
        // TODO Auto-generated method stub
        System.out.println(-8 << -1);
        System.out.println(-8 >> -1);
    }

}

the result is
0
-1

but if not -8, what will be the value?and why???

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James McGill - 10 May 2006 04:30 GMT

> hi ,
>
> what is the value of

without considering java, -8 in 2's complement is (pretending it's a 16
bit value for the moment)

1111 1111 1111 1000

and -1 is

1111 1111 1111 1111

Then when you take these signed values and feed them to an unsigned
operation like shift,

> -8>>-1

You're asking to shift right by the maximum number that can be
represented in you bit width, and I would expect that to be a
sign-extended negative number, and if you evaluate it as a signed
result, I'd expect to see -1.

Likewise

> -8<<-1

You're asking to shift left by way more bits than exist in the value, so
you end up with all zeros, and I'd expect the answer to be zero.

> System.out.println(-8 << -1);
> System.out.println(-8 >> -1);

> the result is
> 0
> -1

Yep.

I don't understand your last question. "If not -8..."

There is a well defined behavior for this sort of operation and I don't
see anything strange or unexpected from your example.

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Matt Atterbury - 10 May 2006 05:03 GMT

> what is the value of
>
[quoted text clipped - 21 lines]
>
> but if not -8, what will be the value?and why???

Presumably "<< -1" means "left shift many times" and not "right shift
by 1", due to -1 being converted to an unsigned value (with all bits
set).

So, a negative number right shifted enough times must become -1,
and a negative number left shifted enough times must become 0.

m.

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a quizz question about java bit operation???

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