a quizz question about java bit operation???

[tyshanchn]

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???

 

[James McGill]

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.

 

[Matt Atterbury]

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???

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.