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>>> operator

 
Sean Walker
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I encoutered the following question:
Which of the following results in a negative value of a?
A: int a = -1; a = a >>> 8;
B: byte a = -1; a = (byte)(a >>> 5);
C: int a = -1; a = a >> 5;
D: int a = -1; a = a >>> 32

The answer is supposed to be A, B and D.
I don't see this at all. I would think the only negative one would B.
I believe that A, C and D are incorrect because any time an int is unsigend-right-shifted, a positive number results.
In the case of B above though things are sort of different. As I recall when a non-int is acted on by any shift operation, it is first cast to an int and then the shifting is performed. This converts the byte of -1 (11111111) into the int form of -1 (11111111111111111111111111111111). This is unsigned-right-shifted by 5, which results in some positive number (00000111111111111111111111111111). Which is then cast back into the form of a byte which leaves us with -1 again (11111111 that is).
Am I right here?
 
Adam Altmann
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I wrote the following program:
public class Test{
public static void main(String[] args){
int A = -1;
A = A >>> 8;

byte B = -1;
B = (byte)(B >>> 5);

int C = -1;
C = C >> 5;

int D = -1;
D = D >>> 32;

System.out.println("A = " + A);
System.out.println("B = " + B);
System.out.println("C = " + C);
System.out.println("D = " + D);
}
}
And got these results:
A = 16777215
B = -1
C = -1
D = -1

Honestly, this makes no sense to me. I would think that C is the only right answer, since >>> is the unsigned right shift (it fills the bits to the left with 0) and >> fills the left with whatever the sign bit is. In Two's Complement, negative numbers have a 1 as their Most Significant Bit. Since each of these start as negative (with MSB of 1), only choice C should stay negative, since it will be the only one left with a MSB of 1 (because A, C, D fill with 0).
Can someone please explain this? I'm thoroughly confused.
 
Bert Bates
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Just taking a quick look without getting out my pencil and paper...
B *could* be correct since you're doing a cast after the shift and just grabbing the rightmost 8 bits - the 8th bit might be a 1
C is using the signed shift operator - so the sign bit will be replicated
D - The right hand shift operator can only be, in practice, from 0-31 for ints and from 0-63 for longs. Think about it like using the remainder operator - in this case the right operand (32) becomes 0, so no shifting occurs!
good luck in your studies!
 
Davy Kelly
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The signed bit is the left most bit so >>> changes a 1 to 0 but not from 0 to 1, the >> keeps the sign bit so if it was 1 all bit shifted will be 1's and 0 with 0's, hopefully the bold has worked below

Which of the following results in a negative value of a?
A: int a = -1; a = a >>> 8;
B: byte a = -1; a = (byte)(a >>> 5);
C: int a = -1; a = a >> 5;
D: int a = -1; a = a >>> 32

A is:
11111111 11111111 11111111 11111111 >>> 8 gives you
00000000 11111111 11111111 11111111 this results in a big poistive int number.
B is:
converts the byte to an int then does the shifting, then casts back to a byte so bits look like this:
byte 11111111 promoted to 11111111 11111111 11111111 11111111 then shift to get 00000111 11111111 11111111 11111111 but we convert back to get the last 8 bits on the right hand side which is 11111111 byte -1
C is:
11111111 11111111 11111111 11111111 then shift with which ever the left most bit was which is 1 to get the same result
D is:
11111111 11111111 11111111 11111111 by 32 bits, I needed the pencil first time round, but then i noticed that it can only shift 31 times to give you +1 but 32 times is shifting 0 times so hence the same result of -1.
I normally find it easier writing out the bits first then trace out what will happen to the bits.
I hope this helps guys
Davy
[ February 18, 2004: Message edited by: Davy Kelly ]
 
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