Question

byte x = -1; for(int i = 0; i < 8; i++) {     x = (byte) (x >>> 1);     System.out.println('X: ' + x); } 

As I understand it, java stores data in two’s-complement, meaning -1 = 11111111 (according to wikipedia).

Also, from the java docs: ‘The bit pattern is given by the left-hand operand, and the number of positions to shift by the right-hand operand. The unsigned right shift operator ‘>>>’ shifts a zero into the leftmost position, while the leftmost position after ‘>>’ depends on sign extension. ‘

Which means that >>> would shift a 0 to the left most bit every time. So I expect this code to be

iteration: bit representation of x

0: 11111111

1: 01111111

2: 00111111

3: 00011111

…so on

However, my output is always X: -1, meaning (I guess) that >>> is putting the sign bit in the left most position. So I then try >>, and same result.

What’s going on? I would expect my output to be: X: -1, x: 127, x: 63, etc.

Answer 1

Whoever thought that bytes should be signed when Java was invented should be taken out and beaten with a wet stick of celery until they cry 🙂

You can do what you want by casting up to an int and ensuring you never shift a 1 into the top bit, something like this:

byte x = -1; int x2 = ((int)x) & 0xff; for(int i = 0; i < 8; i++) {     x2 = (x2 >>> 1);     System.out.println('X: ' + x2); } 

Your particular problem is because >>> is casting up to an int to do the shift, then you’re casting it back to a byte, as shown here:

byte x = -1; int x2 = ((int)x) & 0xff; int x3; int x4 = x2; for(int i = 0; i < 8; i++) {     x2 = (x2 >>> 1);     System.out.println('X2: ' + x2);     x3 = (x >>> 1);     x = (byte)x3;     x4 = (x4 >>> 1);     System.out.println('X: ' + x3 + ' ' + x + ' ' + x4); } 

Which outputs:

X2: 127 X: 2147483647 -1 127 X2: 63 X: 2147483647 -1 63 X2: 31 X: 2147483647 -1 31 X2: 15 X: 2147483647 -1 15 X2: 7 X: 2147483647 -1 7 X2: 3 X: 2147483647 -1 3 X2: 1 X: 2147483647 -1 1 X2: 0 X: 2147483647 -1 0 

You can clearly see that x and x3 don’t work (even though x3 shifts correctly, casting it back to byte in x sets it to -1 again). x4 works perfectly.