Weird behaviour of bit-shifting with byte in Java

Question

As I was using bit-shifting on byte, I notice I was getting weird results when using unsigned right shift (>>>). With int, both right shift (signed:>> and unsigned:>>>) behave as expected:

    int min1 = Integer.MIN_VALUE>>31; //min1 = -1
    int min2 = Integer.MIN_VALUE>>>31; //min2 = 1

But when I do the same with byte, strange things happen with unsigned right shift:

    byte b1 = Byte.MIN_VALUE; //b1 = -128
    b1 >>= 7; //b1 = -1

    byte b2 = Byte.MIN_VALUE; //b2 = -128
    b2 >>>= 7; //b2 = -1; NOT 1!
    b2 >>>= 8; //b2 = -1; NOT 0!

I figured that it could be that the compiler is converting the byte to int internally, but does not seem quite sufficient to explain that behaviour.

Why is bit-shifting behaving that way with byte in Java?

Answer 1

This happens exactly because byte is promoted to int prior performing bitwise operations. int -128 is presented as:

11111111 11111111 11111111 10000000

Thus, shifting right to 7 or 8 bits still leaves 7-th bit 1, so result is narrowed to negative byte value.

Compare:

System.out.println((byte) (b >>> 7));           // -1
System.out.println((byte) ((b & 0xFF) >>> 7));  //  1

By b & 0xFF, all highest bits are cleared prior shift, so result is produced as expected.