Question

I have this code

byte[] b = new byte[]{-33,-4,20,30};
System.err.println(Arrays.toString(b));

int x =  (b[0] << 24) + (b[1] << 16) + (b[2] << 8) + b[3];

b = new byte[]{(byte)(x >> 24), (byte)(x >> 16), (byte)(x >> 8), (byte)(x)};

System.err.println(Arrays.toString(b));

Output:

[-33, -4, 20, 30]
[-34, -4, 20, 30]

I cannot figure out why this operations are not invers.

Answer 1

Your problem is unwanted sign extension.

Specifically, b[1] is -4, or 0xfc which is sign-extended to 0xfffffffc then left-shifted to 0xfffc0000. This has the effect of decrementing the most-significant byte by 1.

Try:

int x =  ((b[0] & 0xff) << 24) +
         ((b[1] & 0xff) << 16) +
         ((b[2] & 0xff) << 8) +
          (b[3] & 0xff);