FindBugs: INT_VACUOUS_BIT_OPERATION

Question

In order to convert from int to IP String I am using approach in Going from 127.0.0.1 to 2130706433, and back again

private static final byte BYTE_MASK = (byte)0xff;

protected byte[] unpack(int bytes) {
    return new byte[] {
        (byte)((bytes >>> 24) & BYTE_MASK),
        (byte)((bytes >>> 16) & BYTE_MASK),
        (byte)((bytes >>>  8) & BYTE_MASK),
        (byte)((bytes       ) & BYTE_MASK)
   };
}

But FindBugs in Eclipse generates bugs: INT_VACUOUS_BIT_OPERATION.

INT_VACUOUS_BIT_OPERATION: bit operations that don't do any meaningful work.

Why is that and how to fix it?

score 4 · Accepted Answer · answered Mar 12 '13 at 11:38

I suspect it's because you don't need the & BYTE_MASK if you're also casting to byte. I'm assuming that BYTE_MASK is 0xff... in which case it's basically pointless. Just casting will have the same effect.

From section 5.1.3 of the JLS:

A narrowing conversion of a signed integer to an integral type T simply discards all but the n lowest order bits, where n is the number of bits used to represent type T. In addition to a possible loss of information about the magnitude of the numeric value, this may cause the sign of the resulting value to differ from the sign of the input value.

FindBugs: INT_VACUOUS_BIT_OPERATION

1 Answers1