Question

I have written the following code but findbugs is shwowing this error: BIT_ADD_OF_SIGNED_BYTE. I tried a lot but may be I am not getting the concept of left shift correctly.

void problem() {
    byte [] byteArray = {1, 2, 3, 4, 5};
    int localOne = 0;
    for(int i = 0; i < 4; i++) {
        localOne = (localOne<<8) + byteArray[i]; 
    }
}

Answer 1

You’r doing the shift correctly, your (possible) error is when adding a signed byte to an int

Because of sign extension you need to do this:

localOne = (localOne<<8) + (0xFF & byteArray[i]); 

Say you have the byte 80 (hex), which is 1000 0000 (binary), this is -128 (decimal) because of the two’s complement representation. Now, when adding it to an int it first gets converted to an int. The resulting int is not

0000 0000  0000 0000  0000 0000  1000 0000

(binary) it will be

1111 1111  1111 1111  1111 1111  1000 0000

(binary) because of sign extension. To get the first, you have to apply a bitwise and with 0xFF wich is this in binary:

0000 0000  0000 0000  0000 0000  1111 1111