Question

When:

byte[] b = {-128, 0, 0, 0};
long total = 0;

The first expression returns -2,147,483,648:

for (int i = 0; i < b.length; i++) {
   int shift = (b.length - 1 - i) * 8;
   total += (b[i] & 255) << shift;
}

The second returns 2,147,483,648:

for (int i = 0; i < b.length; i++) {
   int shift = (b.length - 1 - i) * 8;
   long tmp = (b[i] & 255);
   total += tmp << shift;
}

My question is; why is the first statement positive and the second negative when they appear to be the same statement?

Answer 1

In this line

total += (b[i] & 255) << shift;

the parenthesized expression is of type int and the left shift sets its leftmost bit to one, making it a negative number. The conversion to long happens only after all the calculation is done.

long tmp = (b[i] & 255);

Here the expression is long and the leftmost bit will stay zero after the shift.

If you want to keep the first expression, just add a cast to long for the parenthesized expression or use a long constant 255L.