How does Java handle integer underflows and overflows and how would you check for it?

Question

How does Java handle integer underflows and overflows?

Leading on from that, how would you check/test that this is occurring?

Answer 1

Having just kinda run into this problem myself, here's my solution (for both multiplication and addition):

static boolean wouldOverflowOccurwhenMultiplying(int a, int b) {
    // If either a or b are Integer.MIN_VALUE, then multiplying by anything other than 0 or 1 will result in overflow
    if (a == 0 || b == 0) {
        return false;
    } else if (a > 0 && b > 0) { // both positive, non zero
        return a > Integer.MAX_VALUE / b;
    } else if (b < 0 && a < 0) { // both negative, non zero
        return a < Integer.MAX_VALUE / b;
    } else { // exactly one of a,b is negative and one is positive, neither are zero
        if (b > 0) { // this last if statements protects against Integer.MIN_VALUE / -1, which in itself causes overflow.
            return a < Integer.MIN_VALUE / b;
        } else { // a > 0
            return b < Integer.MIN_VALUE / a;
        }
    }
}

boolean wouldOverflowOccurWhenAdding(int a, int b) {
    if (a > 0 && b > 0) {
        return a > Integer.MAX_VALUE - b;
    } else if (a < 0 && b < 0) {
        return a < Integer.MIN_VALUE - b;
    }
    return false;
}

feel free to correct if wrong or if can be simplified. I've done some testing with the multiplication method, mostly edge cases, but it could still be wrong.