java short how can I tell if a carry is needed

Question

if I add or subtract two short values, how can I tell if I would need to flag a carry condition

Answer 1

In the case of adding and subtracting only, arithmetic overflow has occurred when both operands are positive and the result is negative and vice versa.

class OverflowTest
{
        public static void main (String[] args)
        {
                System.out.println(isOverflow((short)32767, (short)32767, '+'));
                System.out.println(isOverflow((short)-32767, (short)-32767, '+'));
                System.out.println(isOverflow((short)32767, (short)-32767, '+'));       
        }

        private static boolean isOverflow(short a, short b, char op) {  
                short c = (op == '+') ? (short)(a+b) : (short)(a-b);
                if((a > 0 && b > 0 && c < 0) || (a < 0 && b < 0 && c > 0))
                        return true;
                return false;
        }
}

Answer 2

You can do the addition or subtraction using a larger type such as int, cast it to a short, and test if the cast changes the value.

int i = s1 + s2;
short s = (short)i;
if (i != s) { /* overflow */ }