static bool isPrime(int n) {
  if ((n > 2 && (n & 1) == 0) // check is it even
     || n <= 1  //check for -ve
     || (n > 3 && (n % 3 ==  0)))  // check for 3 divisable
    false;

  int maxLookup = (int) Math.sqrt(n);
  for (int i = 3; (i+2) <= maxLookup; i = i + 6)
    if (n % (i+2) == 0 || n % (i+4) == 0) false;
  true;
}