What can I say, I’m a performance nerd.

That’s a platform dependent change. Overflow is undefined behavior. I’d rather have my code portable so it can run on my Univac 1101.

Fixed

boolean isOdd(int num) {
  if(num == 1)
    return true;
  if(num > 0)
    return isEven(num - 1);
  else
    return isEven(num + 1);
}

boolean isEven(int num) {
  if(num > 0)
    return isOdd(num - 1);
  else
    return isOdd(num + 1);
}