docs: public static boolean isInteger(String string)
    {
      if(string == null || string.isEmpty()) return false;
      int startIndex = string.charAt(0) == '-' ? 1 : string.charAt(0) == '+' ? 1 : 0;
      for(int i = startIndex, l = string.length(); i < l; i++) {
        if(string.charAt(i) == GROUPING_SEPARATOR) continue;
        if(!Character.isDigit(string.charAt(i))) {
          return false;
        }
      }
      return true;
    }