code: public static boolean isSignedLong(String s) {
          if (isEmpty(s)) return defaultEmptyOK;
  
          try {
              Long.parseLong(s);
  
              return true;
          } catch (Exception e) {
              return false;
          }
      }
docs: Returns true if all characters are numbers;
  first character is allowed to be + or - as well.
  
  Does not accept floating point, exponential notation, etc.