code: public static boolean isDigitsOnly(String s) {
          if (isEmpty(s)) return false;
          String rslt = s.trim();
          if (isEmpty(rslt)) return false;
          for (int i = 0; i < rslt.length(); i++) {
              char ch = rslt.charAt(i);
              if (!Character.isDigit(ch)) return false;
          }
          return true;
      }
docs: Returns true if specified string contains only digits