code: public static boolean isDigits(String str) {
      if (Strings.isEmpty(str)) return false;
      for (int i = 0; i < str.length(); i++) {
        if (!Character.isDigit(str.charAt(i))) return false;
      }
      return true;
    }
docs: <p>
  Checks whether the <code>String</code> contains only digit characters.
  </p>
  <p>
  <code>Null</code> and empty String will return <code>false</code>.
  </p>
  
  @param str the <code>String</code> to check
  @return <code>true</code> if str contains only Unicode numeric