code: public static String randomAlphaNumString(int length) {
      StringBuilder sb = new StringBuilder();
      for (int i = 0; i < length; i++) {
        sb.append(ALPHANUM.charAt(RANDOM.nextInt(ALPHANUM.length())));
      }
      return sb.toString();
    }
docs: Generates a random alphanumeric string of the given length.
  
  @param length the length
  @return a random string