code: public static String randomIdentifer(int len) {
          final String AB = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
  
          StringBuilder sb = new StringBuilder( len );
          for (int i = 0; i < len; i++)
              sb.append( AB.charAt( rnd.nextInt(AB.length()) ) );
          return sb.toString();
      }
docs: Produce a random identifer of the given length, consisting only of uppercase letters.
  @param len desired length of the string
  @return an identifier of the desired length