code: public static String generateAuthorizationCode() {
          StringBuilder buf = new StringBuilder(AUTHORIZATION_CODE_LENGTH);
          SecureRandom rand = new SecureRandom();
          for (int i = 0; i < AUTHORIZATION_CODE_LENGTH; i++) {
              buf.append(allowedCharacters[rand.nextInt(allowedCharacters.length)]);
          }
          return buf.toString();
      }
docs: Generated Unique Authorization Code
  @return Generated Authorization Code