code: private static void validateArguments(String[] args) {
      if (ArrayUtils.isEmpty(args)) {
        System.err.printf("$ java -server ... %s <port>%n", EchoServer.class.getName());
        System.exit(1);
      }
    }
docs: Validates the array of {@link String arguments} passed into the {@link EchoServer} program from the command-line.
  
  @param args array of {@link String arguments} passed into this program from the command-line.