Just a rough first hack at Hangman over MQTT... players should subscribe to hangman/result and publish their single character guesses to hangman/guess. A client for playing is left as an exercise for the reader

MqttHangman.java

package org.eclipse.pahodemo;

import org.eclipse.paho.client.mqttv3.MqttCallback;
import org.eclipse.paho.client.mqttv3.IMqttDeliveryToken;
import org.eclipse.paho.client.mqttv3.MqttClient;
import org.eclipse.paho.client.mqttv3.MqttConnectOptions;
import org.eclipse.paho.client.mqttv3.MqttException;
import org.eclipse.paho.client.mqttv3.MqttMessage;
import org.eclipse.paho.client.mqttv3.MqttPersistenceException;

public class MqttHangman implements MqttCallback {

  MqttClient client;
	MqttConnectOptions options;
	
	String word;
	String guess;
	
	String resulttopic="hangman/result";
	String guesstopic="hangman/guess";
	boolean newguess=false;
	boolean done=false;
	
	public MqttHangman() {}

	public static void main(String[] args) {
		new MqttHangman().doDemo();
	}

	
	public void setupHangman() {
		word="hangman";
		guess="_______";
		newguess=false;
		done=false;
	}
	
	
	public void doDemo() {
		try {
			
			
			client = new MqttClient("tcp://localhost:1883", "hangman_master");
			options = new MqttConnectOptions();
			client.setCallback(this);
			client.connect(options);
			client.subscribe(guesstopic);
			
			while(true) {
			setupHangman();
			publishResult();

			while(!done) {
				try {
					Thread.sleep(1000);
					if(newguess) {
						publishResult();
						newguess=false;
					}
				} catch (InterruptedException e) {}
			}
			
			publishResult();
			}
			
			//client.disconnect();
		} catch (MqttException e) {
			e.printStackTrace();
		}
	}

	private void publishResult() throws MqttException, MqttPersistenceException {
		MqttMessage message = new MqttMessage();
		message.setPayload(guess.getBytes());
		client.publish(resulttopic, message);
	}

	private void guessLetter(String s) {
		char c=s.toLowerCase().charAt(0);
		StringBuilder sb=new StringBuilder();
		
		for(int i=0;i<word.length();i++) {
			if(word.charAt(i)==c) {
				sb.append(c);
			} else {
				sb.append(guess.charAt(i));
			}
		}
		
		guess=sb.toString();
		done=guess.indexOf('_')==-1;
		newguess=true;
	}
	
	@Override
	public void connectionLost(Throwable cause) {}

	@Override
	public void messageArrived(String topic, MqttMessage message)
			throws Exception {
		if(topic.equals(guesstopic)) {
			guessLetter(new String(message.getPayload()));
		}
	}

	@Override
	public void deliveryComplete(IMqttDeliveryToken token) {
	}
}