import org.eclipse.paho.client.mqttv3.*;
import org.eclipse.paho.client.mqttv3.persist.MemoryPersistence;

public class MqttServer {

    public static void main(String[] args) {
        String broker = "tcp://localhost:1883";
        String clientId = "MqttServer";
        MemoryPersistence persistence = new MemoryPersistence();

        try {
            MqttClient mqttClient = new MqttClient(broker, clientId, persistence);

            MqttConnectOptions connectOptions = new MqttConnectOptions();
            connectOptions.setCleanSession(true);

            mqttClient.setCallback(new MqttCallback() {
                public void connectionLost(Throwable cause) {
                    System.out.println("Connection lost: " + cause.getMessage());
                }

                public void messageArrived(String topic, MqttMessage message) {
                    System.out.println("Received message: " + new String(message.getPayload()));
                }

                public void deliveryComplete(IMqttDeliveryToken token) {
                    // Not used in this example
                }
            });

            mqttClient.connect(connectOptions);
            mqttClient.subscribe("mytopic");

            System.out.println("MqttServer is running");

        } catch (MqttException e) {
            System.out.println("Error: " + e.getMessage());
        }
    }
}

import org.eclipse.paho.client.mqttv3.*;
import org.eclipse.paho.client.mqttv3.persist.MemoryPersistence;

public class MqttClientExample {
    public static void main(String[] args) {
        String broker = "tcp://:1883";
        String clientId = "JavaMqttClient";
        MemoryPersistence persistence = new MemoryPersistence();

        try {
            MqttClient client = new MqttClient(broker, clientId, persistence);
            MqttConnectOptions connOpts = new MqttConnectOptions();
            connOpts.setCleanSession(true);

            System.out.println("Connecting to broker: " + broker);
            client.connect(connOpts);
            System.out.println("Connected");

            String topic = "test/topic";
            int qos = 1;

            System.out.println("Subscribing to topic: " + topic);
            client.subscribe(topic, qos);

            System.out.println("Publishing message: Hello, MQTT!");
            String message = "Hello, MQTT!";
            MqttMessage mqttMessage = new MqttMessage(message.getBytes());
            mqttMessage.setQos(qos);
            client.publish(topic, mqttMessage);

            client.setCallback(new MqttCallback() {
                public void connectionLost(Throwable cause) {
                    System.out.println("Connection lost");
                }

                public void messageArrived(String topic, MqttMessage message) throws Exception {
                    System.out.println("Received message: " + new String(message.getPayload()));
                }

                public void deliveryComplete(IMqttDeliveryToken token) {
                    System.out.println("Message delivered");
                }
            });

            // Wait for some time to receive messages
            Thread.sleep(5000);

            client.disconnect();
            System.out.println("Disconnected");
        } catch (MqttException | InterruptedException e) {
            e.printStackTrace();
        }
    }
}