MQTTManager.java

     /**
     * We can subscribe topic from here
     */
public void subscribeToTopic() {
        try {
            /**
             * @param topic_name
             * @param QOS_level (0,1,2)
             * @param userContext
             * @param Listener
             */
            mqttAndroidClient.subscribe(topic, QOS, null, new IMqttActionListener() {
                @Override
                public void onSuccess(IMqttToken asyncActionToken) {
                    addToHistory("Subscribed!");
                }

                @Override
                public void onFailure(IMqttToken asyncActionToken, Throwable exception) {
                    addToHistory("Failed to subscribe");
                }
            });

            /**
             * @param topic_name
             * @param QOS_level (0,1,2)
             * @param Listener
             */
            mqttAndroidClient.subscribe(topic, QOS, new IMqttMessageListener() {
                @Override
                public void messageArrived(String topic, MqttMessage message) throws Exception {
                    /*
                    message Arrived!
                    * */
                    System.out.println("Message: " + topic + " : " + new String(message.getPayload()));
                }
            });

        } catch (MqttException ex) {
            System.err.println("Exception whilst subscribing");
            ex.printStackTrace();
        }
    }