MQTTClient.java

private void subscribe() {
    String subscribeTopic = "inbox";
        try {
            client.subscribe(subscribeTopic, 0, new IMqttMessageListener() {
                @Override
                public void messageArrived(final String topic, final MqttMessage message) throws Exception {
                    runOnUiThread(new Runnable() {
                        @Override
                        public void run() {
                            Toast.makeText(MainActivity.this, message.toString(), Toast.LENGTH_SHORT).show();
                        }
                    });
                }
            });
        } catch (MqttException e) {
            e.printStackTrace();
        }
    }