Android : as grouped by the type of notification?

I have 2 types of notifications. such as "hello world" and "goodbye world". and here comes from the server "hello world" - a notification appears. comes again "hello world" - notification does not appear as an old stated that the two of them. comes "as the world" and now the two notifications. one with the number two with a second number one. How is that done?

now all each notification is displayed separately:

private static void generateNotification(Context context, String message) {
        NotificationManager nm = (NotificationManager)context.getSystemService(Context.NOTIFICATION_SERVICE);

        Intent intent = new Intent(context,MyActivity.class);
        PendingIntent pendingIntent = PendingIntent.getActivity(context, 0, intent, PendingIntent.FLAG_CANCEL_CURRENT);

        NotificationCompat.Builder builder = new NotificationCompat.Builder(context);
        builder
                .setContentIntent(pendingIntent)
                .setSmallIcon(R.drawable.ic_stat_gcm)
                .setLargeIcon(BitmapFactory.decodeResource(context.getResources(), R.drawable.ic_stat_gcm))
                .setTicker("New message")
                .setWhen(System.currentTimeMillis())
                .setAutoCancel(true)
                .setContentTitle("message")
                .setContentText(message);

        Notification notification = builder.build();
        nm.notify(id++,notification);
    }

method call is triggered in the method of service:

@Override
    protected void onMessage(Context context, Intent intent) {
        String message = intent.getStringExtra("content");
        generateNotification(context, message);
    }