NotificationService.java

public class NotificationService extends FirebaseMessagingService { 
  @Override
  public void onMessageReceived(RemoteMessage remoteMessage) {
    super.onMessageReceived(remoteMessage);
    Notification notification = new NotificationCompat.Builder(this)
      .setContentTitle(remoteMessage.getNotification().getTitle())
      .setContentText(remoteMessage.getNotification().getBody())
      .setSmallIcon(R.mipmap.ic_launcher)
      .build();
     NotificationManagerCompat manager = NotificationManagerCompat.from(getApplicationContext());
     manager.notify(123, notification);
   }
}