code: public static synchronized void subscribe(android.content.Context context,
                                              java.lang.String channel, java.lang.Class<? extends android.app.Activity> cls) {
      startServiceIfRequired(context, cls);
      final String finalChannel = channel;
      AVInstallation.getCurrentInstallation().addUnique("channels", finalChannel);
      _installationSaveHandler.sendMessage(Message.obtain());
  
      if (cls != null) {
        AVNotificationManager manager = AVPushMessageListener.getInstance().getNotificationManager();
        manager.addDefaultPushCallback(channel, cls.getName());
  
        // set default push callback if it's not exist yet
        if (manager.getDefaultPushCallback(AVOSCloud.getApplicationId()) == null) {
          manager.addDefaultPushCallback(AVOSCloud.getApplicationId(), cls.getName());
        }
      }
    }
docs: Helper function to subscribe to push notifications with the default application icon.
  
  @param context This is used to access local storage to cache the subscription, so it must
  currently be a viable context.
  @param channel A string identifier that determines which messages will cause a push
  notification to be sent to this client. The channel name must start with a letter and
  contain only letters, numbers, dashes, and underscores.
  @param cls     This should be a subclass of Activity. An instance of this Activity is started when
  the user responds to this push notification. If you are not sure what to use here,
  just
  use your application's main Activity subclass.