public class BroadCastReceiver extends BroadcastReceiver {
    @Override
    public void onReceive(Context context, Intent intent) {
        NotifikationManager notifikationManager = NotifikationManager.INSTANCE;

        if (notifikationManager.isAppInForeground()) {
            // You can send the notification to the current activity by calling "notify"
            notifikationManager.notify("Hello current Activity!");
        } else {
            // send notification
        }
    }
}

public class MyActivity extends AppCompatActivity implements Observer {

    @Override
    protected void onResume() {
        super.onResume();
        NotifikationManager.INSTANCE.addObserver(this);
    }

    @Override
    protected void onPause() {
        super.onPause();
        NotifikationManager.INSTANCE.deleteObserver(this);
    }

    @Override
    public void update(Observable observable, Object o) {
        // Do something with object
    }
}

public class MyActivity extends NotifikationActivity {
    @Override
    protected void onNotificationReceived(@NotNull Object any) {
        // Here, you will receive the notification
    }
}