HelloWorldActivity.java

public class HelloWorldActivity extends AppCompatActivity {

    @Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        int notificationId = 0;

        NotificationManager notificationManager = new NotificationManager(this, notificationId);
        notificationManager.notify(new HelloWorldNotification());
    }
}

HelloWorldNotification.java

public class HelloWorldNotification implements Notification {

    @NonNull
    @Override
    public android.app.Notification onCreateNotification(Context context) {
        
        String notificationChannel = "test";

        NotificationCompat.Builder builder = new NotificationCompat.Builder(context, notificationChannel)
                .setSmallIcon(R.drawable.ic_app)
                .setContentTitle("HELLO WORLD!");

        return builder.build();
    }
}

how to use.txt

1. implement Notification
2. create a NotifcationManager
3. call NotificationManager.notify() giving it an instance of Notification