Why are observers not notified?

Question

I am trying to implement observer pattern in Java Swing application I am working on as my school project. I have these two very simple classes, one states as a singleton observable model and second is an observer.

Observable model:

public class Model extends Observable {

    private static Model instance = null;
    private File selectedImageFile;
    private int colorsCount;

    public static Logger LOG = Logger.getLogger(Model.class);

    private Model() {
        this.initialize();
    }

    private void initialize() {
        addObserver(new ModelObserver());
    }

    public static Model instance() {
        if (Model.instance == null) {
            Model.instance = new Model();
        }
        return Model.instance;
    }

    public void setColorsCount(int colorsCount) {
        this.colorsCount = colorsCount;
        notifyObservers(Actions.COLORS_COUNT);
    }

    public void selectedImage(File imageFile) {
        this.selectedImageFile = imageFile;
        notifyObservers(Actions.SELECTED_IMAGE);
    }

    public enum Actions {
        SELECTED_IMAGE, COLORS_COUNT
    }

}

Observer

public class ModelObserver implements Observer {

    public static Logger LOG = Logger.getLogger(ModelObserver.class);

    @Override
    public void update(Observable o, Object arg) {

        if (arg instanceof Model.Actions) {

            Model.Actions action = (Actions) arg;

            switch (action) {

            case SELECTED_IMAGE:
                selectedImage();
                break;

            case COLORS_COUNT:
                colorsCount();
                break;

            default:
                LOG.warn("Not supported action: " + action);
                break;
            }

        } else {
            LOG.warn("Not supported action: " + String.valueOf(arg));
        }

    }

    private void colorsCount() {
        LOG.info("Colors count has been changed....");
    }

    private void selectedImage() {
        LOG.info("Image has been changed....");
    }

}

Everything works - Model instance register change but observers (only one in my case) are not notified. In a method ModeObserver.update(Observable o, Object arg) there is no mention that model has been changed. So my problem is that observers are not notified.

What am I doing wrong? Thank you.

Answer 1

You forgot to call setChanged() to mark the observable as changed, which is required by the notifyObservers to actually perform the notification.