Using static method as JavaFX change listener

Question

Can someone explain to me why this isn't working?

public class ListenerTest {
    public static void addListener(Node node) {
        node.visibleProperty().addListener(ListenerTest::handleVisibleChanged);
    }

    private static void handleVisibleChanged(ObservableValue<? extends Boolean> observable,
            Boolean oldValue, Boolean newValue) {
        // Do something
    }
}

As you can see, I am using Java 8 method reference to assign a static method as a JavaFX change listener. It compiles just fine, but handleVisibleChanged() method is not invoked when visible property changes.

Please don't offer workarounds or ask why am I doing this. Think of it as an exercise. :-)

Answer 1

Here is a simple example that uses your two methods and works as expected: when the button is clicked, a message is printed.

public class ListenerTest extends Application {

    @Override
    public void start(Stage stage) throws Exception {
        Label label = new Label("You can see me");
        addListener(label);
        Button button = new Button("Hide/Show");
        button.setOnAction(e -> label.setVisible(!label.isVisible()));

        Scene scene = new Scene(new VBox(20, label, button));
        stage.setScene(scene);
        stage.show();
    }
    public static void addListener(Node node) {
        node.visibleProperty().addListener(ListenerTest::handleVisibleChanged);
    }
    private static void handleVisibleChanged(ObservableValue<?> a, Boolean b, Boolean newValue) {
        System.out.println("new value: " + newValue);
    }

    public static void main(String[] args) {
        launch();
    }
}