how do I get an element in JavaFx using an id?

Question

I am new to FXML and I am trying to create a handler for all of the button clicks using a switch. However, in order to do so, I need to get the elements using and id. I have tried the following but for some reason (maybe because I am doing it in the controller class and not on the main) I get a stack overflow exception.

public class ViewController {
    public Button exitBtn;

    public ViewController() throws IOException {
         Parent root = FXMLLoader.load(getClass().getResource("mainWindow.fxml"));
         Scene scene = new Scene(root);

         exitBtn = (Button) scene.lookup("#exitBtn");
    }
}

So how will I get an element (for example a button) using it's id as a reference?

The fxml block for the button is:

<Button fx:id="exitBtn" contentDisplay="CENTER" mnemonicParsing="false"
        onAction="#handleButtonClick" text="Exit" HBox.hgrow="NEVER" HBox.margin="$x1"/>

Answer 1

Use a controller class, so that you don't need to use a lookup. The FXMLLoader will inject the fields into the controller for you. The injection is guaranteed to happen before the initialize() method (if you have one) is called

public class ViewController {

    @FXML
    private Button exitBtn ;

    @FXML
    private Button openBtn ;

    public void initialize() {
        // initialization here, if needed...
    }

    @FXML
    private void handleButtonClick(ActionEvent event) {
        // I really don't recommend using a single handler like this,
        // but it will work
        if (event.getSource() == exitBtn) {
            exitBtn.getScene().getWindow().hide();
        } else if (event.getSource() == openBtn) {
            // do open action...
        }
        // etc...
    }
}

Specify the controller class in the root element of your FXML:

<!-- imports etc... -->
<SomePane xmlns="..." fx:controller="my.package.ViewController">
<!-- ... -->
    <Button fx:id="exitBtn" contentDisplay="CENTER" mnemonicParsing="false" onAction="#handleButtonClick" text="Exit" HBox.hgrow="NEVER" HBox.margin="$x1" />
    <Button fx:id="openBtn" contentDisplay="CENTER" mnemonicParsing="false" onAction="#handleButtonClick" text="Open" HBox.hgrow="NEVER" HBox.margin="$x1" />
</SomePane>

Finally, load the FXML from a class other than your controller class (maybe, but not necessarily, your Application class) with

Parent root = FXMLLoader.load(getClass().getResource("path/to/fxml"));
Scene scene = new Scene(root);   
// etc...