JavaFX FXML: Constructor vs. `initialize()` Method – What's the Difference?

JavaFX FXML Controller: Distinguishing the Roles of Constructor and Initialize Method

In JavaFX, FXML is utilized to declaratively construct user interfaces. A controller class, as specified in the fx:controller attribute of the FXML file, serves as the interaction logic for the GUI elements. Two lifecycle methods, the constructor and the initialize() method, play vital roles in initializing the controller.

The constructor is invoked first, followed by the injection of @FXML fields with their corresponding Java objects. Subsequently, the initialize() method is executed.

Constructor vs. Initialize Method

The key distinction between the constructor and the initialize() method lies in the timing of their execution and their access to FXML annotations. The constructor executes first, but it cannot access fields annotated with @FXML. These fields are only accessible within the initialize() method.

This allows the constructor to perform tasks prior to the FXML component injection, while the initialize() method can access and manipulate these injected components.

Practical Use

The constructor should be used for tasks that need to be performed before the GUI is fully loaded, such as reading data from a database. The initialize() method, on the other hand, is suitable for initializing the GUI components, such as setting event handlers or modifying their default values.

Example

Consider the following code:

public class MainViewController {

    // Constructor
    public MainViewController() {
        System.out.println("first");
    }

    // Initialize method
    @FXML
    public void initialize() {
        initializeComponents();
        System.out.println("second");
    }

    private void initializeComponents() {
        // Initialize GUI components, e.g., add event handlers
    }
}

In this example, the constructor prints "first" before any Java objects are injected into the FXML-annotated fields. Then, the initialize() method prints "second" after these fields have been injected. This demonstrates the sequential execution of these lifecycle methods.

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