JavaFX FXML Controllers: Constructor vs. `initialize()` Method – When Should I Use Which?

JavaFX FXML Controller: Constructor vs. Initialize Method

In JavaFX, the construction and initialization of FXML controllers involve two distinct methods: the default constructor and the initialize() method. This article delves into the differences between these methods and their respective roles in controller initialization.

Constructor: First Step in Controller Creation

The default constructor of the controller class is the first method called during its instantiation. It is responsible for creating an instance of the controller and performing any necessary initialization that does not require access to GUI components defined in the FXML file.

initialize() Method: Post-Processing and GUI Interaction

The initialize() method is called after the default constructor and the population of @FXML-annotated fields. Its primary purpose is to perform post-processing actions or setup operations that require access to these GUI components.

Key Difference: Access to @FXML Fields

The main distinction between the constructor and the initialize() method lies in their ability to access @FXML-annotated fields. These fields refer to components defined in the FXML file, and while the constructor does not have access to them, the initialize() method does.

This difference allows you to use the constructor for initializing properties of the controller that are unrelated to the GUI, such as data models, while the initialize() method can be used to handle interactions with GUI components, such as event handlers or data binding.

Example:

Consider the following code:

public class MainViewController {

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

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

The output of this code will be:

first
second

This demonstrates that the constructor is called first, followed by the initialize() method, which can access the @FXML-annotated field.

Conclusion

In summary, the constructor and initialize() method serve different purposes in JavaFX FXML controller initialization. The constructor initializes properties that do not require access to GUI components, while the initialize() method handles post-processing actions and interactions with GUI components. By understanding these distinctions, you can effectively leverage these methods to manage the state of your FXML controllers.

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