How to access a variable from one class controller file to another in JavaFX?

So I have an integer number called “size” saved to a controller class called SettingsStageController.java and I want that variable to be accessed through my other controller class file called GameStageController.java but I can’t seem to find out how.

SettingsStageController.java

/* has the int size variable stored in this file */
int size = 5;

public void startGame(ActionEvent event) throws IOException {

        FXMLLoader loader = new FXMLLoader(getClass().getResource("gameStage.fxml"));
        root = loader.load();
        stage = (Stage) ((Node) event.getSource()).getScene().getWindow(); // ti ston poutso
        scene = new Scene(root);
        stage.setTitle("DnB: " + Integer.toString(size) + "x" + Integer.toString(size) + " Game");
        stage.setScene(scene);
        stage.show();

        GameStageController gameStageController = loader.getController();
        gameStageController.showPane();

    }

GameStageController.java

    public class GameStageController implements Initializable {
    
    
    @FXML
    Text testText;

    @FXML
    AnchorPane twoXtwoPane;
    
    @FXML
    AnchorPane threeXthreePane;
    
    @FXML
    AnchorPane fourXfourPane;
    
    @FXML
    AnchorPane fiveXfivePane;
    
    
    public void showPane() {
        switch (/* I WANT TO PUT THE "SIZE" NUMBER HERE" */) {
            case 2:
                twoXtwoPane.setDisable(false);
                twoXtwoPane.setVisible(true);
                break;
            case 3:
                threeXthreePane.setDisable(false);
                threeXthreePane.setVisible(true);
                break;
            case 4:
                fourXfourPane.setDisable(false);
                fourXfourPane.setVisible(true);
                break;
            case 5:
                fiveXfivePane.setDisable(false);
                fiveXfivePane.setVisible(true);
                break;
            default:
                twoXtwoPane.setDisable(false);
                twoXtwoPane.setVisible(true);
                break;
        }
    } 
}

Answer

If a method needs data to perform its functionality, then that data should be a parameter to the method. You should do:

public class GameStageController implements Initializable {

    // ...

    public void showPane(int size) {
        switch (size) {
            // ...
        }
    }
}

and then of course

private int size = 5;

public void startGame(ActionEvent event) throws IOException {

    // ...

    GameStageController gameStageController = loader.getController();
    gameStageController.showPane(size);

}

If your GameStageController instance needs the size variable later on, you can create an instance variable in that class, and set it in the showPane method to the value passed as the parameter.