JavaFX, Label null pointer exception

Question

I am having the following problem with a program that I am currently writing, and I have searched on the internet, but I couldn't really find anything to help me understand the following problem

So inside another class I have written a method that executes this whenever the search button is clicked and the method looks like this:

public void searchButton(){
        try {
            new SearchController().display();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

And then the SearchController class looks something like this (I simplified it here):

public class SearchController {

    @FXML
    private Button cancelButton;

    @FXML
    private Label what;

    private static Stage stage;

    private static BorderPane borderPane;

    @FXML
    public void initialize(){
        what.setText("Testing"); // this woks
        cancelButton.setOnAction(e -> stage.close());
    }

    public void display() throws IOException {

        stage = new Stage();
        stage.setResizable(false);
        stage.setTitle("Product search");
        stage.initModality(Modality.APPLICATION_MODAL);
        FXMLLoader loader = new FXMLLoader();
        loader.setLocation(SearchController.class.getResource("Search.fxml"));
        borderPane = loader.load();
        Scene scene = new Scene(borderPane);
        stage.setScene(scene);
        //what.setText("Testing") and this doesn't work
        stage.showAndWait();

    }



}

Can someone please tell me why it is possible to write text on the initialize method (that method gets called after the borderPane = loader.load(); line...so why doesn't it work if I try to write on the label after that line?)

Thank you in advance

As an aside, note that your `static` fields are going to be a disaster if you ever end up loading the FXML file twice (and having both displayed at the same time). You don't need these at all, and they are bugs waiting to happen. You can use `cancelButton.setOnAction(e -> cancelButton.getScene().getWindow().hide());` to close the window (and get rid of the `stage` field. If you really need the `borderPane` field, make it an instance variable and inject it from the fxml file (i.e. remove `static` and just put `fx:id="borderPane"` on the root element of the fxml). — James_D, Mar 23 '16 at 19:53

score 6 · Accepted Answer · answered Mar 23 '16 at 19:44

6

The FXMLLoader creates an instance of the class specified in the fx:controller attribute of the FXML root element. It then injects the elements defined in the FXML file into the controller instance it created when the fx:id attributes match the field names. Then it calls the initialize() method on that instance.

You create an instance of the controller "by hand" with new SearchController(). This is not the same object that is created by the FXMLLoader. So now when you have loaded the fxml file you have two different instances of SearchController. So if you call what.setText(...) from the display() method, you are not calling it on the controller instance created by the FXMLLoader. Consequently, what has not been initialized in the instance on which you are calling what.setText(...), and you get a null pointer exception.

Since initialize() is invoked by the FXMLLoader on the instance it created, when you call what.setText(...) from the initialize() method, you are calling it on the instance created by the FXMLLoader, and so the FXML-injected fields for that instance have been initialized.

answered Mar 23 '16 at 19:44

James_D

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Thank you for the explanation, I have just started JavaFX so it is not that clear to me yet. What would be a good way to open a new window then? Not just in this case, in general? Because until now I thought that I should do that by creating a new Object of the controller class. – TimNeutron Mar 23 '16 at 20:59
1

Using the default setup, the `FXMLLoader` creates the controller from the fxml file, so in that setup you should never create the controller objects yourself. The code you have in the `display()` method is exactly the code you need to create a new window and display the content of the fxml file in it, but it just doesn't make sense to make this an instance method in the controller class; you would usually just have this code at the point in your application where you needed to create the window. – James_D Mar 23 '16 at 21:11
Oh I see. Thanks again, it looks like I misunderstood the concept. Just another quick question : This : `SearchController a = loader.getController()` doesn't create a new instance right? I think I got the concept now, thanks to you :D – TimNeutron Mar 23 '16 at 21:20
1

Yes, that's correct: `loader.getController()` gives you the controller that the loader created (you need to call it after you call `load()`, for reasons that become obvious if you think about it a bit) – James_D Mar 23 '16 at 21:25

JavaFX, Label null pointer exception

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