Calling a function from another class - help!

Question

I realize that this is probably a basic thing for people who have been working with JavaFX for a while, but it is eluding me, and I have been working on it for over a week.
I need to call a function that is in another class.  Here's the deal.  In EntryDisplayController.java, there are 2 possible passwords that can be accepted - one will give full access to the car, the second will limit functions on the car.  So when a password is entered and verified as to which one it is, I need to call a function in MainDisplayController.java to set a variable that will let the system know which password is being used - full or restricted.
So in MainDisplayController.java I have this snippet to see
public class MainDisplayController implements Initializable, ControlledScreen {
    ScreensController myController;
    public static char restrict;
     * Initializes the controller class.
    @Override
    public void initialize(URL url, ResourceBundle rb) {
        // TODO
    public void setRestriction(){
        restrict = 0;
        restrictLabel.setText("RESTRICTED");
    public void clearRestriction(){
        restrict = 1;
        restrictLabel.setText("");
And in EntryScreenDisplay.java I have this snippet:
public class EntryDisplayController implements Initializable, ControlledScreen {
     @FXML
    private Label passwordLabel ;
    static String password = new String();
    static String pwd = new String("");
     ScreensController myController;
     private MainDisplayController controller2;
     * Initializes the controller class.
    @Override
    public void initialize(URL url, ResourceBundle rb) {
        setPW(pwd);    // TODO
     @FXML
    private void goToMainDisplay(ActionEvent event){
        if(password.equals ("123456")){
          controller2.clearRestriction();
           myController.setScreen(ScreensFramework.MainDisplayID);
           pwd = "";
           password = "";
           setPW(pwd);
        else if(password.equals ("123457")){
           controller2.setRestriction();
            myController.setScreen(ScreensFramework.MainDisplayID);
           pwd = "";
           password = "";
           setPW(pwd);
        else{
            password = "";
            pwd = "";
            setPW(pwd);
When I enter the restricted (or full) password, I get a long list of errors, ending with
Caused by: java.lang.NullPointerException
  at velocesdisplay.EntryDisplayController.goToMainDisplay(EntryDisplayController.java:60)
Line 60 is where "controller2.setRestriction(); is.
As always, thanks for any help.
Kind regards,
David

Answer 1

You never set the controller2 variable to anything, which is why you get a null pointer exception when you try to reference it.
public static variables (and even singletons) are rarely a good idea as they introduce global state (sometimes they can be, but usually not).
If a static recorder like this can only have a binary value, use a boolean, if it can have multiple values, use an enum.
Some of the data sharing solutions in dependency injection - Passing Parameters JavaFX FXML - Stack Overflow might be more appropriate.
I also created a small framework for roll based solutions in JavaFX which perhaps might be useful:
http://stackoverflow.com/questions/19666982/is-there-a-way-to-implement-a-property-like-rendered-on-javafx
https://gist.github.com/jewelsea/7229260
It was just something I hacked together, so it's not a fully polished solution and you probably don't need something quite so complex.
What you need is a model class shared between the components of your application.  Dependency injection frameworks such as afterburner.fx can help achieve this in a fairly simple to use way.
If you don't want to go with a dependency injection framework, then you could use either a static singleton model class for your settings or pass the model class using the mechanism in defined in the passing parameters link I provided.  For a model class example, see the ClickCounter class from this example.   That example doesn't use FXML, but to put it together with the passing parameters solution, for your restriction model, you would have something like the following:
   private class Restricted {
     private final ReadOnlyBooleanWrapper restricted;
     public Restricted(boolen isRestricted) {
       restricted = new ReadOnlyBooleanWrapper(isRestricted);  
     public boolean isRestricted() {
       return restricted.get();
     public ReadOnlyBooleanProperty restrictedProperty() {
       return restricted.getReadOnlyProperty();
Then in your EntryDisplayController you have:
@FXML
public void goToMainDisplay(ActionEvent event) {
     // determine the restriction model as per your original code...
     RestictionModel restrictionModel = new RestrictionModel(<appropriate value>);
  FXMLLoader loader = new FXMLLoader(
  getClass().getResource(
   "mainDisplay.fxml"
  Stage stage = new Stage(StageStyle.DECORATED);
  stage.setScene(
   new Scene(
     (Pane) loader.load()
  MainDisplayController controller =
    loader.<MainDisplayController>getController();
  controller.initRestrictionModel(restrictionModel);
  stage.show();
class MainDisplayController() {
  @FXML private RestrictionModel restrictionModel;
  @FXML private RestrictLabel restrictLabel;
  public void initialize() {}
  // naming convention (if the restriction model should only be set once per controller, call the method init, otherwise call it set).
  public void initRestrictionModel(RestrictionModel restrictionModel) {
    this.restrictionModel = restrictionModel;
     // take some action based on the new restriction model, for example
    restrictLabel.textProperty.bind(
      restrictionModel.restrictedProperty().asString()
If you have a centralized controller for your navigation and instantiation of your FXML (like in this small FXML navigation framework), then you can handle the setting of restrictions on new screens centrally within the framework at the point where it loads up FXML or navigates to a screen (it seems like you might already have something like this with your ScreensController class).
If you do this kind of stuff a lot, then you would probably benefit from a move to a framework like afterburner.  If it is just a few times within your application, then perhaps something like the above outline will give you enough guidance to implement a custom solution.