Could a Label be modified by another method inside an extended class?

Arduino Mandarelli :

I'm trying to create a Cinema booking system that allows the User to purchase tickets and snacks, and at each click refresh the label.

I've two classes:

public class DashboardUserController{

  public Label subtotal;
  public static Double subtotalCounter = 0.0;
  public static Double filmPrice;

  @FXML
  public void onWaterBottleClick(){
    subtotalCounter = subtotalCounter + 1.50;
    orderRecipt = orderRecipt + "Water Bottle 1.50£ \n";
    setSubtotal();
  }
  // and similar methods for each type of snack 

public void setSubtotal(){
    subtotal.setText(subtotalCounter.toString() + " £");
  }

// set the price for each click on the label

  public void addTicket() {
    System.out.println(subtotalCounter);
    subtotalCounter = subtotalCounter + filmPrice;
    setSubtotal();
  } 
}

And another class which extends the DashboardUserController

public class TheatresController extends DashboardUserController {

  @FXML
  private void onClick(MouseEvent event) {                  //method for selection/deselection of seats
    ImageView imageView = (ImageView) event.getSource();
    if (imageView.getImage() == redSeat) {
      Alert alert = new Alert(Alert.AlertType.CONFIRMATION, "Do you want to proceed with the unbooking?", ButtonType.YES, ButtonType.NO);
      if (alert.showAndWait().orElse(ButtonType.NO) == ButtonType.YES) {
        imageView.setImage(imageView.getImage() == redSeat ? greenSeat : redSeat);
      }
    } else {
      Alert alert = new Alert(Alert.AlertType.CONFIRMATION, "Do you want to proceed with the booking?", ButtonType.YES, ButtonType.NO);
      if (alert.showAndWait().orElse(ButtonType.NO) == ButtonType.YES) {
        imageView.setImage(imageView.getImage() == redSeat ? greenSeat : redSeat);
        addTicket();
      }
    }
  }

Now if I Click on the food buttons the subtotalCounter and the Label correctly refreshed:

But when I click the seats, the price of the film is correctly added but the Label became "null" and give me a NullPointerException.

The strange thing here is that if I click again any snack the Label display correctly the price of the selected food plus the price of the film for each seat selected.

Firstly I thought that the problem was on the nullPointException so I checked this question. Then I tried to understand if an extended class as thiscan call a Label and modify it, the answer seem yes but for an awful reason doesn't seem apply to my scenario.

Thanks to everybody.

Matt :

So there is a lot wrong(Exaggeration for effect) but you are learning so I will do my best to help. Please read it all before copying and pasting into your program so you understand it

Starting off the reason you were getting a null pointer was due to the fact that you had extend DashBoardUserController which means that the DashBoardUserController that the program was initialized with was not talking to your TheatresController and when you tried to access it it gave you a null pointer because nothing was initialized in the extended class

So Step 1 remove the extend DashBoardUserController

So since that is gone we need to get a reference of DashBoardUserController to that class so that it can call the necessary methods we can do that on initialization of TheatresController like so

        FXMLLoader loader = new FXMLLoader(getClass().getResource(resourceLocation));
        Pane pane = loader.load();
        TheatresController controller = loader.getController();
        controller.setDashBoardUserControllerReference(this);//We'll get here

Next is to make necessary methods and variables in TheatresController

private DashboardUserController dashboardUserController;//Set at top of class

public void setDashBoardUserControllerReference(DashboardUserController dashboardUserController){
    this.dashboardUserController = dashboardUserController;
}

Done this will fix the null pointer moving on to generify your method because there is a lot of code replication which for you is unnecessary typing

@FXML
public void onjosephclick() {
    //weekPickerInput();                                //You can delete from here
    //addFilmsInList();
    //filmList.setShowLocation("Joseph P");
    //System.out.println("joseph button pressed");
    //try {
    //    Pane pane = FXMLLoader.load(getClass().getResource("scenes/josephpane.fxml"));
    //    seatsSelection.getChildren().setAll(pane);
    //} catch (IOException e) {
    //    System.out.println(e);
    //}
    //setStuff();
    //seatsavailable.setText(Integer.toString(seatsJoseph));
    //filmPrice = filmList.searchFilm().get().getPrice();
    //System.out.println("Price:"+filmPrice);            //Down to here
    genericOnClick("Joseph P","scenes/josephpane.fxml",seatsJoseph);
    //change the strings for each call and remove the unnecessary stuff 
    //in the methods it will clean it up a lot
}

private void genericOnClick(String location, String resourceLocation, int seats) {
    weekPickerInput();
    addFilmsInList();
    filmList.setShowLocation(location);
    System.out.println("Location:"+location);
    try {
        FXMLLoader loader = new FXMLLoader(getClass().getResource(resourceLocation));
        Pane pane = loader.load();
        TheatresController controller = loader.getController();
        controller.setDashBoardUserControllerReference(this);
        seatsSelection.getChildren().setAll(pane);
    } catch (IOException e) {
        System.out.println(e);
    }
    setStuff();
    seatsavailable.setText(Integer.toString(seats));
    filmPrice = filmList.searchFilm().get().getPrice();
    System.out.println("Price:"+filmPrice);
}

Some other things learn to learn eventually are java naming conventions, dealing with code replication I saw a good amount of code that can be generified like I showed you in the above method

Good luck code on good sir. If its not working I can post the full class but I think you should try and figure it out as its good practice and I gave you all the code you need let me know if you need more help