How to pass Class1 variables to Class2 if in Class1 they are associated with FXML?

Question

Java

the .fxml file has a Text object with its own fx:id

<Text fx:id="system_messages" layoutX="202.0" layoutY="638.0" strokeType="OUTSIDE" strokeWidth="0.0" text="сообщений пока нет..." wrappingWidth="889.69921875">

in the main controller I declared this object with the same name as fx:id.

public class MainController implements Initializable {
    @FXML
    Text system_messages;
...

Now I need to change the text of the system_messages object , for example to "An error occurred", but not in the MainController but in another class, for example in the SystemMessagesController class in a dedicated controller that will change the textbox texts ... I'm trying to do it like this:

public class SystemMessagesController {
    MainController main = new MainController();
    
    public void system_messages(String type) {
        switch (type) {
            case "UnknownHostException": {
                FXMLLoader fxmlLoader = new FXMLLoader(getClass().getResource("main.fxml"));
                main = (MainController) fxmlLoader.getController ();
                main.system_messages.setText("Ошибка получения данных. Проверьте подключение к интернету.");
            }
            default: {
                main.system_messages.setText("Ошибка получения данных. Ошибка неизвестна.");
            }
        }
    }
}

But an error occurs

Caused by: java.lang.NullPointerException: Cannot read field "system_messages" because "this.main" is null

I understand it has something to do with @FXML before declaring the System_messages Text object; but I don't have enough knowledge to understand how it can be fixed... Still, I would like not to litter in the main controller, but to do it in the second one... Help please.

Answer 1

You are getting an error in the SystemMessagesController class with a value nullfor the reason that yours system_messagesis in another class, namely MainController.
You need to pass system_messagesfrom the MainController class to the SystemMessagesController class! And in the class itself, get system_messagesand substitute in replacements for the text of the message. And also in yours main.fxmlshould be

<Text fx:id="get_system_messages" layoutX="202.0" layoutY="638.0" strokeType="OUTSIDE" strokeWidth="0.0" text="сообщений пока нет..." wrappingWidth="889.69921875">

Should be something like this

main.get_system_messages.setText("сюда передаём system_messages");