How to call functions on the stage in JavaFX's controller file

Question

I am using javafx along with fxml, so I use the controller for the real coding. I need to do a few operations on the stage, such as getting

Answer 1

Sample Solution

You can initialize the stage in the controller using the technique from: Passing Parameters JavaFX FXML.

Here is a sample program which creates a utility window which tracks the x and y co-ordinates of the screen as you drag the utility window around. The contents of the utility window are rendered in an fxml defined pane.

StageTrackingSample.java

import javafx.application.Application;
import javafx.fxml.FXMLLoader;
import javafx.scene.*;
import javafx.stage.*;

public class StageTrackingSample extends Application {

  @Override public void start(final Stage stage) throws Exception {
    final FXMLLoader loader = new FXMLLoader(
      getClass().getResource(
        "stagetracking.fxml"
      )
    );

    final Parent root = (Parent) loader.load();
    final StageTrackingController controller = loader.getController();
    controller.initData(stage);

    stage.initStyle(StageStyle.UTILITY);
    stage.setResizable(false);
    stage.setScene(new Scene(root));
    stage.show();
  }

  public static void main(String[] args) { launch(args); }
}

StageTrackingController.java

import javafx.beans.binding.Bindings;
import javafx.fxml.FXML;
import javafx.scene.control.*;
import javafx.stage.Stage;

public class StageTrackingController {
  @FXML private Label stageX;
  public void initialize() {}  
  public void initData(final Stage stage) {
    stageX.textProperty().bind(
      Bindings.format(
        "(%1$.2f, %2$.2f)", 
        stage.xProperty(), 
        stage.yProperty()
      )
    );
  }
}

stagetracking.fxml

Alternate Solutions

tarrsalah's answer of just getting the stage from an @FXML component is also a good way if you know that the root component of the controller has already been added to a scene which has already been added to a stage (which is often the case when something like a button event handler is fired).

Another way to do it is similar to tarrsalah's answer, but to use ChangeListeners on the scene property of an @FXML node and the window property of the changed scene. This allows you to track changes to the scene and stage in case the pane is moved to a new scene or stage. Most of the time you don't need to track those changes though as most panes are just added to a single scene which stays on one stage.

Answers to Additional Questions and Comments

Can I get a simpler answer?

tarrsalah already provided a simpler answer.

The only problem with a simpler answer in this case is that it might not provide enough context required for you to replicate the answer's solution and adapt it to your work.

I made my current answer as simple as I could, but, unfortunately, even the most basic JavaFX FXML application requires quite a bit code and markup to work.

I am a mere beginner in java

Don't use FXML when you are first starting to develop your initial Java and JavaFX applications. Instead, just stick with the standard Java API in your JavaFX code, for which there are many more tutorials as well as the excellent Ensemble Sample to refer to.

Make sure before beginning JavaFX, that you have completed all of the Java Tutorial Trails Covering the Basics. Only the basics of Java are required to start using JavaFX, you don't need to branch off into learning Java Enterprise Edition and can forget about Swing.

Consider using SceneBuilder and FXML for larger applications once you have written a few basic JavaFX applications, hand-coded some layouts according to the Java API and reached a level of comfort with the core technologies. At that time you will likely find that learning FXML is quite straightforward. FXML attributes and elements are just a reflection of the Java APIs.

please explain the other-than-usual bits of your code

I can't really do that as I don't know what is unusual for you.

If there are particular parts of the code that you cannot understand through your own knowledge or research, create a new StackOverflow question for each difficult concept.