JavaFX CheckBox handle selection event

import javafx.application.Application;
import javafx.stage.Stage;
import javafx.scene.Scene;
import javafx.scene.control.RadioButton;
import javafx.scene.control.ToggleGroup;
import javafx.scene.control.CheckBox;
import javafx.scene.layout.StackPane;
import javafx.scene.layout.BorderPane;
import javafx.scene.layout.HBox;
import javafx.scene.shape.Circle;
import javafx.scene.shape.Rectangle;
import javafx.scene.shape.Ellipse;
import javafx.geometry.Pos;
import javafx.scene.paint.Color;

public class Main extends Application {
   protected Circle circle = new Circle(50); // Create a circle
   protected Rectangle rectangle = new Rectangle(); // Create a Rectangle
   protected Ellipse ellipse = new Ellipse(); // Crate an Ellipse


   @Override/*  w w  w  .ja v  a 2  s  .c o m*/
   public void start(Stage primaryStage) {
      // Set shape properties
      circle.setStroke(Color.BLACK);
      circle.setFill(Color.WHITE);
      rectangle.setStroke(Color.BLACK);
      rectangle.setWidth(150);
      rectangle.setFill(Color.WHITE);
      rectangle.setHeight(100);
      ellipse.setFill(Color.WHITE);
      ellipse.setStroke(Color.BLACK);
      ellipse.setRadiusX(100);
      ellipse.setRadiusY(50);

      // Create a hbox to hold buttons
      HBox paneForButtons = new HBox(5);
      paneForButtons.setAlignment(Pos.CENTER);
      RadioButton rbCircle = new RadioButton("Circle"); 
      RadioButton rbRectangle = new RadioButton("Rectangle"); 
      RadioButton rbEllipse = new RadioButton("Ellipse"); 

      // Create a toggle group for shapes
      ToggleGroup group = new ToggleGroup();
      rbCircle.setToggleGroup(group);
      rbRectangle.setToggleGroup(group);
      rbEllipse.setToggleGroup(group);

      // Create a check box
      CheckBox chkFill = new CheckBox("Fill");

      // Place buttons in the hbox
      paneForButtons.getChildren().addAll(rbCircle, 
         rbRectangle, rbEllipse, chkFill);

      // Create a stack pane to hold the shapes
      StackPane paneForShapes = new StackPane();
      paneForShapes.setStyle("-fx-border-color: black");

      // Create a border pane to hold nodes
      BorderPane pane = new BorderPane();
      pane.setBottom(paneForButtons);
      pane.setCenter(paneForShapes);

      // Create and register handlers
      rbCircle.setOnAction(e -> {
         if (rbCircle.isSelected()) {
            paneForShapes.getChildren().clear();
            paneForShapes.getChildren().add(circle);
         }
      });

      rbRectangle.setOnAction(e -> {
         if (rbRectangle.isSelected()) {
            paneForShapes.getChildren().clear();
            paneForShapes.getChildren().add(rectangle);
         }
      });

      rbEllipse.setOnAction(e -> {
         if (rbEllipse.isSelected()) {
            paneForShapes.getChildren().clear();
            paneForShapes.getChildren().add(ellipse);
         }
      });

      chkFill.setOnAction(e -> {
         if (chkFill.isSelected()) {
            circle.setFill(Color.BLACK);
            rectangle.setFill(Color.BLACK);
            ellipse.setFill(Color.BLACK);
         } else {
            circle.setFill(Color.WHITE);
            rectangle.setFill(Color.WHITE);
            ellipse.setFill(Color.WHITE);       }
      });

      // Create a scene and place it in the stage
      Scene scene = new Scene(pane, 400, 150);
      primaryStage.setTitle("java2s.com");
      primaryStage.setScene(scene);
      primaryStage.show();
   }
}