JavaFX Circle move

import javafx.application.Application;
import javafx.stage.Stage;
import javafx.scene.Scene;
import javafx.scene.layout.Pane;
import javafx.scene.layout.HBox;
import javafx.scene.layout.BorderPane;
import javafx.scene.control.Button;
import javafx.scene.paint.Color;
import javafx.scene.shape.Circle;
import javafx.geometry.Insets;
import javafx.geometry.Pos;

public class Main extends Application {
  @Override/*www. j ava 2 s  .c o m*/
  public void start(Stage primaryStage) {
    BallPane ballPane = new BallPane();

    HBox buttonPane = new HBox(5);
    Button btLeft = new Button("Left");
    Button btRight = new Button("Right");
    Button btUp = new Button("Up");
    Button btDown = new Button("Down");
    buttonPane.getChildren().addAll(btLeft, btRight, btUp, btDown);

    btLeft.setOnAction(e -> ballPane.moveLeft());
    btRight.setOnAction(e -> ballPane.moveRight());
    btUp.setOnAction(e -> ballPane.moveUp());
    btDown.setOnAction(e -> ballPane.moveDown());

    BorderPane pane = new BorderPane();
    pane.setCenter(ballPane);
    pane.setBottom(buttonPane);
    pane.setPadding(new Insets(0, 20, 0, 20));
    BorderPane.setAlignment(buttonPane, Pos.CENTER);

    Scene scene = new Scene(pane, 250, 250);
    primaryStage.setTitle("E15_03");
    primaryStage.setScene(scene);
    primaryStage.show();
  }

  

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

class BallPane extends Pane {
   private double radius = 15;
   private double width = 200;
   private double height = 200;
   Circle ball = new Circle(width / 2, height / 2, radius);

   BallPane() {
     ball.setFill(Color.WHITE);
     ball.setStroke(Color.BLACK);
     getChildren().add(ball);
   }

   public void moveLeft() {
     if (ball.getRadius() < ball.getCenterX()) {
       ball.setCenterX(ball.getCenterX() - 10);
     }
   }

   public void moveRight() {
     if (ball.getCenterX() < width - ball.getRadius()) {
       ball.setCenterX(ball.getCenterX() + 10);
     }
   }

   public void moveUp() {
     if (ball.getRadius() < ball.getCenterY()) {
       ball.setCenterY(ball.getCenterY() - 10);
     }
   }

   public void moveDown() {
     if (ball.getCenterY() < height - ball.getRadius()) {
       ball.setCenterY(ball.getCenterY() + 10);
     }
   }
 }