How does the code in Listing 1 BallPane.java change the direction of the ball movement?LIS...

How does the code in Listing 1 BallPane.java change the direction of the ball movement?

LISTING 1 BallPane.java

1 import javafx.animation.KeyFrame;
2 import javafx.animation.Timeline;
3 import javafx.beans.property.DoubleProperty;
4 import javafx.scene.layout.Pane;
5 import javafx.scene.paint.Color;
6 import javafx.scene.shape.Circle;
7 import javafx.util.Duration;
8
9 public class BallPane extends Pane {
10 public final double radius = 20;
11 private double x = radius, y = radius;
12 private double dx = 1, dy = 1;
13 private Circle circle = new Circle(x, y, radius);
14 private Timeline animation;
15
16 public BallPane() {
17 circle.setFill(Color.GREEN); // Set ball color
18 getChildren().add(circle); // Place a ball into this pane
19
20 // Create an animation for moving the ball
21 animation = new Timeline(
22 new KeyFrame(Duration.millis(50), e -> moveBall()));
23 animation.setCycleCount(Timeline.INDEFINITE);
24 animation.play(); // Start animation
25 }
26
27 public void play() {
28 animation.play();
29 }
30
31 public void pause() {
32 animation.pause();
33 }
34
35 public void increaseSpeed() {
36 animation.setRate(animation.getRate() + 0.1);
37 }
38
39 public void decreaseSpeed() {
40 animation.setRate(
41 animation.getRate() > 0 ? animation.getRate() - 0.1 : 0);
42 }
43
44 public DoubleProperty rateProperty() {
45 return animation.rateProperty();
46 }
47
48 protected void moveBall() {
49 // Check boundaries
50 if (x getWidth() - radius) {
51 dx *= -1; // Change ball move direction
52 }
53 if (y getHeight() - radius) {
54 dy *= -1; // Change ball move direction
55 }
56
57 // Adjust ball position
58 x += dx;
59 y += dy;
60 circle.setCenterX(x);
61 circle.setCenterY(y);
62 }
63 }