Chapter 20 Exercise 5, Introduction to Java Programming, Tenth Edition Y. Daniel LiangY.

20.5 (Combine colliding bouncing balls)
 The example in Section 20.7 displays multiple bouncing balls.
 Extend the example to detect collisions. Once two balls collide,
 remove the later ball that was added to the pane and add its radius
 to the other ball, as shown in Figure 20.17b. Use the Suspend button
 to suspend the animation and the Resume button to resume the animation.
 Add a mouse pressed handler that removes a ball when the mouse
 is pressed on the ball.

import javafx.animation.KeyFrame;
import javafx.animation.Timeline;
import javafx.application.Application;
import javafx.beans.property.DoubleProperty;
import javafx.geometry.Pos;
import javafx.stage.Stage;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.control.ScrollBar;
import javafx.scene.layout.BorderPane;
import javafx.scene.layout.HBox;
import javafx.scene.layout.Pane;
import javafx.scene.paint.Color;
import javafx.scene.shape.Circle;
import javafx.util.Duration;

public class MultipleBounceBall extends Application {
    @Override // Override the start method in the Application class
    public void start(Stage primaryStage) {
        MultipleBallPane ballPane = new MultipleBallPane();
        ballPane.setStyle("-fx-border-color: yellow");

        Button btAdd = new Button("+");
        Button btSubtract = new Button("-");
        HBox hBox = new HBox(10);
        hBox.getChildren().addAll(btAdd, btSubtract);
        hBox.setAlignment(Pos.CENTER);

        // Add or remove a ball
        btAdd.setOnAction(e -> ballPane.add());
        btSubtract.setOnAction(e -> ballPane.subtract());

        // Pause and resume animation
        ballPane.setOnMousePressed(e -> ballPane.pause());
        ballPane.setOnMouseReleased(e -> ballPane.play());

        // Use a scroll bar to control animation speed
        ScrollBar sbSpeed = new ScrollBar();
        sbSpeed.setMax(20);
        sbSpeed.setValue(10);
        ballPane.rateProperty().bind(sbSpeed.valueProperty());

        BorderPane pane = new BorderPane();
        pane.setCenter(ballPane);
        pane.setTop(sbSpeed);
        pane.setBottom(hBox);

        // Create a scene and place the pane in the stage
        Scene scene = new Scene(pane, 250, 150);
        primaryStage.setTitle("MultipleBounceBall"); // Set the stage title
        primaryStage.setScene(scene); // Place the scene in the stage
        primaryStage.show(); // Display the stage
    }

    protected class MultipleBallPane extends Pane {
        protected Timeline animation;

        public MultipleBallPane() {
            // Create an animation for moving the ball
            animation = new Timeline(
                    new KeyFrame(Duration.millis(50), e -> moveBall()));
            animation.setCycleCount(Timeline.INDEFINITE);
            animation.play(); // Start animation
        }

        public void add() {
            Color color = new Color(Math.random(),
                    Math.random(), Math.random(), 0.5);

            // Create ball and add listener
            Ball ball = new Ball(30, 30, 20, color);
            ball.setOnMouseClicked(e-> getChildren().remove(ball));

            // Add ball to pane
            getChildren().add(ball);
        }

        public void add(Ball ball) {
            ball.setOnMouseClicked(e-> getChildren().remove(ball));

            // Add ball to pane
            getChildren().add(ball);
        }

        public void subtract() {
            if (getChildren().size() > 0) {
                getChildren().remove(getChildren().size() - 1);
            }
        }

        public void play() {
            animation.play();
        }

        public void pause() {
            animation.pause();
        }

        public void increaseSpeed() {
            animation.setRate(animation.getRate() + 0.1);
        }

        public void decreaseSpeed() {
            animation.setRate(
                    animation.getRate() > 0 ? animation.getRate() - 0.1 : 0);
        }

        public DoubleProperty rateProperty() {
            return animation.rateProperty();
        }

        protected void moveBall() {
            for (int i = 0; i < this.getChildren().size(); i++) {
                Ball ball = (Ball)this.getChildren().get(i);

                // Check boundaries
                if (ball.getCenterX() < ball.getRadius() ||
                        ball.getCenterX() > getWidth() - ball.getRadius()) {
                    ball.dx *= -1; // Change ball move direction
                }
                if (ball.getCenterY() < ball.getRadius() ||
                        ball.getCenterY() > getHeight() - ball.getRadius()) {
                    ball.dy *= -1; // Change ball move direction
                }


                for (int j = i + 1; j < this.getChildren().size(); j++) {
                    Ball b = (Ball)this.getChildren().get(j);
                    if (b.intersects(ball.getBoundsInLocal())) {
                        this.getChildren().removeAll(ball, b);

                        double radius = b.getRadius() + ball.getRadius();

                        double x = (b.getCenterX() - radius < 0 || b.getCenterX() + radius > getWidth()) ? radius + 1 : b.getCenterX();
                        double y = (b.getCenterY() - radius < 0 ||  b.getCenterY() + radius > getHeight()) ? radius + 1 : b.getCenterY();
                        Ball newBall = new Ball(x, y, radius, ball.color);

                        this.add(newBall);
                    }
                }

                // Adjust ball position
                ball.setCenterX(ball.dx + ball.getCenterX());
                ball.setCenterY(ball.dy + ball.getCenterY());
            }
        }

    }

    class Ball extends Circle {
        protected double dx = 1, dy = 1;
        protected Color color;
        Ball(double x, double y, double radius, Color color) {
            super(x, y, radius);
            this.color = color;
            setFill(this.color); // Set ball color
        }
    }

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

import javafx.application.Application;
import javafx.geometry.Insets;
import javafx.geometry.Pos;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.control.ScrollBar;
import javafx.scene.layout.BorderPane;
import javafx.scene.layout.HBox;
import javafx.stage.Stage;

public class Exercise_05 extends MultipleBounceBall {

    @Override
    public void start(Stage primaryStage) {
        MultipleBallPane ballPane = new MultipleBallPane();
        ballPane.setStyle("-fx-border-color: yellow");

        Button btAdd = new Button("+");
        Button btSubtract = new Button("-");
        Button btSuspend = new Button("Suspend");
        Button btResume = new Button("Resume");

        HBox hBox = new HBox(10, btSuspend, btResume, btAdd, btSubtract);
        hBox.setPadding(new Insets(5));
        hBox.setAlignment(Pos.CENTER);

        // Add or remove a ball
        btAdd.setOnAction(e -> ballPane.add());
        btSubtract.setOnAction(e -> ballPane.subtract());

        // Pause and resume animation
        btSuspend.setOnAction(e -> ballPane.pause());
        btResume.setOnAction(e -> ballPane.play());

        // Use a scroll bar to control animation speed
        ScrollBar sbSpeed = new ScrollBar();
        sbSpeed.setMax(50);
        sbSpeed.setValue(10);
        ballPane.rateProperty().bind(sbSpeed.valueProperty());

        BorderPane pane = new BorderPane();
        pane.setCenter(ballPane);
        pane.setTop(sbSpeed);
        pane.setBottom(hBox);

        // Create a scene and place the pane in the stage
        Scene scene = new Scene(pane, 400, 400);
        primaryStage.setTitle("MultipleBounceBall"); // Set the stage title
        primaryStage.setScene(scene); // Place the scene in the stage
        primaryStage.show(); // Display the stage
    }
    public static void main(String[] args) {
        Application.launch(args);
    }
}