How to make the ball bounce off the walls in JavaFX?

Question

I am new to Javafx and I am creating a simple program. What I'm trying to achieve is get the ball to bounce off the walls, but I haven't figured out how to do that yet. Also, feel free to leave other suggestions about my code.

Here's the source code:

public class GamePractice extends Application {

    public static Circle circle;
    public static Pane canvas;
    private long counter = 0;

    @Override
    public void start(Stage primaryStage) {

        canvas = new Pane();
        Scene scene = new Scene(canvas, 800, 600);

        primaryStage.setTitle("Game");
        primaryStage.setScene(scene);
        primaryStage.show();

        circle = new Circle(15,Color.BLUE);
        circle.relocate(100, 100);         

        canvas.getChildren().addAll(circle);


        Timeline loop = new Timeline(new KeyFrame(Duration.millis(10), new EventHandler<ActionEvent>() {

            @Override
            public void handle(ActionEvent t) {
                if (counter++ % 10 == 0)
                {
                    circle.setLayoutX(circle.getLayoutX() + 10);
                    circle.setLayoutY(circle.getLayoutY() + 10);


                    //This is what I currently have, am I headed the right direction?
                    //Check X and Y for collision with the ball
                    if ((circle.getTranslateX() == canvas.getWidth() - 10) || (circle.getTranslateY() == canvas.getHeight() - 10)) {
                        //How do I make the ball bounce?
                    }

                }
            }

        }));

        loop.setCycleCount(Timeline.INDEFINITE);
        loop.play();
    }

I appreciate the help.

Instead of adding 10, subtract 10? Of course you would need a boolean variable to identify whether or not the ball should move to the left or right (would change once it hits one of the walls) For example, if it hits the left wall, set `movingRight = true` and when it hits the right wall, set `movingRight = false`. — Josh M, Nov 16 '13 at 19:41
I'm going to write a really simple example of this but not in JavaFX, just so you can see how you can do it. And then you could take some ideas from it and use it in yours. — Josh M, Nov 16 '13 at 19:45

score 13 · Accepted Answer · edited May 23 '17 at 11:53

One hint: you should avoid comparing double values for exact equality a == b.

With some small changes of your code you're already there:

package learn.javafx;

import javafx.animation.KeyFrame;
import javafx.animation.Timeline;
import javafx.application.Application;
import javafx.event.ActionEvent;
import javafx.event.EventHandler;
import javafx.geometry.Bounds;
import javafx.scene.Scene;
import javafx.scene.layout.Pane;
import javafx.scene.paint.Color;
import javafx.scene.shape.Circle;
import javafx.stage.Stage;
import javafx.util.Duration;

public class GamePractice extends Application {

    public static Circle circle;
    public static Pane canvas;

    @Override
    public void start(final Stage primaryStage) {

        canvas = new Pane();
        final Scene scene = new Scene(canvas, 800, 600);

        primaryStage.setTitle("Game");
        primaryStage.setScene(scene);
        primaryStage.show();

        circle = new Circle(15, Color.BLUE);
        circle.relocate(100, 100);

        canvas.getChildren().addAll(circle);

        final Timeline loop = new Timeline(new KeyFrame(Duration.millis(10), new EventHandler<ActionEvent>() {

            double deltaX = 3;
            double deltaY = 3;

            @Override
            public void handle(final ActionEvent t) {
                circle.setLayoutX(circle.getLayoutX() + deltaX);
                circle.setLayoutY(circle.getLayoutY() + deltaY);

                final Bounds bounds = canvas.getBoundsInLocal();
                final boolean atRightBorder = circle.getLayoutX() >= (bounds.getMaxX() - circle.getRadius());
                final boolean atLeftBorder = circle.getLayoutX() <= (bounds.getMinX() + circle.getRadius());
                final boolean atBottomBorder = circle.getLayoutY() >= (bounds.getMaxY() - circle.getRadius());
                final boolean atTopBorder = circle.getLayoutY() <= (bounds.getMinY() + circle.getRadius());

                if (atRightBorder || atLeftBorder) {
                    deltaX *= -1;
                }
                if (atBottomBorder || atTopBorder) {
                    deltaY *= -1;
                }
            }
        }));

        loop.setCycleCount(Timeline.INDEFINITE);
        loop.play();
    }

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

How to make the ball bounce off the walls in JavaFX?

1 Answers1

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