Java BounceBall mouse event

问题

I am working with the example below. And I have a difficulty. Necessary to make the balls escape the mouse pointer. Change direction when they find the pointer.

I tried to pass as a parameter to the mouse position to the function "move" but get an error: "Exception in thread" Thread-2 "java.lang.NullPointerException"

Can anyone give me a hint how to do this? I'm lost.

Code:

public class SimpleBalls {

    private Point mousePoint;

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

    public SimpleBalls() {
        EventQueue.invokeLater(new Runnable() {
            @Override
            public void run() {
                try {
                    UIManager.setLookAndFeel(UIManager
                            .getSystemLookAndFeelClassName());
                } catch (ClassNotFoundException ex) {
                } catch (InstantiationException ex) {
                } catch (IllegalAccessException ex) {
                } catch (UnsupportedLookAndFeelException ex) {
                }

                JFrame frame = new JFrame("Spot");
                frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                frame.setLayout(new BorderLayout());
                Balls balls = new Balls();
                frame.add(balls);
                frame.setSize(400, 400);
                frame.setVisible(true);

                new Thread(new BounceEngine(balls)).start();

            }
        });
    }

    public static int random(int maxRange) {
        return (int) Math.round((Math.random() * maxRange));
    }

    public class Balls extends JPanel {

        private List<Ball> ballsUp;

        public Balls() {
            ballsUp = new ArrayList<Ball>(25);

            MouseAdapter handler = new MouseAdapter() {

                @Override
                public void mouseMoved(MouseEvent e) {
                    mousePoint = e.getPoint();
                    // System.out.println(mousePoint);
                }

                @Override
                public void mouseExited(MouseEvent e) {
                    mousePoint = e.getPoint();
                }

            };

            addMouseListener(handler);
            addMouseMotionListener(handler);

            for (int index = 0; index < 10 + random(10); index++) {
                ballsUp.add(new Ball(new Color(random(255), random(255),
                        random(255))));
            }

        }

        @Override
        protected void paintComponent(Graphics g) {
            super.paintComponent(g);
            Graphics2D g2d = (Graphics2D) g.create();
            g2d.setRenderingHint(RenderingHints.KEY_ANTIALIASING,
                    RenderingHints.VALUE_ANTIALIAS_ON);
            for (Ball ball : ballsUp) {
                ball.paint(g2d);
            }
            g2d.dispose();
        }

        public List<Ball> getBalls() {
            return ballsUp;
        }
    }

    public class BounceEngine implements Runnable {

        private Balls parent;

        public BounceEngine(Balls parent) {
            this.parent = parent;
        }

        @Override
        public void run() {

            int width = getParent().getWidth();
            int height = getParent().getHeight();

            for (Ball ball : getParent().getBalls()) {
                int x = random(width);
                int y = random(height);

                Dimension size = ball.getSize();

                if (x + size.width > width) {
                    x = width - size.width;
                }
                if (y + size.height > height) {
                    y = height - size.height;
                }

                ball.setLocation(new Point(x, y));

            }

            while (getParent().isVisible()) {

                SwingUtilities.invokeLater(new Runnable() {
                    @Override
                    public void run() {
                        getParent().repaint();
                    }
                });

                for (Ball ball : getParent().getBalls()) {
                    move(ball, mousePoint);
                }

                try {
                    Thread.sleep(100);
                } catch (InterruptedException ex) {
                    ex.printStackTrace();
                }

            }

        }

        public Balls getParent() {
            return parent;
        }

        public void move(Ball ball, Point mouse) {

            try {
                Point p = ball.getLocation();
                Point speed = ball.getSpeed();
                Dimension size = ball.getSize();

                System.out.println(mouse.x);

                int vx = speed.x;
                int vy = speed.y;

                int x = p.x;
                int y = p.y;

                if (x + vx < 0 || x + size.width + vx > getParent().getWidth()) {
                    vx *= -1;
                }
                if (y + vy < 0 || y + size.height + vy > getParent().getHeight()) {
                    vy *= -1;
                }
                x += vx;
                y += vy;

                ball.setSpeed(new Point(vx, vy));
                ball.setLocation(new Point(x, y));

            } catch (Exception e) {
                e.printStackTrace();
            }           

        }

    }

    public class Ball {

        private Color color;
        private Point location;
        private Dimension size;
        private Point speed;
        private int dimeter;

        public Ball(Color color) {
            Random rnd = new Random();
            dimeter = 10 + rnd.nextInt(35);

            setColor(color);

            speed = new Point(10 - random(20), 10 - random(20));
            size = new Dimension(dimeter, dimeter);

        }

        public Dimension getSize() {
            return size;
        }

        public void setColor(Color color) {
            this.color = color;
        }

        public void setLocation(Point location) {
            this.location = location;
        }

        public Color getColor() {
            return color;
        }

        public Point getLocation() {
            return location;
        }

        public Point getSpeed() {
            return speed;
        }

        public void setSpeed(Point speed) {
            this.speed = speed;
        }

        protected void paint(Graphics2D g2d) {

            Point p = getLocation();
            if (p != null) {
                g2d.setColor(getColor());
                Dimension size = getSize();
                g2d.fillOval(p.x, p.y, size.width, size.height);
            }

        }
    }
}

Thanks.

EDIT: Error..

java.lang.NullPointerException
    at balls.SimpleBalls$BounceEngine.move(SimpleBalls.java:170)
    at balls.SimpleBalls$BounceEngine.run(SimpleBalls.java:146)
    at java.lang.Thread.run(Unknown Source)

回答1:

The problem is mousepoint may not be set by the time move() is called. Just check to see if it is null in your move() method. It doesn't look like you've started to add the code to "escape the mouse pointer" -- I'm guessing you can handle figuring that part out on your own.

public void move(Ball ball, Point mouse) {
     Point p = ball.getLocation();
     Point speed = ball.getSpeed();
     Dimension size = ball.getSize();

     if (mouse != null) {
         System.out.println(mouse.x);  // here 
     }

     int vx = speed.x;
     int vy = speed.y;
     // ...

来源：https://stackoverflow.com/questions/25816205/java-bounceball-mouse-event