class BounceFrame extends JFrame {
    private BallCanvas canvas;

    public BounceFrame() {
        setSize(600, 500);
        setTitle("Bounce Ball");
        Container contentPane = getContentPane();
        canvas = new BallCanvas();
        contentPane.add(canvas, BorderLayout.CENTER); // add canvas to teh
                                                        // container of frame

        JPanel buttonPanel = new JPanel();

        addButton(buttonPanel, "Start", new ActionListener() {
            public void actionPerformed(ActionEvent evt) {
                addBall(); // call method
            }
        });

        addButton(buttonPanel, "Close", new ActionListener() {
            public void actionPerformed(ActionEvent evt) {
                System.exit(0);
            }
        });

        contentPane.add(buttonPanel, BorderLayout.SOUTH);
    }

    public void addButton(Container c, String title, ActionListener listener) {
        JButton button = new JButton(title);
        c.add(button); // add button to panel
        button.addActionListener(listener);
    }

    public void addBall() {
        // try{
        Ball b = new Ball(canvas);
        canvas.add(b); // add ball to canvas
        for (int i = 1; i <= 1000; i++) {
            b.move();
            // main()单线程运行，无法每次move()后，sleep()等一会儿，减慢速度。只有想别的办法，比如改变步长。但立即会有一蹿一蹿的跳跃感。
            
            
            for (int t = 0; t < 100; t++) { // 只有：每次move()一个小步长，然后for()空循环耗去一段时间。
                System.out.println();
            }
            
            
                try {
                    Thread.currentThread().sleep(5);
                } catch (InterruptedException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }    //main()也是一个线程。一样可以用线程操控的招。
            
            // 还可以：Timer（），用定时器，每次xxx毫秒后，触发事件处理器，事件处理方法中.move();
        }
        // }
        // catch (InterruptedException e) {}
    }
}

class BallCanvas extends JPanel {
    private ArrayList balls = new ArrayList();

    public void add(Ball b) {
        balls.add(b); // add to list
    }

    public void paintComponent(Graphics g) {
        super.paintComponent(g);
        Graphics2D g2 = (Graphics2D) g;
        for (int i = 0; i < balls.size(); i++) {
            Ball b = (Ball) balls.get(i);
            b.draw(g2);
        }
    }

}

class Ball {
    private Component canvas;

    private int x = 0;

    private int y = 0;

    private int dx = 2; // 小球每次移动步长。循环----调用.move()----x=x+dx;

    private int dy = 2; // 因为是单线程，独占cpu，所以跑得快。

    public Ball(Component c) {
        canvas = c;
    }

    public void draw(Graphics2D g2) {
        g2.fill(new Ellipse2D.Double(x, y, 15, 15));
    }

    public void move() {
        x += dx;
        y += dy;
        if (x < 0) {
            x = 0;
            dx = -dx;
        }
        if (x + 15 >= canvas.getWidth()) {
            x = canvas.getWidth() - 15;
            dx = -dx;
        }
        if (y < 0) {
            y = 0;
            dy = -dy;
        }
        if (y + 15 >= canvas.getHeight()) {
            y = canvas.getHeight() - 15;
            dy = -dy;
        }
        canvas.paint(canvas.getGraphics());
    }

}
*/