使用定时器实现弹弹球

         今天模拟书上的一个例题做了一个弹弹球，是在画布上的指定位置画多个圆，经过一段的延时后，在附近位置重新画。使球看起来是动，通过JSpinner组件调节延时，来控制弹弹球的移动速度.

        BallsCanvas.java

public class BallsCanvas extends Canvas implements ActionListener,
    FocusListener {

  private Ball balls[]; // 多个球
  private Timer timer;

  private static class Ball {
    int x, y; // 坐标
    Color color; // 颜色
    boolean up, left; // 运动方向

    Ball(int x, int y, Color color) {
      this.x = x;
      this.y = y;
      this.color = color;
      up = left = false;
    }
  }

  public BallsCanvas(Color colors[], int delay) { // 初始化颜色、延时
    this.balls = new Ball[colors.length];
    for (int i = 0, x = 40; i < colors.length; i++, x += 40) {
      balls[i] = new Ball(x, x, colors[i]);
    }
    this.addFocusListener(this);
    timer = new Timer(delay, this); // 创建定时器对象，delay指定延时
    timer.start();

  }

  // 设置延时
  public void setDelay(int delay) {
    timer.setDelay(delay);
  }

  // 在canvas上面作画
  public void paint(Graphics g) {
    for (int i = 0; i < balls.length; i++) {
      g.setColor(balls[i].color); // 设置颜色
      balls[i].x = balls[i].left ? balls[i].x - 10 : balls[i].x + 10;
      if (balls[i].x < 0 || balls[i].x >= this.getWidth()) { // 到水平方向更改方向
        balls[i].left = !balls[i].left;
      }

      balls[i].y = balls[i].up ? balls[i].y - 10 : balls[i].y + 10;
      if (balls[i].y < 0 || balls[i].y >= this.getHeight()) { // 到垂直方向更改方向
        balls[i].up = !balls[i].up;
      }
      g.fillOval(balls[i].x, balls[i].y, 20, 20); // 画指定直径的圆
    }
  }

  // 定时器定时执行事件
  @Override
  public void actionPerformed(ActionEvent e) {
    repaint(); // 重画
  }

  // 获得焦点
  @Override
  public void focusGained(FocusEvent e) {
    timer.stop(); // 定时器停止

  }

  // 失去焦点
  @Override
  public void focusLost(FocusEvent e) {
    timer.restart(); // 定时器重启动

  }
}

        BallsJFrame.java

class BallsJFrame extends JFrame implements ChangeListener {

    private BallsCanvas ball;
    private JSpinner spinner;

    public BallsJFrame() {
      super("弹弹球");
      this.setBounds(300, 200, 480, 360);
      this.setDefaultCloseOperation(EXIT_ON_CLOSE);
      Color colors[] = { Color.red, Color.green, Color.blue,
          Color.magenta, Color.cyan };
      ball = new BallsCanvas(colors, 100);
      this.getContentPane().add(ball);

      JPanel panel = new JPanel();
      this.getContentPane().add(panel, "South");
      panel.add(new JLabel("Delay"));
      spinner = new JSpinner();
      spinner.setValue(100);
      panel.add(spinner);
      spinner.addChangeListener(this);
      this.setVisible(true);
    }

    @Override
    public void stateChanged(ChangeEvent e) {
      // 修改JSpinner值时，单击JSpinner的Up或者down按钮时，或者在JSpinner中按Enter键
      ball.setDelay(Integer.parseInt("" + spinner.getValue()));

    }

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

}