java为什么这个线程在睡觉前执行了两次？

Question

我现在正在写一个2D游戏来学习java，我使用一个线程作为计数器来动画播放器（它增加了一个整数，它告诉paintComponent方法绘制哪个图片）。但由于某种原因，线程在睡眠前执行了两次。相关的线程是船舶动画线程。

public class Frame extends JFrame implements KeyListener{
    public static Frame frame;
private JPanel contentPane;
RenderPanel panel;
public Ship ship = new Ship(36, 72);
int y = 0;
int time;
int animation = 0;
/**
 * Launch the application.
 */
public static void main(String[] args) {
    EventQueue.invokeLater(new Runnable() {
        public void run() {
            try {
                frame = new Frame();
                frame.setVisible(true);
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
    });
}

/**
 * Create the frame.
 */
public Frame() {
    setResizable(false);
    setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    panel = new RenderPanel();
    setBounds(100, 100, 256*3, 276*3);
    ship.setPos((this.getWidth()/2), (this.getHeight()/4)*3);
    contentPane = new JPanel();
    contentPane.setBorder(new EmptyBorder(0, 0, 0, 0));
    contentPane.setLayout(new BorderLayout(0, 0));
    setContentPane(contentPane);
    Thread backgr = new Thread("Hintergrund"){
        public void run(){
            try {
            while(true){

                if(frame.y < 0){
                    frame.y = 300;
                }
                frame.y -= 1;
                Thread.sleep(50);
                panel.repaint();
                }
            }
                catch (InterruptedException e) {

                    e.printStackTrace();
            }
        }

    };

    Thread anim = new Thread("shipanimation"){
         public void run(){
             try {
                 while(true){
                 if(Frame.frame.animation > 7){
                     Frame.frame.animation = 0;
                 }else{
                Frame.frame.animation++;
                 }
            System.out.println(Frame.frame.animation);
                panel.repaint();
                Thread.sleep(500);
                }
            } catch (Exception e) {
                e.printStackTrace();
            }
         }
    };
    backgr.start();
    anim.start();
    contentPane.add(panel);
    panel.setLayout(null);

}

谁能告诉我为什么会这样？该程序似乎增加了两次，并打印两次，然后睡眠500ms然后再次相同。

编辑：感谢您的提示，所以我将代码更改为：

public Frame() {
    aTimer = new Timer(400, this);
    aTimer.start();
    backTimer = new Timer(25, this);
    backTimer.start();
            setResizable(false);
    setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    panel = new RenderPanel();
    setBounds(100, 100, 256*3, 276*3);
    ship.setPos((this.getWidth()/2), (this.getHeight()/4)*3);
    contentPane = new JPanel();
    contentPane.setBorder(new EmptyBorder(0, 0, 0, 0));
    contentPane.setLayout(new BorderLayout(0, 0));
    setContentPane(contentPane);
    contentPane.add(panel);
    panel.setLayout(null);

} 
@Override
public void actionPerformed(ActionEvent e) {

    if(e.getSource() == aTimer){    
    if(Frame.frame.animation > 7){
         Frame.frame.animation = 0;
     }else{
    Frame.frame.animation++;
     }
    System.out.println(Frame.frame.animation);
    panel.repaint();
    }
    if(e.getSource() == backTimer){
        if(frame.y < 0){
            frame.y = 300;
        }
        frame.y -= 1;
        panel.repaint();
    }

}

但它仍然相同，计时器执行所有操作两次然后等待时间。