KeyListener未激活

Question

我在代码中找不到什么问题...我尝试通过输入sysout进行调试，但似乎keylistener是罪魁祸首。为什么不注册任何按键？

public class Pong extends Applet implements Runnable, KeyListener {

public void init() {
    this.resize(WIDTH, HEIGHT);
    this.addKeyListener(this);

    p1 = new HumanPaddle(1);

    thread = new Thread(this);
    thread.start();
}

public void update(Graphics g) {
    paint(g);
}

public void keyPressed(KeyEvent e) {

    if(e.getKeyCode() == KeyEvent.VK_UP) {
        p1.setUpAccel(true);
    }
    if(e.getKeyCode() == KeyEvent.VK_DOWN) {
        p1.setDownAccel(true);
    }
}

public void keyReleased(KeyEvent e) {
    int key = e.getKeyCode();

    if(key == KeyEvent.VK_UP) {
        p1.setUpAccel(false);
    }
    if(key == KeyEvent.VK_DOWN) {
        p1.setDownAccel(false);
    }
}

public void keyTyped(KeyEvent e) {

}
}