密钥适配器在JComponent中不起作用

Question

我尝试做一个小菜单，你可以用键盘更改选项，创建一个从键适配器扩展的内部类，然后将其添加到菜单类，但是我没有得到任何响应键盘，有人可以帮我解决这个问题吗？

public class MenuWindow extends JComponent 
{
    private Rectangle2D.Float returnButton;
    private Rectangle2D.Float mainMenuButton;
    private Rectangle2D.Float restartButton;
    private Rectangle2D.Float highScoreButton;
    private int selectedButton;

    public MenuWindow  ()
    {
        selectedButton=0;
        mainMenuButton = new Rectangle2D.Float(25, 25, 200, 75);
        restartButton = new  Rectangle2D.Float(25, 125, 200, 75);
        highScoreButton = new Rectangle2D.Float( 25, 225, 200, 75);
        returnButton = new  Rectangle2D.Float( 25, 325, 200, 75);           
        this.addKeyListener(new MenuKeyInput());
        this.requestFocus();            
    }

    public void paint (Graphics g)
    {
        Graphics2D g2 = (Graphics2D ) g;        
        g2.setComposite(AlphaComposite.getInstance(AlphaComposite.SRC_OVER, 0.6f));         
        g2.setColor(Color.BLACK);           
        g2.fillRect(0, 0, 250, 425);
        g2.setStroke(new BasicStroke(14));
        g2.setColor(Color.BLUE);
        g2.drawRect(0, 0, 250, 425);            
        //draw buttons 
        g2.setComposite(AlphaComposite.getInstance(AlphaComposite.SRC_OVER, 0.2f));
        g2.setColor(Color.BLUE);            
        g2.fill(mainMenuButton);
        g2.fill(restartButton);
        g2.fill(highScoreButton);
        g2.fill(returnButton);          
        // paint selected button
        paintSelectedButton(mainMenuButton, restartButton, highScoreButton, returnButton, g2);
    }

    public void paintSelectedButton(Rectangle2D.Float a,Rectangle2D.Float b,Rectangle2D.Float c,Rectangle2D.Float d, Graphics2D g)
    {
        g.setComposite(AlphaComposite.getInstance(AlphaComposite.SRC_OVER, 0.45f));
        g.setColor(Color.YELLOW);
        switch (selectedButton)
        {
        case 0:
            g.fill(a);
            break;
        case 1:
            g.fill(b);
            break;
        case 2:
            g.fill(c);
            break;
        case 3:
            g.fill(d);
            break;  
        }
    }
     //inner class
    public class MenuKeyInput extends KeyAdapter
    {
        public void keyPressed (KeyEvent e)
        {
            int keyCode = e.getKeyCode();

            if (keyCode == KeyEvent.VK_ESCAPE)System.exit(0);
            if (keyCode == KeyEvent.VK_UP)
            {
                selectedButton--;

                if (selectedButton == -1)
                {
                    selectedButton=3;
                }
                repaint();
            }
            else if (keyCode == KeyEvent.VK_DOWN)
            {
                selectedButton++;
                if (selectedButton ==4)
                {
                    selectedButton=0;
                }
                repaint();
            }               
        }
    }
}

这是我正在使用的主要课程：

public static void main (String arg[])
{

    JFrame frame = new JFrame ();
    MenuWindow window = new MenuWindow();
    frame.add(window);

    frame.setSize(new Dimension(250, 425));
    frame.setResizable(false);
    frame.setUndecorated(true);

    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.setResizable(false);
    frame.setLocationRelativeTo(null);

    frame.setVisible(true);

}