如何简单地实现KeyListener？

Question

public class MyPanel extends JPanel implements KeyListener {
    private char c = 'e';
    public MyPanel() {
        this.setPreferredSize(new Dimension(500,500));
        addKeyListener(this);
    }
    public void paintComponent(Graphics g) {
        super.repaint();
        g.drawString("the key that pressed is" + c, 250,250);
    }

    public void keyPressed(KeyEvent e) {
        c=e.getKeyChar();
        repaint();

    }

    public void keyReleased(KeyEvent e) {
    }


    public void keyTyped(KeyEvent e) {
        c=e.getKeyChar();
        repaint();
    }

    public static void main(String[] s) {
        JFrame f=new JFrame();
        f.getContentPane().add(new MyPanel());
        f.pack();
        f.setVisible(true);
    }
}

我尝试阅读这个但没有管理以了解如何简单地实现KeyListener.那么我需要改变什么呢？

Answer 1

以下是它不起作用的原因:

1. 在JPanel没有键盘焦点.(框架有它.)您可能希望requestFocus将面板添加到屏幕时.

2. 您需要repaint在图形更改时调用.

3. 你不能打电话repaint给paintComponent方法.

4. 您需要在再次绘制字符串之前清除绘图区域(否则所有字符将最终在彼此之上).

这是一个完整的工作示例:

class MyPanel extends JPanel implements KeyListener {
    private char c = 'e';

    public MyPanel() {
        this.setPreferredSize(new Dimension(500, 500));
        addKeyListener(this);
    }

    public void addNotify() {
        super.addNotify();
        requestFocus();
    }

    public void paintComponent(Graphics g) {
        g.clearRect(0, 0, getWidth(), getHeight());
        g.drawString("the key that pressed is " + c, 250, 250);
    }

    public void keyPressed(KeyEvent e) { }
    public void keyReleased(KeyEvent e) { }
    public void keyTyped(KeyEvent e) {
        c = e.getKeyChar();
        repaint();
    }

    public static void main(String[] s) {
        JFrame f = new JFrame();
        f.getContentPane().add(new MyPanel());
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        f.pack();
        f.setVisible(true);
    }
}

哦,您可能希望添加f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE)以在关闭窗口时终止应用程序.:-)