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);
    }
}

I tried reading this yet didnt mange to understand how to simply implement a KeyListener. so what do i need to change for this to work?

Answer 1

Here are the reasons why it doesn’t work:

1. The JPanel does not have the keyboard focus. (The frame has it.) You probably want to requestFocus when the panel is added to the screen.

2. You need to call repaint when the graphic should change.

3. You mustn’t call repaint in the paintComponent method.

4. You need to clear the drawing area before drawing the string again (otherwise all characters will end up on top of each other).

Here’s a complete working example:

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);
    }
}

Oh, and you may want to add f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE) to make the application terminate when you close the window. 🙂