添加计时器时，JPanel不响应KeyListener吗？

Question

I don't Understand why JPanel doesn't response to KeyListener when i add the following :

timer = new Timer(80,this);
timer.start();

and it works when i comment it ??

import java.awt.*;
import java.awt.event.*;

import javax.swing.*;

public class Test extends JPanel implements KeyListener,ActionListener{

    private static final long serialVersionUID = 1L;
    private Timer timer ;
    public Test( )  {
        addKeyListener(this);
        setFocusable(true);
        setBackground(Color.RED);
        setFocusTraversalKeysEnabled(false);
        setPreferredSize(new Dimension(700, 700));
        //timer = new Timer(80,this);
        //timer.start();
    }
    public static void main(String[] args) {
        JFrame frame = new JFrame();
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setVisible(true);

        Test t = new Test();
        frame.add(t);
        frame.pack();

    }
    @Override
    public void keyTyped(KeyEvent e) {
        // TODO Auto-generated method stub

    }
    @Override
    public void keyPressed(KeyEvent e) {
        System.out.println("pressed");

    }
    @Override
    public void keyReleased(KeyEvent e) {
        // TODO Auto-generated method stub

    }
    @Override
    public void actionPerformed(ActionEvent e) {
        //timer.start();

    }
}

Answer 1

Nothing happens because KeyEvents are only generated for the component with focus. Your panel doesn't have focus.

The order of the code should be:

    Test t = new Test();
    frame.add(t);
    frame.pack();
    frame.setVisible(true);

That is you need to add the components to the frame before the frame is visible, then focus will be placed on the panel.

However, that is still not the proper solution because the panel could lose focus in which case your code will stop working.

The better solution when using Swing is to use Key Bindings . Key Bindings allow you to bind a KeyStroke to an Action even when the component doesn't have focus. Read the section from the Swing tutorial on How to Use Key Bindings for more information.