KeyEvent不会更改JPanels的背景颜色

Question

I want the KeyEvent to change the background color of my JPanels. Nothing happens when I press anything on the keyboard. One of my applications specifications is that I need a 'Customized component extended from JPanel.' which is why I have another class for my graphics panel.

My problem is when G is pressed nothing happens but my center panel should turn green...

Here is code for part of my application.

public class Maths extends JFrame implements KeyListener
{    
private JPanel pNorth = new JPanel();
private JPanel pSouth = new JPanel();
private JPanel pCenter = new JPanel();
private JPanel pEast = new JPanel();
private JPanel pWest = new JPanel();
private File file;
private JPanel pDraw = new GraphicsPanel();

public static void main(String args[]) 
{
    new Maths();

}

public Maths()
{
    mainFrame = new JFrame();

    mainFrame.setTitle("Maths Test Game");
    mainFrame.setLayout(new BorderLayout());
    mainFrame.setSize(1200, 800);
    mainFrame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

    mainFrame.add(pNorth, BorderLayout.NORTH);
    mainFrame.add(pSouth, BorderLayout.SOUTH);
    mainFrame.add(pCenter, BorderLayout.CENTER);
    mainFrame.add(pEast, BorderLayout.EAST);
    mainFrame.add(pWest, BorderLayout.WEST);

    pNorth.setLayout(new FlowLayout());
    pSouth.setLayout(new FlowLayout());
    pCenter.setLayout(new FlowLayout());
    pEast.setLayout(new FlowLayout());
    pWest.setLayout(new FlowLayout());

    addKeyListener(this);
    setFocusable(true);

    mainFrame.setVisible(true);
}

class GraphicsPanel extends JPanel 
{
    GraphicsPanel() 
    {
        // set a preferred size for the custom panel.
        setPreferredSize(new Dimension(250, 300));
    }
    @Override
    public void paint(Graphics g) 
    {
        super.paint(g);
        // set blue color for drawing
        g.setColor(Color.blue);
        // face
        g.drawOval(90, 70, 80, 80);
        // eyes
        g.drawOval(110, 95, 5, 5);
        g.drawOval(145, 95, 5, 5);
        // nose
        g.drawLine(130, 95, 130, 115);
        // mouth
        g.drawArc(113, 115, 35, 20, 0, -180);

    }
}

@Override
public void keyPressed(KeyEvent e) 
{
    if(e.getKeyCode() == KeyEvent.VK_G)
    {
        pCenter.setBackground(Color.green);
    }
    repaint();
}

@Override
public void keyReleased(KeyEvent e) 
{

}

@Override
public void keyTyped(KeyEvent e) 
{

}

}

Answer 1

There are a multitude of reasons this is probably not working, generally, you don't want to attach KeyListener s to top level containers like JFrame , as they is simply to many things that can get in the way and prevent the frame from raising key events.

Instead, use the key bindings API. See How to Use Key Bindings for more details