一个关于j2se中canvas的问题,在线等,急!

//问题在注释,就是想获得canvas上某一点的颜色值.麻烦大家帮帮我啊,急.
//程序可以运行的
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import javax.swing.event.*;
import javax.swing.border.*;
import java.lang.*;
import java.util.*;public class a
{
public a()
{
    MainFrame frame=new MainFrame();
}
public static void main(String args[])
{
new a();
}
}
class MainFrame extends JFrame
{

JPanel panel=new JPanel();
JPanel canvasPanel=new JPanel();
Canvas canvas=new Canvas();
JButton btn=new JButton("getRGB(50,50)");

MainFrame()
{
addWindowListener(new WindowAdapter()
{public void windowClosing(WindowEvent e){dispose();System.exit(0);}});

this.setLayout(new BorderLayout());
canvas.setPreferredSize(new Dimension(200,200));
btn.addActionListener(new ActionListener()
{
public void actionPerformed(ActionEvent e)
{
//输出canvas上(50,50)的颜色???????
}
});
panel.add(btn);
this.add("North",panel);
this.add(canvas);

this.pack();
this.show();
}
}