窗口里画线

[YSS_YSS2013]

请教各位大虾两个问题。一是为什么下面的程序线画不出来了？二是为什么红色那一行必须那么做？

import javax.swing.*;
import java.awt.geom.*;
import java.awt.*;

class DrawTest
{
public static void main(String[] args)
{
MyFrame frame=new MyFrame();
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.setVisible(true);
}
}

class MyFrame extends JFrame
{
public MyFrame()
{
setSize(FRAME_WIDTH,FRAME_HEIGHT);
Toolkit kit=Toolkit.getDefaultToolkit();
Dimension screenSize=kit.getScreenSize();
setLocation((screenSize.width-FRAME_WIDTH)/2,(screenSize.height-FRAME_HEIGHT)/2);
setTitle("my first window");
add(new MyPanel());
}
public static final int FRAME_WIDTH=600;
public static final int FRAME_HEIGHT=400;
}

class MyPanel extends JPanel
{
public void paintComonent(Graphics g)
{
super.paintComponent(g);
Graphics2D g2=(Graphics2D)g;
Point2D pointBegin=new Point2D.Double(100,100);
Point2D pointEnd=new Point2D.Double(200,250);
Line2D line=new Line2D.Double(pointBegin,pointEnd);
g2.draw(line);
}
}