java 画图

这几天看Javacore 运行上面的画图的例子 出来了一个窗口 但是没有得到想要的图像 面板上什么也没有 大家帮忙看下啊
import javax.swing.JFrame;

@SuppressWarnings("serial")
public class DrawFrame extends JFrame {

public static final int DEFAULT_WIDTH = 400;
public static final int DEFAULT_HEIGHT = 400;
public DrawFrame(){
setTitle("DrawTest");
setSize(DEFAULT_WIDTH, DEFAULT_HEIGHT);
DrawComponent component = new DrawComponent();
add(component);
}
}

import java.awt.Color;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.geom.Ellipse2D;
import java.awt.geom.Line2D;
import java.awt.geom.Rectangle2D;

import javax.swing.JComponent;

public class DrawComponent extends JComponent{
public DrawComponent(){
super.doLayout();
}
public void painComponent(Graphics g){
Graphics2D g2 = (Graphics2D) g;
double leftX = 100;
double topY = 100;
double width = 200;
double height = 150;

Rectangle2D rect = new Rectangle2D.Double(leftX, topY, width, height);
g2.draw(rect);

Ellipse2D ellipse = new Ellipse2D.Double();
ellipse.setFrame(rect);
g2.draw(ellipse);
g2.setPaint(Color.RED);
g2.drawString("Warning!", 100, 1