一个简单的java图形绘制

题目大意：做一个简单的交通灯，要求框架里有三个单选按钮（red，green，yellow），一组交通指示灯（三个灯泡）。选择单选按钮后，相应的灯会亮，一次只能亮一种灯。
以下是我的程序，执行以后面板上有按钮，可就是没有图像，求各位指点啊
import javax.swing.*;
import java.awt.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
public class TrafficLight extends JFrame
{
PaintPanel paintPanel=new PaintPanel();//申请画图面板，按钮面板和三个单选按钮
JPanel buttons=new JPanel();
JRadioButton red=new JRadioButton("red");
JRadioButton green=new JRadioButton("green");
JRadioButton yellow=new JRadioButton("yellow");
TrafficLight()
{
ButtonGroup ni=new ButtonGroup();//将单选按钮加入按钮组
ni.add(red);
ni.add(green);
ni.add(yellow);
buttons.setLayout(new FlowLayout());//将单选按钮加入按钮面板
buttons.add(red);
buttons.add(green);
buttons.add(yellow);
setLayout(new BorderLayout());//将按钮面板和画图面板加入框架
add(paintPanel,BorderLayout.NORTH);
add(buttons,BorderLayout.SOUTH );
red.addActionListener(new ActionListener(){//为三个单选按钮设置监听
public void actionPerformed(ActionEvent e){repaint();}});
yellow.addActionListener(new ActionListener(){
public void actionPerformed(ActionEvent e){repaint();}});

green.addActionListener(new ActionListener(){
public void actionPerformed(ActionEvent e){repaint();}});
}
public class PaintPanel extends JPanel
{
protected void paintComponent(Graphics g)
{
super.paintComponents(g);
g.drawRect(getWidth()/2-10,20, 20, 60);//按要求绘制交通灯
if(!red.isSelected())
g.drawOval(getWidth()/2,30,15,15);//亮红灯
else
g.fillOval(getWidth()/2,30,15,15);//熄灭红灯
if(!green.isSelected())
g.drawOval(getWidth()/2,50,15,15);//亮绿灯
else
g.fillOval(getWidth()/2,50,15,15);//熄灭绿灯
if(!yellow.isSelected())
g.drawOval(getWidth()/2,70,15,15);//亮黄灯
else
g.fillOval(getWidth()/2,70,15,15);//熄灭黄灯
}

}
public static void main(String [] args)//主函数
{
    TrafficLight mine=new TrafficLight();
    mine.setTitle("TrafficLight");
    mine.setSize(800,500);
    mine.setVisible(true);
}

}

[code="java"]

import javax.swing.*;
import java.awt.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

public class TrafficLight extends JFrame {
PaintPanel paintPanel = new PaintPanel();// 申请画图面板，按钮面板和三个单选按钮
JPanel buttons = new JPanel();
JRadioButton red = new JRadioButton("red", true);
JRadioButton green = new JRadioButton("green");
JRadioButton yellow = new JRadioButton("yellow");

TrafficLight() {
    paintPanel.setPreferredSize(new Dimension(600, 400));
    ButtonGroup ni = new ButtonGroup();// 将单选按钮加入按钮组
    ni.add(red);
    ni.add(green);
    ni.add(yellow);
    buttons.setLayout(new FlowLayout());// 将单选按钮加入按钮面板
    buttons.add(red);
    buttons.add(green);
    buttons.add(yellow);
    setLayout(new BorderLayout());// 将按钮面板和画图面板加入框架
    add(paintPanel, BorderLayout.NORTH);
    add(buttons, BorderLayout.SOUTH);
    red.addActionListener(new ActionListener() {// 为三个单选按钮设置监听
                public void actionPerformed(ActionEvent e) {
                    repaint();
                }
            });
    yellow.addActionListener(new ActionListener() {
        public void actionPerformed(ActionEvent e) {
            repaint();
        }
    });
    green.addActionListener(new ActionListener() {
        public void actionPerformed(ActionEvent e) {
            repaint();
        }
    });
}

public class PaintPanel extends JPanel {
    protected void paintComponent(Graphics g) {
        super.paintComponents(g);
        g.clearRect(0, 0, getWidth(), getHeight());
        g.drawRect(getWidth() / 2, 20, 30, 80);// 按要求绘制交通灯
        if (red.isSelected()) {
            lightRed(g);
        } else if (green.isSelected()) {
            lightGreen(g);
        } else
            lightYellow(g);
    }

}

private void lightRed(Graphics g) {
    g.setColor(Color.red);
    g.fillOval(getWidth() / 2, 30, 15, 15);
    g.setColor(Color.black);
    g.drawOval(getWidth() / 2, 50, 15, 15);
    g.drawOval(getWidth() / 2, 70, 15, 15);
}

private void lightGreen(Graphics g) {
    g.setColor(Color.green);
    g.fillOval(getWidth() / 2, 50, 15, 15);
    g.setColor(Color.black);
    g.drawOval(getWidth() / 2, 30, 15, 15);
    g.drawOval(getWidth() / 2, 70, 15, 15);
}

private void lightYellow(Graphics g) {
    g.setColor(Color.yellow);
    g.fillOval(getWidth() / 2, 70, 15, 15);
    g.setColor(Color.black);
    g.drawOval(getWidth() / 2, 30, 15, 15);
    g.drawOval(getWidth() / 2, 50, 15, 15);
}

public static void main(String[] args)// 主函数
{
    TrafficLight mine = new TrafficLight();
    mine.setTitle("TrafficLight");
    mine.setSize(800, 600);
    mine.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    mine.setVisible(true);
}

}
[/code]

代码还是有很多地方待改进