Java实训——编写一个窗体应用程序模拟交通信号灯。

实训要求：

代码：

import java.awt.*;  
import java.awt.event.*;  
import javax.swing.*;  
  
import java.awt.Graphics;  
  
public class MainClass extends JFrame {  
    public JComboBox box;  
    int flag = 0;  
    jpNewPanel jpNewPanel;  
  
    public static void main(String[] args) {  
    	MainClass frame = new MainClass();  
    	frame.setBounds(650,300,550,550);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setTitle("信号灯"); 
        frame.setVisible(true);  
    }  
  
    public MainClass() {  
        box = new JComboBox();  
        box.addItem("请选择");  
        box.addItem("红灯");  
        box.addActionListener(new ActionListener() {  
            public void actionPerformed(ActionEvent e) {  
                flag = box.getSelectedIndex(); 
                jpNewPanel.repaint();  
            }  
        });  
        bo