ComboBox的设置和监听

import java.applet.*;
import java.awt.*;
import java.awt.event.*;

import javax.swing.*;

public class ComboBox extends Applet{
    String str[]={"计算机1班","计算机2班","计算机3班","计算机4班"};
    Label l1=new Label("班级选项：");
    Label l2=new Label("选中的班级为：");
    Label l3=new Label("计算机1班");
    JComboBox cb=new JComboBox(str);
    JPanel row1=new JPanel();
    JPanel row2=new JPanel();
    FlowLayout layout = new FlowLayout();
    public void init(){
        row1.setLayout(layout);
        row1.add(l1);
        row1.add(cb);
        this.add(row1);
        row2.setLayout(layout);
        row2.add(l2);
        row2.add(l3);
        this.add(row2);
    }
    public void start(){
        cb.addItemListener(new ItemListener(){

            public void itemStateChanged(ItemEvent arg0) {
                l3.setText(arg0.getItem().toString());
            }

        });
    }

}