java之swing下拉菜单实现方法

import java.awt.*;import javax.swing.*;import java.awt.event.*;public class test extends JAppletimplements ItemListener{  JLabel jtf;  ImageIcon a1, a2, a3;  public void init(){    Container contentPane = getContentPane();    contentPane.setLayout(new FlowLayout());    JComboBox jc = new JComboBox();    jc.addItem("6");    jc.addItem("7");    jc.addItem("8");    jc.addItemListener(this);    contentPane.add(jc);    jtf = new JLabel(new ImageIcon("D:/data/Images/6.gif"));    contentPane.add(jtf);  }  public void itemStateChanged(ItemEvent ie){    String s = (String)ie.getItem();    jtf.setIcon(new ImageIcon("D:/data/Images/"+s+".gif"));  }}