java下拉框内容监听

import javax.swing.*;
import java.awt.event.ItemListener;
import java.awt.event.ItemEvent;
import java.awt.*;

import java.awt.event.ActionListener;
import java.awt.event.ActionEvent;

public class da1 extends JFrame{
        public da1()
        {
            setLayout(null);
            setTitle("JFrame结课");
            Container c = getContentPane();
            
            //文本显示框
            JTextField jt1 = new JTextField();
            JTextField jt2 = new JTextField();
            jt1.setBounds(150, 220, 100, 50);
            jt2.setBounds(150, 280, 100, 50);
            
            c.add(jt1);
            c.add(jt2);
                    
            
            //颜色下拉框
            String[] b1 = new String[]{"红色","蓝色","绿色"};
            final JComboBox<String> lib = new JComboBox<String>(b1);
            lib.setBounds(150, 10, 100, 50);
            c.add(lib);
            
            
            lib.addItemListener(new ItemListener()
            {        
                    //开始itemStateChanged方法，该方法在选择一种颜色时自动调用
                    public void itemStateChanged(ItemEvent e) {
                    //首先获得选择的列表的索引号
                            int index = lib.getSelectedIndex();                    
                            //判断所选的内容
                            switch (index)
                            {
                                    case 0: //红
                                    jt1.setText("红");
                                    break;
                                    case 1: //蓝
                                    jt1.setText("蓝");
                                    break;
                                    case 2: //绿
                                    jt1.setText("绿");
                                    break;
                                    default: //其它情况
                                    break;
                            }
                    } 
            }); // 事件监听添加完成 
            
            
            
            
            
            
            //男女单选框
            JRadioButton db1 = new JRadioButton("男");
            JRadioButton db2 = new JRadioButton("女");
            ButtonGroup bu1 = new ButtonGroup();
            db1.setBounds(150, 80, 50, 20);
            db2.setBounds(150, 110, 50, 20);
            bu1.add(db1);
            bu1.add(db2);
            
            db1.addActionListener(new ActionListener()
            {
                    public void actionPerformed(ActionEvent e)
                    {
                        jt2.setText("");
                        jt2.setText("男");
                    }
            });
            
            db2.addActionListener(new ActionListener()
            {
                    public void actionPerformed(ActionEvent e)
                    {
                        jt2.setText("");
                        jt2.setText("女");
                    }
            });
            
            
            
            c.add(db1);
            c.add(db2);
            
            
            //确认取消按钮
            JButton jb1 = new JButton("确认");
            JButton jb2 = new JButton("取消");
            jb1.setBounds(80, 160, 100, 50);
            jb2.setBounds(190, 160, 100, 50);
            
            jb1.addActionListener(new ActionListener()
                    {
                            public void actionPerformed(ActionEvent e)
                            {
                                String s=jt1.getText().toString();                                
                                System.out.println(s);                            
                                String s1=jt2.getText().toString();
                                System.out.println(s1);
                                
                                JOptionPane.showMessageDialog(null, "提交成功");
                            }
                    });
                            
            c.add(jb1);
            c.add(jb2);
            
        
            setBounds(800,300,400,400);
            setVisible(true);
        }
    
        public static void main(String args[])
        {
            new da1();
        }
}

lib.addItemListener(new ItemListener()//添加选择监听事件
            {        
                    //开始itemStateChanged方法，该方法在选择一种颜色时自动调用
                    public void itemStateChanged(ItemEvent e) {
                    //首先获得选择的列表的索引号
                            int index = lib.getSelectedIndex();                    
                            //判断所选的内容
                            switch (index)
                            {
                                    case 0: //红
                                    jt1.setText("红");
                                    break;
                                    case 1: //蓝
                                    jt1.setText("蓝");
                                    break;
                                    case 2: //绿
                                    jt1.setText("绿");
                                    break;
                                    default: //其它情况
                                    break;
                            }
                    } 
            }); // 事件监听添加完成