下拉列表

package ComboBoxDemo;
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import java.util.*;


public class ComboBoxDemo extends JFrame {
private String[] cities = {"北京","上海","南京","苏州","珠海","广州"};
private JTextField textField = new JTextField(15);
private JComboBox comboBox = new JComboBox();
private JButton button = new JButton("添加更多城市选项");
private int count = 0;

public ComboBoxDemo(String title) {
super(title);
for(int i = 0; i < 4; i++)
comboBox.addItem(cities[count++]);

button.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
if(count < cities.length)
comboBox.addItem(cities[count++]);
}
});

comboBox.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
textField.setText("index: " + comboBox.getSelectedIndex() + " " + comboBox.getSelectedItem());
}
});

comboBox.setEditable(true);
textField.setEditable(false);
Container contentPane =  getContentPane();
contentPane.setLayout(new FlowLayout());

contentPane.add(comboBox);
contentPane.add(button);
contentPane.add(textField);

setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
pack();
setVisible(true);
}
public static void main(String[] args) {
// TODO Auto-generated method stub
new ComboBoxDemo("Hello");
}
}