Swing中如何实现二级联动下拉列表

import javax.swing.*;
import java.awt.*;
import java.awt.event.ItemEvent;
public class MyFrame extends JFrame {
    //    声明下来列表JComboBox
    private JComboBox choice1;
    private JComboBox choice2;
    //定义一个数组
    private static String[] s1 = {"中国","美国","日本","澳大利亚","法国","德国","英国"};
    private static String[][] s2 = {
            {"北京","上海","广州","深圳"},
            {"华盛顿","纽约","芝加哥","旧金山"},
            {"东京","大阪","名古屋","北海道"},
            {"悉尼","堪培拉","珀斯"},
            {"巴黎","南特","德勒","鲁昂"},
            {"柏林","华沙","卢布林","波兹南"},
            {"伦敦","曼切斯特","诺丁汉","牛津"}
    };
    public MyFrame(String title){
        super(title);
        //        设置布局管理
        getContentPane().setLayout(new GridLayout(2,2,0,0));
//        创建标签
        JLabel label1 = new JLabel("选择你喜欢的国家：");
        label1.setHorizontalAlignment(SwingConstants.RIGHT);
        getContentPane().add(label1);
        //        实例化JComboBox对象
        choice1 = new JComboBox(s1);
        // 为组合框的选择动作注册监听事件,当此组合框的选择有变化时,另一个组合框自动更新内容
        choice1.addActionListener(e -> {
            choice2.removeAllItems();
            JComboBox cb = (JComboBox) e.getSource();
//            获得选择项目
            String itemString = (String) cb.getSelectedItem();
            System.out.println(itemString);
            int index = choice1.getSelectedIndex();
            for(int i=0; i<s2[index].length; i++) {
                choice2.addItem(s2[index][i]);
            }
        });
        getContentPane().add(choice1);
        //        创建标签
        JLabel label2 = new JLabel("选择城市：");
        label2.setHorizontalAlignment(SwingConstants.RIGHT);
        getContentPane().add(label2);
        //        实例化JComboBox对象
        choice2 = new JComboBox(s2[0]);
        //        注册Action事件监听，采用Lambda表达式
        choice2.addItemListener(e -> {
//            获得选择项目
            if (e.getStateChange() == ItemEvent.SELECTED){
//                获得选择项目
                String itemString = (String) e.getItem();
                System.out.println(itemString);
            }
        });
        getContentPane().add(choice2);
        //        设置窗口大小
        setSize(400,150);
//        设置窗口可见
        setVisible(true);
    }
}

public class HelloWorld {
    public static void main(String[] args) {
        MyFrame myFrame = new MyFrame("下拉列表");
    }
}