java swing下拉框两级联动

例如第一个下拉框选中“湖北”，则在第二个下拉框中列出“武汉，荆州，宜昌，襄樊”。
第一个下拉框选中“江苏”，则在第二个下拉框中列出“南京，苏州，镇江，无锡”。
等等......
请高手给出源代码

//:ComboBoxText.java
import java.awt.FlowLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.JComboBox;
import javax.swing.JFrame;

public class ComboBoxText {
private String[][] xxx = {
{"浙江","杭州","温州","绍兴","舟山",},
{"江苏","南京","无锡","徐州","蒙眬",},
{"上海","闵行区","普陀区","金山区",},
};

private Object[] a(){
Object[] os = new Object[xxx.length];
for(int i=0; i<xxx.length; i++){
os[i]=xxx[i][0];
}
return os;
}

private Object[] b(Object a){
for(int i=0; i<xxx.length; i++){
if(xxx[i][0].equals(a)){
Object[] o = new Object[xxx[i].length-1];
for(int j=0; j<o.length; j++)
o[j]=xxx[i][j+1];
return o;
}
}