Jcombobox加item到固定位置，覆盖原有的item？怎么搞？

import java.awt.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.util.Vector;
import javax.swing.*;

public class ItemChange {
public static void main(String[] args){
final Vector v = new Vector();
v.add("1");
v.add("3");
v.add("2");
v.add("4");

JComboBox combo = new JComboBox(v);
JFrame f = new JFrame("我的Frame");
f.setSize(new Dimension(300,400));
f.getContentPane().add(combo,BorderLayout.NORTH);
f.getContentPane().add(new JLabel("test"),BorderLayout.CENTER);
f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
JButton btn = new JButton("替换");
f.getContentPane().add(btn,BorderLayout.SOUTH);
btn.addActionListener(new ActionListener(){

public void actionPerformed(ActionEvent a) {
v.remove(1);//把位置2的元素拿掉
v.add