如何使JTree中的跟节点不显示（急急急）

Question

//你看看这个是不是对你有帮助？
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import java.util.*;public class TreeDemo1
{
    public TreeDemo1()
    {
        JFrame f = new JFrame("TreeDemo");
        Container contentPane = f.getContentPane();
        
        String[] s1 = {"公司文件","个人信件","私人文件"};
        String[] s2 = {"本机磁盘(C:)","本机磁盘(D:)","本机磁盘(E:)"};
        String[] s3 = {"奇摩站","职棒消息","网络书店"};
        Hashtable hashtable1 = new Hashtable();
        Hashtable hashtable2 = new Hashtable();
        hashtable1.put("我的公文包",s1);
        hashtable1.put("我的电脑",s2);
        hashtable1.put("收藏夹",hashtable2);
        hashtable2.put("网站列表",s3);
        JTree tree = new JTree(hashtable1);
        JScrollPane scrollPane = new JScrollPane();
        scrollPane.setViewportView(tree);
        
        contentPane.add(scrollPane);
        f.pack();
        f.setVisible(true);
        
        f.addWindowListener(new WindowAdapter() {
            public void windowClosing(WindowEvent e) {
                System.exit(0);
            }
        });    }    public static void main(String args[]) {
    
        new TreeDemo1();
    }
}