怎样修改JTree中每级节点前面的图标

import java.awt.*;

import java.awt.event.*;

import javax.swing.*;

import javax.swing.tree.*;

public class TreeDemo4

{

    public TreeDemo4()

    {

        JFrame f = new JFrame("TreeDemo");

        Container contentPane = f.getContentPane();

        

        DefaultMutableTreeNode root = new DefaultMutableTreeNode("资源管理器");

        DefaultMutableTreeNode node1 = new DefaultMutableTreeNode("我的公文包");

        DefaultMutableTreeNode node2 = new DefaultMutableTreeNode("我的电脑");

        DefaultMutableTreeNode node3 = new DefaultMutableTreeNode("收藏夹");

        DefaultMutableTreeNode node4 = new DefaultMutableTreeNode("Readme");

        

        DefaultTreeModel treeModel = new DefaultTreeModel(root);

        treeModel.insertNodeInto(node1, root, root.getChildCount());

        treeModel.insertNodeInto(node2, root, root.getChildCount());

        treeModel.insertNodeInto(node3, root, root.getChildCount());

        treeModel.insertNodeInto(node4, root, root.getChildCount());

        

        DefaultMutableTreeNode leafnode = new 

                DefaultMutableTreeNode("公司文件");

        treeModel.insertNodeInto(leafnode, node1, node1.getChildCount());

        leafnode = new DefaultMutableTreeNode("个人信件");

        treeModel.insertNodeInto(leafnode, node1, node1.getChildCount());

        leafnode = new DefaultMutableTreeNode("私人文件");

        treeModel.insertNodeInto(leafnode, node1, node1.getChildCount());

        

        leafnode = new DefaultMutableTreeNode("本机磁盘(C:)");

        treeModel.insertNodeInto(leafnode, node2, node2.getChildCount());

        leafnode = new DefaultMutableTreeNode("本机磁盘(D:)");

        treeModel.insertNodeInto(leafnode, node2, node2.getChildCount());

        leafnode = new DefaultMutableTreeNode("本机磁盘(E:)");

        treeModel.insertNodeInto(leafnode, node2, node2.getChildCount());

        

        DefaultMutableTreeNode node31 = new DefaultMutableTreeNode("网站列表");

        treeModel.insertNodeInto(node31, node3, node3.getChildCount());

        leafnode = new DefaultMutableTreeNode("奇摩站");

        treeModel.insertNodeInto(leafnode, node3, node3.getChildCount());

        leafnode = new DefaultMutableTreeNode("职棒消息");

        treeModel.insertNodeInto(leafnode, node3, node3.getChildCount());

        leafnode = new DefaultMutableTreeNode("网络书店");

        treeModel.insertNodeInto(leafnode, node3, node3.getChildCount());

        

        JTree tree = new JTree(treeModel);

        tree.setRowHeight(20); 

        

        DefaultTreeCellRenderer cellRenderer = (DefaultTreeCellRenderer)tree.getCellRenderer();

        

        cellRenderer.setLeafIcon(new ImageIcon(".\\icons\\leaf.gif"));

        cellRenderer.setOpenIcon(new ImageIcon(".\\icons\\open.gif"));

        cellRenderer.setClosedIcon(new ImageIcon(".\\icons\\close.gif"));

        cellRenderer.setFont(new Font("宋体",Font.PLAIN,12));

        cellRenderer.setBackgroundNonSelectionColor(Color.white);

        cellRenderer.setBackgroundSelectionColor(Color.yellow);

        cellRenderer.setBorderSelectionColor(Color.red);

        cellRenderer.setTextNonSelectionColor(Color.black);

        cellRenderer.setTextSelectionColor(Color.blue);

        

        tree.setEditable(true);

        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 TreeDemo4();

    }

}