Java JTree set user object

import java.awt.Dimension;
import java.awt.GridLayout;
import java.net.URL;

import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JScrollPane;
import javax.swing.JSplitPane;
import javax.swing.JTree;
import javax.swing.event.TreeSelectionEvent;
import javax.swing.event.TreeSelectionListener;
import javax.swing.tree.DefaultMutableTreeNode;

public class Main extends JPanel implements TreeSelectionListener {
   private JTree tree;
   private static String lineStyle = "Horizontal";

   public Main() {
      super(new GridLayout(1, 0));

      // Create the nodes.
      DefaultMutableTreeNode top = new DefaultMutableTreeNode("The Java Series");
      createNodes(top);/*from ww  w  .  j  av  a 2 s.co  m*/

      tree = new JTree(top);
      tree.addTreeSelectionListener(this);

      tree.putClientProperty("JTree.lineStyle", lineStyle);

      JScrollPane treeView = new JScrollPane(tree);

      JSplitPane splitPane = new JSplitPane(JSplitPane.VERTICAL_SPLIT);
      splitPane.setTopComponent(treeView);

      Dimension minimumSize = new Dimension(100, 50);
      treeView.setMinimumSize(minimumSize);
      splitPane.setDividerLocation(100);
      splitPane.setPreferredSize(new Dimension(500, 300));

      add(splitPane);
   }

   public void valueChanged(TreeSelectionEvent e) {
      DefaultMutableTreeNode node = (DefaultMutableTreeNode) tree.getLastSelectedPathComponent();

      if (node == null)
         return;

      Object nodeInfo = node.getUserObject();
      if (node.isLeaf()) {
         BookInfo book = (BookInfo) nodeInfo;
         System.out.print(book.bookURL);
      }
      System.out.println(nodeInfo.toString());
   }

   private void createNodes(DefaultMutableTreeNode top) {
      DefaultMutableTreeNode category = null;
      DefaultMutableTreeNode book = null;

      category = new DefaultMutableTreeNode("Books Programmers");
      top.add(category);

      book = new DefaultMutableTreeNode(new BookInfo("C++", "Cpp"));
      category.add(book);

      book = new DefaultMutableTreeNode(new BookInfo("C", "c.html"));
      category.add(book);

      book = new DefaultMutableTreeNode(new BookInfo("Python", "python.html"));
      category.add(book);

      book = new DefaultMutableTreeNode(new BookInfo("SQL", "sql.html"));
      category.add(book);

      book = new DefaultMutableTreeNode(new BookInfo("HTML", "html.html"));
      category.add(book);

      book = new DefaultMutableTreeNode(new BookInfo("Javascript", "javascript.html"));
      category.add(book);

      category = new DefaultMutableTreeNode("B1");
      top.add(category);

      book = new DefaultMutableTreeNode(new BookInfo("CSS", "css.html"));
      category.add(book);

      book = new DefaultMutableTreeNode(new BookInfo("Java", "java.html"));
      category.add(book);
   }

   public static void main(String[] args) {
      JFrame frame = new JFrame("TreeDemo");
      frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

      frame.add(new Main());

      frame.pack();
      frame.setVisible(true);
   }
}

class BookInfo {
   public String bookName;
   public URL bookURL;

   public BookInfo(String book, String filename) {
      bookName = book;
      bookURL = getClass().getResource(filename);
      if (bookURL == null) {
         System.err.println("Couldn't find file: " + filename);
      }
   }
   public String toString() {
      return bookName;
   }
}