Adopt Adapter

import java.io.File;
import javax.swing.tree.*;
class FileNode extends DefaultMutableTreeNode 
               implements ExplorableTreeNode {
   private boolean explored = false;
   public FileNode(File file) { 
      setUserObject(file); 
   }
   public boolean getAllowsChildren() { return isDirectory(); }
   public boolean isLeaf()     { return !isDirectory(); }
   public File getFile()       { return (File)getUserObject(); }
   public boolean isDirectory() {
      File file = getFile();
      return file.isDirectory();
   }
   public String toString() {
      File file = (File)getUserObject();
      String filename = file.toString();
      int index = filename.lastIndexOf(File.separator);
      return (index != -1 && index != filename.length()-1) ?
                           filename.substring(index+1) : 
                           filename;
   }
   public boolean isExplored() { 
       return explored; 
   }
   public void explore() {
      if(!isDirectory())
         return;
      if(!isExplored()) {
         File file = getFile();
         File[] children = file.listFiles();
         for(int i=0; i < children.length; ++i) 
            add(new FileNode(children[i]));
         explored = true;
      }
   }
}

import java.io.File;
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import javax.swing.event.*;
import javax.swing.tree.*;
abstract class TreePanel extends JPanel {
   abstract TreeModel createTreeModel();
   public void createTree() {
      final JTree tree = new JTree(createTreeModel());
      JScrollPane scrollPane = new JScrollPane(tree);
      setLayout(new BorderLayout());
      add(scrollPane, BorderLayout.CENTER);
      tree.addTreeExpansionListener(new TreeExpansionListener() {
         public void treeCollapsed(TreeExpansionEvent e) {
            // Don't care about collapse events.
         }
         public void treeExpanded(TreeExpansionEvent e) {
            ...
            TreePath path = e.getPath();
            ExplorableTreeNode node = (ExplorableTreeNode)
                           path.getLastPathComponent();
            if( ! node.isExplored()) {
               DefaultTreeModel model = (DefaultTreeModel)tree.getModel();
               ...   
               node.explore();
               model.nodeStructureChanged(node);
            }
         }
         ...
      });
   }
}
public class Test extends JFrame {
   ...
   public Test() {
      ...
      updateButton.addActionListener(new ActionListener() {
         private boolean treeCreated = false;
         public void actionPerformed(ActionEvent event) {
            if(!treeCreated) {
               TreePanel treePanel = new TreePanel() {
                  public TreeModel createTreeModel() {
                     FileNode rootNode = new FileNode(new File("/"));
                     rootNode.explore();
                     return new DefaultTreeModel(rootNode);
                  }
               };
               ...
            }
         }
      });
   }
   public static void main(String args[]) {
      GJApp.launch(new Test(),"UI Builder Prototype", 300,300,600,175);
   }
}
// Class GJApp omitted for brevity.