Java Swing - 如何处理JTree路径选择事件

import java.awt.BorderLayout;

import javax.swing.JFrame;
import javax.swing.JScrollPane;
import javax.swing.JTextField;
import javax.swing.JTree;
import javax.swing.event.TreeSelectionEvent;
import javax.swing.event.TreeSelectionListener;
import javax.swing.tree.DefaultMutableTreeNode;
import javax.swing.tree.TreePath;

public class Main extends JFrame implements TreeSelectionListener {

  public static void main(String[] args) {
    new Main();
  }

  DefaultMutableTreeNode root;
  DefaultMutableTreeNode e1 = new DefaultMutableTreeNode("1");
  DefaultMutableTreeNode e2 = new DefaultMutableTreeNode("2");
  DefaultMutableTreeNode e3 = new DefaultMutableTreeNode("3");
  DefaultMutableTreeNode e11 = new DefaultMutableTreeNode("1.1");
  DefaultMutableTreeNode e22 = new DefaultMutableTreeNode("2.2");
  DefaultMutableTreeNode e33 = new DefaultMutableTreeNode("3.3");
  DefaultMutableTreeNode e111 = new DefaultMutableTreeNode("1.1.1");
  DefaultMutableTreeNode e222 = new DefaultMutableTreeNode("2.2.2");
  DefaultMutableTreeNode e333 = new DefaultMutableTreeNode("3.3.3");
  DefaultMutableTreeNode aChild;
  private JTree tree;
  private JTextField currentSelectionField;

  public Main() {
    setDefaultCloseOperation(EXIT_ON_CLOSE);
    root = new DefaultMutableTreeNode("Root");
    root.add(e1);
    root.add(e2);
    root.add(e3);
    e1.add(e11);
    e2.add(e22);
    e3.add(e33);
    e11.add(e111);
    e22.add(e222);
    e33.add(e333);

    tree = new JTree(root);
    tree.addTreeSelectionListener(this);
    add(new JScrollPane(tree), BorderLayout.CENTER);
    currentSelectionField = new JTextField("Current Selection: NONE");
    add(currentSelectionField, BorderLayout.SOUTH);
    setSize(250, 275);
    setLocationRelativeTo(null);
    setVisible(true);
  }

  @Override
  public void valueChanged(TreeSelectionEvent event) {
    tree.expandPath(new TreePath(e11.getPath()));
    currentSelectionField.setText(event.getPath().toString());
  }
}