问JTree在重新加载后避免崩溃

EN

import java.awt.BorderLayout;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JScrollPane;
import javax.swing.UIManager;

import javax.swing.JTree;
import javax.swing.tree.DefaultMutableTreeNode;
import javax.swing.tree.TreeSelectionModel;
import javax.swing.event.TreeSelectionEvent;
import javax.swing.event.TreeSelectionListener;

import java.awt.GridLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.JButton;
import javax.swing.UnsupportedLookAndFeelException;
import javax.swing.tree.DefaultTreeModel;
import javax.swing.tree.TreePath;

public class TestTree extends JPanel {

    private JTree tree;
    private DefaultTreeModel model;
    private JButton btnAdd;
    private int childCount;

    public TestTree() {
        super(new BorderLayout());

        //Create the nodes.
        DefaultMutableTreeNode top = new DefaultMutableTreeNode("The Java Series");
        createNodes(top);

        model = new DefaultTreeModel(top);

        //Create a tree that allows one selection at a time.
        tree = new JTree(model);
        tree.getSelectionModel().setSelectionMode(TreeSelectionModel.SINGLE_TREE_SELECTION);

        //Create the scroll pane and add the tree to it. 
        JScrollPane treeView = new JScrollPane(tree);

        //Add the split pane to this panel.
        add(treeView);

        btnAdd = new JButton("Add");
        btnAdd.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
                TreePath treePath = tree.getSelectionPath();
                if (treePath != null) {
                    DefaultMutableTreeNode node = (DefaultMutableTreeNode) treePath.getLastPathComponent();
                    DefaultMutableTreeNode child = new DefaultMutableTreeNode("Child " + (++childCount));
                    model.insertNodeInto(child, node, node.getChildCount());
                }
            }
        });

        add(btnAdd, BorderLayout.SOUTH);
    }

    private class BookInfo {

        public String bookName;

        public BookInfo(String book) {
            bookName = book;
        }

        public String toString() {
            return bookName;
        }
    }

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

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

        //original Tutorial
        book = new DefaultMutableTreeNode(new BookInfo("The Java Tutorial: A Short Course on the Basics"));
        category.add(book);

        //Tutorial Continued
        book = new DefaultMutableTreeNode(new BookInfo("The Java Tutorial Continued: The Rest of the JDK"));
        category.add(book);

        //JFC Swing Tutorial
        book = new DefaultMutableTreeNode(new BookInfo("The JFC Swing Tutorial: A Guide to Constructing GUIs"));
        category.add(book);

        //Bloch
        book = new DefaultMutableTreeNode(new BookInfo("Effective Java Programming Language Guide"));
        category.add(book);

        //Arnold/Gosling
        book = new DefaultMutableTreeNode(new BookInfo("The Java Programming Language"));
        category.add(book);

        //Chan
        book = new DefaultMutableTreeNode(new BookInfo("The Java Developers Almanac"));
        category.add(book);

        category = new DefaultMutableTreeNode("Books for Java Implementers");
        top.add(category);

        //VM
        book = new DefaultMutableTreeNode(new BookInfo("The Java Virtual Machine Specification"));
        category.add(book);

        //Language Spec
        book = new DefaultMutableTreeNode(new BookInfo("The Java Language Specification"));
        category.add(book);
    }

    /**
     * Create the GUI and show it. For thread safety, this method should be
     * invoked from the event dispatch thread.
     */
    private static void createAndShowGUI() {
        try {
            UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
        } catch (ClassNotFoundException | InstantiationException | IllegalAccessException | UnsupportedLookAndFeelException ex) {
            ex.printStackTrace();
        }

        //Create and set up the window.
        JFrame frame = new JFrame("TreeDemo");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        //Add content to the window.
        frame.add(new TestTree());

        //Display the window.
        frame.pack();
        frame.setVisible(true);
    }

    public static void main(String[] args) {
        //Schedule a job for the event dispatch thread:
        //creating and showing this application's GUI.
        javax.swing.SwingUtilities.invokeLater(new Runnable() {
            public void run() {
                createAndShowGUI();
            }
        });
    }

}