DefaultTreeModel.removeNodeFromParent throwing NullPointerException

Question

I have an applet which involves editing data represented by a JTree. However, I'm getting this exception when I try to remove a node from the tree (to move it by subsequently inserting it at +/-1 position):
java.lang.NullPointerException
at javax.swing.plaf.basic.BasicTreeUI.completeEditing(Unknown Source)
at javax.swing.plaf.basic.BasicTreeUI.completeEditing(Unknown Source)
at javax.swing.plaf.basic.BasicTreeUI$TreeSelectionHandler.valueChanged(Unknown Source)
at javax.swing.tree.DefaultTreeSelectionModel.fireValueChanged(Unknown Source)
at javax.swing.tree.DefaultTreeSelectionModel.notifyPathChange(Unknown Source)
at javax.swing.tree.DefaultTreeSelectionModel.removeSelectionPaths(Unknown Source)
at javax.swing.JTree.removeDescendantSelectedPaths(Unknown Source)
at javax.swing.JTree.removeDescendantSelectedPaths(Unknown Source)
at javax.swing.JTree$TreeModelHandler.treeNodesRemoved(Unknown Source)
at javax.swing.tree.DefaultTreeModel.fireTreeNodesRemoved(Unknown Source)
at javax.swing.tree.DefaultTreeModel.nodesWereRemoved(Unknown Source)
at javax.swing.tree.DefaultTreeModel.removeNodeFromParent(Unknown Source)
at ...[my code from here]This is the code that moves the node:
DefaultMutableTreeNode selectedNode = (DefaultMutableTreeNode)myTree.getLastSelectedPathComponent();
factbookTree.setEnabled(false);
DefaultMutableTreeNode parent = (DefaultMutableTreeNode)selectedNode.getParent();
DefaultTreeModel treeModel = (DefaultTreeModel)factbookTree.getModel();
int oldIndex = treeModel.getIndexOfChild(parent,selectedNode);
int newIndex = oldIndex + move;
//throws exception here:
treeModel.removeNodeFromParent(selectedNode);
//This also causes exception:
//parent.remove(selectedNode);
//treeModel.nodesWereRemoved(parent, new int[] {oldIndex}, new Object[] {selectedNode});Any ideas as to what the problem is?
It started throwing this error when I changed my objects overriding JTree and TreeNode, but I'm not doing anything far out here, just getting the CellRenderer to include a JCheckBox and adding code to store the state of the JCheckBox. There is no multithreading that I'm adding. The node is being visibly removed from the applet before the exception. My only thought was that I needed to initialise the tree model somwhere myself - the tree is being constructed by passing it the root node.
I'm using JDK 1.4.1 as a JApplet with the Java Plug-In on W2K.
cheers,
Andy

Answer 1

Yeah, I checked that it wasn't null. I've hacked the code down and here's a complete listing which shows the problem, apologies for dodgy formatting. The code is modified from the JCheckTree implementation I found at http://www.fawcette.com/archives/premier/mgznarch/javapro/2001/01jan01/vc0101/vc0101.asp
// FactbookBrowser.java
import java.applet.*;
import java.awt.*;
import java.awt.event.*;
import java.net.*;
import javax.swing.*;
import javax.swing.tree.*;
import javax.swing.event.*;
public class FactbookBrowser extends JApplet implements ActionListener {
     JButton upButton;
     JButton downButton;
     JCheckTree factbookTree;
     public FactbookBrowser() {
          //Set up the tree display
          CheckTreeNode root = new CheckTreeNode("Root");
          CheckTreeNode one = new CheckTreeNode("one");
          CheckTreeNode two = new CheckTreeNode("two");
          CheckTreeNode three = new CheckTreeNode("three");
          root.add(one);
          root.add(two);
          root.add(three);
          factbookTree = new JCheckTree(root);
          factbookTree.getSelectionModel().setSelectionMode(
               TreeSelectionModel.SINGLE_TREE_SELECTION
          factbookTree.putClientProperty("JTree.lineStyle", "Angled");
          JPanel moveButtons = new JPanel();
          upButton = new JButton("up");
          downButton = new JButton("down");
          moveButtons.add(upButton);
          moveButtons.add(downButton);
          upButton.addActionListener(this);
          downButton.addActionListener(this);
          //Add everything to scrollpanes, split panes and this
          JScrollPane treeScroll = new JScrollPane(factbookTree);
          JPanel treePanel = new JPanel();
          treePanel.setLayout(new BorderLayout());
          treePanel.add(treeScroll, BorderLayout.CENTER);
          treePanel.add(moveButtons, BorderLayout.SOUTH);
          this.getContentPane().setLayout(new BorderLayout());
          this.getContentPane().add(treePanel, BorderLayout.CENTER);
     private void moveNode(int move) {
          DefaultMutableTreeNode selectedNode = (DefaultMutableTreeNode)factbookTree.getLastSelectedPathComponent();
        if ((move == -1) && (selectedNode.getPreviousSibling() == null)) {
               System.out.println("Node already at start of section, can't move up");
               return;
          } else if ((move == 1) && (selectedNode.getNextSibling() == null)) {
               System.out.println("Node already at end of section, can't move down");
               return;
          } else if ((move != 1) && (move != -1)) {
               System.out.println("Can't move node more than one place");
               return;
          DefaultMutableTreeNode parent = (DefaultMutableTreeNode)selectedNode.getParent();
          DefaultTreeModel treeModel = (DefaultTreeModel)factbookTree.getModel();
          int oldIndex = treeModel.getIndexOfChild(parent,selectedNode);
          int newIndex = oldIndex + move;
System.out.println("*** treeModel:"+treeModel+" selectedNode:"+selectedNode+" new index:"+newIndex);
//@todo Throwing exception here! WHY?
          treeModel.removeNodeFromParent(selectedNode);
          //parent.remove(selectedNode);
          //treeModel.nodesWereRemoved(parent, new int[] {oldIndex}, new Object[] {selectedNode});
System.out.println("*** Removed node, inserting");
          treeModel.insertNodeInto(selectedNode,parent,newIndex);
System.out.println("*** Inserted Node");
          TreePath newPath = new TreePath(selectedNode.getPath());
          factbookTree.scrollPathToVisible(newPath);
          factbookTree.setSelectionPath(newPath);
          factbookTree.repaint();
     public void actionPerformed(ActionEvent ae) {
          if (ae.getSource() == upButton) {
               //Move the currently selected node up within its folder
               moveNode(-1);
          } else if (ae.getSource() == downButton) {
               //Move the currently selected node up within its folder
               moveNode(1);
    public static void main(String[] args) {
        FactbookBrowser browser = new FactbookBrowser();
// JCheckTree.java
import java.awt.*;
import javax.swing.*;
import javax.swing.tree.*;
public class JCheckTree extends JTree {
  public JCheckTree(TreeNode root) {
    super(root);
    super.setCellRenderer(new CheckTreeCellRenderer(this));
    setCellEditor(new CheckTreeCellEditor(this));
    setEditable(true);
  public void setCellRenderer(TreeCellRenderer renderer) {
    super.setCellRenderer(new CheckTreeCellRenderer(this, renderer));
  public void setEditorRenderer(TreeCellRenderer renderer) {
    setCellEditor(new CheckTreeCellEditor(this, renderer));
// CheckTreeNode.java
import java.util.*;
import javax.swing.tree.*;
public class CheckTreeNode extends DefaultMutableTreeNode {
     protected boolean selected, propagate;
     public CheckTreeNode(Object data) {
          this(data, false, true);
     public CheckTreeNode(Object data, boolean selected) {
          this(data, selected, true);
     public CheckTreeNode(Object data, boolean selected, boolean propagate) {
          super(data);
          this.selected = selected;
          this.propagate = propagate;
     public boolean isSelected() {
          return selected;
     public void setSelected(boolean selected) {
          this.selected = selected;
          if (propagate)
          propagateSelected(selected);
     public void propagateSelected(boolean selected) {
          Enumeration enum = children();
          while (enum.hasMoreElements()) {
               CheckTreeNode node = (CheckTreeNode)enum.nextElement();
               node.setSelected(selected);
     public void setUserObject(Object obj) {
          if (obj == this) return;
          super.setUserObject(obj);
// CheckTreeCellRenderer.java
import java.awt.*;
import java.util.*;
import javax.swing.*;
import javax.swing.tree.*;
public class CheckTreeCellRenderer extends JPanel implements TreeCellRenderer {
     protected CheckTreeNode node;
     protected TreeCellRenderer renderer;
     protected JCheckBox check;
     public CheckTreeCellRenderer(JTree tree) {
          this(tree, new DefaultTreeCellRenderer());
     public CheckTreeCellRenderer(JTree tree, TreeCellRenderer renderer) {
          setLayout(new BorderLayout());
          this.renderer = renderer;
          add(BorderLayout.CENTER,
          renderer.getTreeCellRendererComponent(tree, "", true, true, true, 0, true));
          check = new JCheckBox();
          add(BorderLayout.WEST, check);
     public Component getTreeCellRendererComponent(JTree tree,
                              Object value,
                              boolean selected,
                              boolean expanded,
                              boolean leaf,
                              int row,
                              boolean hasFocus) {
          if (value instanceof CheckTreeNode) {
               node = (CheckTreeNode)value;
               check.setSelected(node.isSelected());
               value = node.getUserObject();
          renderer.getTreeCellRendererComponent(tree, value, selected, expanded, leaf, row, hasFocus);
          return this;
// CheckTreeCellEditor.java
import java.awt.*;
import java.util.*;
import java.awt.event.*;
import javax.swing.*;
import javax.swing.tree.*;
import javax.swing.event.*;
public class CheckTreeCellEditor extends CheckTreeCellRenderer implements TreeCellEditor, ActionListener {
  protected CellEditorListener list;
  public CheckTreeCellEditor(JTree tree)
    this(tree, new DefaultTreeCellRenderer());
  public CheckTreeCellEditor(JTree tree, TreeCellRenderer renderer)
    super(tree, renderer);
    check.addActionListener(this);
  public Component getTreeCellEditorComponent(
    JTree tree, Object value, boolean selected,
    boolean expanded, boolean leaf, int row)
    return getTreeCellRendererComponent(
      tree, value, true, expanded, leaf, row, true);
  public boolean stopCellEditing()
    return true;
  public Object getCellEditorValue()
    node.setSelected(check.isSelected());
    return node;
  public boolean isCellEditable(EventObject event)
    return true;
  public boolean shouldSelectCell(EventObject event)
    return true;
  public void  cancelCellEditing()
    fireEditingCanceled();
     HashSet listeners = new HashSet();
  public void addCellEditorListener(CellEditorListener listener) {
    listeners.add(listener);
  public void removeCellEditorListener(CellEditorListener listener) {
    listeners.remove(listener);
  protected void fireEditingStopped() {
     Iterator i = listeners.iterator();
     while (i.hasNext()) {
          ((CellEditorListener)i.next()).editingStopped(new ChangeEvent(this));
  protected void fireEditingCanceled() {
     Iterator i = listeners.iterator();
     while (i.hasNext()) {
          ((CellEditorListener)i.next()).editingCanceled(new ChangeEvent(this));
  public void actionPerformed(ActionEvent event) {
    fireEditingStopped();
}