/*
 * @(#)DefaultMutableTreeNode.java	1.15 00/02/02
 *
 * Copyright 1997-2000 Sun Microsystems, Inc. All Rights Reserved.
 * 
 * This software is the proprietary information of Sun Microsystems, Inc.  
 * Use is subject to license terms.
 * 
 */

package javax.swing.tree;
   // ISSUE: this class depends on nothing in AWT -- move to java.util?

import java.io.*;
import java.util.*;


/**
 * A <code>DefaultMutableTreeNode</code> is a general-purpose node in a tree data
 * structure. 
 * For examples of using default mutable tree nodes, see
 * <a
 href="http://java.sun.com/docs/books/tutorial/uiswing/components/tree.html">How to Use Trees</a>
 * in <em>The Java Tutorial.</em>
 *
 * <p>
 *
 * A tree node may have at most one parent and 0 or more children.
 * <code>DefaultMutableTreeNode</code> provides operations for examining and modifying a
 * node's parent and children and also operations for examining the tree that
 * the node is a part of.  A node's tree is the set of all nodes that can be
 * reached by starting at the node and following all the possible links to
 * parents and children.  A node with no parent is the root of its tree; a
 * node with no children is a leaf.  A tree may consist of many subtrees,
 * each node acting as the root for its own subtree.
 * <p>
 * This class provides enumerations for efficiently traversing a tree or
 * subtree in various orders or for following the path between two nodes.
 * A <code>DefaultMutableTreeNode</code> may also hold a reference to a user object, the
 * use of which is left to the user.  Asking a <code>DefaultMutableTreeNode</code> for its
 * string representation with <code>toString()</code> returns the string
 * representation of its user object.
 * <p>
 * <b>This is not a thread safe class.</b>If you intend to use
 * a DefaultMutableTreeNode (or a tree of TreeNodes) in more than one thread, you
 * need to do your own synchronizing. A good convention to adopt is
 * synchronizing on the root node of a tree.
 * <p>
 * While DefaultMutableTreeNode implements the MutableTreeNode interface and
 * will allow you to add in any implementation of MutableTreeNode not all
 * of the methods in DefaultMutableTreeNode will be applicable to all
 * MutableTreeNodes implementations. Especially with some of the enumerations
 * that are provided, using some of these methods assumes the
 * DefaultMutableTreeNode contains only DefaultMutableNode instances. All
 * of the TreeNode/MutableTreeNode methods will behave as defined no
 * matter what implementations are added.
 *
 * <p>
 * <strong>Warning:</strong>
 * Serialized objects of this class will not be compatible with
 * future Swing releases.  The current serialization support is appropriate
 * for short term storage or RMI between applications running the same
 * version of Swing.  A future release of Swing will provide support for
 * long term persistence.
 *
 * @see MutableTreeNode
 *
 * @version 1.15 02/02/00
 * @author Rob Davis
 */
public class DefaultMutableTreeNode extends Object implements Cloneable,
       MutableTreeNode, Serializable
{

    /**
     * An enumeration that is always empty. This is used when an enumeration
     * of a leaf node's children is requested.
     */
    static public final Enumeration EMPTY_ENUMERATION
	= new Enumeration() {
	    public boolean hasMoreElements() { return false; }
	    public Object nextElement() {
		throw new NoSuchElementException("No more elements");
	    }
    };

    /** this node's parent, or null if this node has no parent */
    protected MutableTreeNode   parent;

    /** array of children, may be null if this node has no children */
    protected Vector		children;

    /** optional user object */
    transient protected Object	userObject;

    /** true if the node is able to have children */
    protected boolean		allowsChildren;


    /**
     * Creates a tree node that has no parent and no children, but which
     * allows children.
     */
    public DefaultMutableTreeNode() {
	this(null);
    }

    /**
     * Creates a tree node with no parent, no children, but which allows 
     * children, and initializes it with the specified user object.
     * 
     * @param userObject an Object provided by the user that constitutes
     *                   the node's data
     */
    public DefaultMutableTreeNode(Object userObject) {
	this(userObject, true);
    }

    /**
     * Creates a tree node with no parent, no children, initialized with
     * the specified user object, and that allows children only if
     * specified.
     * 
     * @param userObject an Object provided by the user that constitutes
     *        the node's data
     * @param allowsChildren if true, the node is allowed to have child
     *        nodes -- otherwise, it is always a leaf node
     */
    public DefaultMutableTreeNode(Object userObject, boolean allowsChildren) {
	super();
	parent = null;
	this.allowsChildren = allowsChildren;
	this.userObject = userObject;
    }


    //
    //  Primitives
    //

    /**
     * Removes <code>newChild</code> from its present parent (if it has a
     * parent), sets the child's parent to this node, and then adds the child
     * to this node's child array at index <code>childIndex</code>.
     * <code>newChild</code> must not be null and must not be an ancestor of
     * this node.
     *
     * @param	newChild	the MutableTreeNode to insert under this node
     * @param	childIndex	the index in this node's child array
     *				where this node is to be inserted
     * @exception	ArrayIndexOutOfBoundsException	if
     *				<code>childIndex</code> is out of bounds
     * @exception	IllegalArgumentException	if
     *				<code>newChild</code> is null or is an
     *				ancestor of this node
     * @exception	IllegalStateException	if this node does not allow
     *						children
     * @see	#isNodeDescendant
     */
    public void insert(MutableTreeNode newChild, int childIndex) {
	if (!allowsChildren) {
	    throw new IllegalStateException("node does not allow children");
	} else if (newChild == null) {
	    throw new IllegalArgumentException("new child is null");
	} else if (isNodeAncestor(newChild)) {
	    throw new IllegalArgumentException("new child is an ancestor");
	}

	    MutableTreeNode oldParent = (MutableTreeNode)newChild.getParent();

	    if (oldParent != null) {
		oldParent.remove(newChild);
	    }
	    newChild.setParent(this);
	    if (children == null) {
		children = new Vector();
	    }
	    children.insertElementAt(newChild, childIndex);
    }

    /**
     * Removes the child at the specified index from this node's children
     * and sets that node's parent to null. The child node to remove
     * must be a <code>MutableTreeNode</code>.
     *
     * @param	childIndex	the index in this node's child array
     *				of the child to remove
     * @exception	ArrayIndexOutOfBoundsException	if
     *				<code>childIndex</code> is out of bounds
     */
    public void remove(int childIndex) {
	MutableTreeNode child = (MutableTreeNode)getChildAt(childIndex);
	children.removeElementAt(childIndex);
	child.setParent(null);
    }

    /**
     * Sets this node's parent to <code>newParent</code> but does not 
     * change the parent's child array.  This method is called from
     * <code>insert()</code> and <code>remove()</code> to
     * reassign a child's parent, it should not be messaged from anywhere
     * else.
     *
     * @param	newParent	this node's new parent
     */
    public void setParent(MutableTreeNode newParent) {
	parent = newParent;
    }

    /**
     * Returns this node's parent or null if this node has no parent.
     *
     * @return	this node's parent TreeNode, or null if this node has no parent
     */
    public TreeNode getParent() {
	return parent;
    }

    /**
     * Returns the child at the specified index in this node's child array.
     *
     * @param	index	an index into this node's child array
     * @exception	ArrayIndexOutOfBoundsException	if <code>index</code>
     *						is out of bounds
     * @return	the TreeNode in this node's child array at  the specified index
     */
    public TreeNode getChildAt(int index) {
	if (children == null) {
	    throw new ArrayIndexOutOfBoundsException("node has no children");
	}
	return (TreeNode)children.elementAt(index);
    }

    /**
     * Returns the number of children of this node.
     *
     * @return	an int giving the number of children of this node
     */
    public int getChildCount() {
	if (children == null) {
	    return 0;
	} else {
	    return children.size();
	}
    }

    /**
     * Returns the index of the specified child in this node's child array.
     * If the specified node is not a child of this node, returns
     * <code>-1</code>.  This method performs a linear search and is O(n)
     * where n is the number of children.
     *
     * @param	aChild	the TreeNode to search for among this node's children
     * @exception	IllegalArgumentException	if <code>aChild</code>
     *							is null
     * @return	an int giving the index of the node in this node's child 
     *          array, or <code>-1</code> if the specified node is a not
     *          a child of this node
     */
    public int getIndex(TreeNode aChild) {
	if (aChild == null) {
	    throw new IllegalArgumentException("argument is null");
	}

	if (!isNodeChild(aChild)) {
	    return -1;
	}
	return children.indexOf(aChild);	// linear search
    }

    /**
     * Creates and returns a forward-order enumeration of this node's
     * children.  Modifying this node's child array invalidates any child
     * enumerations created before the modification.
     *
     * @return	an Enumeration of this node's children
     */
    public Enumeration children() {
	if (children == null) {
	    return EMPTY_ENUMERATION;
	} else {
	    return children.elements();
	}
    }

    /**
     * Determines whether or not this node is allowed to have children. 
     * If <code>allows</code> is false, all of this node's children are
     * removed.
     * <p>
     * Note: By default, a node allows children.
     *
     * @param	allows	true if this node is allowed to have children
     */
    public void setAllowsChildren(boolean allows) {
	if (allows != allowsChildren) {
	    allowsChildren = allows;
	    if (!allowsChildren) {
		removeAllChildren();
	    }
	}
    }

    /**
     * Returns true if this node is allowed to have children.
     *
     * @return	true if this node allows children, else false
     */
    public boolean getAllowsChildren() {
	return allowsChildren;
    }

    /**
     * Sets the user object for this node to <code>userObject</code>.
     *
     * @param	userObject	the Object that constitutes this node's 
     *                          user-specified data
     * @see	#getUserObject
     * @see	#toString
     */
    public void setUserObject(Object userObject) {
	this.userObject = userObject;
    }

    /**
     * Returns this node's user object.
     *
     * @return	the Object stored at this node by the user
     * @see	#setUserObject
     * @see	#toString
     */
    public Object getUserObject() {
	return userObject;
    }


    //
    //  Derived methods
    //

    /**
     * Removes the subtree rooted at this node from the tree, giving this
     * node a null parent.  Does nothing if this node is the root of its
     * tree.
     */
    public void removeFromParent() {
	MutableTreeNode parent = (MutableTreeNode)getParent();
	if (parent != null) {
	    parent.remove(this);
	}
    }

    /**
     * Removes <code>aChild</code> from this node's child array, giving it a
     * null parent.
     *
     * @param	aChild	a child of this node to remove
     * @exception	IllegalArgumentException	if <code>aChild</code>
     *					is null or is not a child of this node
     */
    public void remove(MutableTreeNode aChild) {
	if (aChild == null) {
	    throw new IllegalArgumentException("argument is null");
	}

	if (!isNodeChild(aChild)) {
	    throw new IllegalArgumentException("argument is not a child");
	}
	remove(getIndex(aChild));	// linear search
    }

    /**
     * Removes all of this node's children, setting their parents to null.
     * If this node has no children, this method does nothing.
     */
    public void removeAllChildren() {
	for (int i = getChildCount()-1; i >= 0; i--) {
	    remove(i);
	}
    }

    /**
     * Removes <code>newChild</code> from its parent and makes it a child of
     * this node by adding it to the end of this node's child array.
     *
     * @see		#insert
     * @param	newChild	node to add as a child of this node
     * @exception	IllegalArgumentException    if <code>newChild</code>
     *						is null
     * @exception	IllegalStateException	if this node does not allow
     *						children
     */
    public void add(MutableTreeNode newChild) {
	if(newChild != null && newChild.getParent() == this)
	    insert(newChild, getChildCount() - 1);
	else
	    insert(newChild, getChildCount());
    }



    //
    //  Tree Queries
    //

    /**
     * Returns true if <code>anotherNode</code> is an ancestor of this node
     * -- if it is this node, this node's parent, or an ancestor of this
     * node's parent.  (Note that a node is considered an ancestor of itself.)
     * If <code>anotherNode</code> is null, this method returns false.  This
     * operation is at worst O(h) where h is the distance from the root to
     * this node.
     *
     * @see		#isNodeDescendant
     * @see		#getSharedAncestor
     * @param	anotherNode	node to test as an ancestor of this node
     * @return	true if this node is a descendant of <code>anotherNode</code>
     */
    public boolean isNodeAncestor(TreeNode anotherNode) {
	if (anotherNode == null) {
	    return false;
	}

	TreeNode ancestor = this;

	do {
	    if (ancestor == anotherNode) {
		return true;
	    }
	} while((ancestor = ancestor.getParent()) != null);

	return false;
    }

    /**
     * Returns true if <code>anotherNode</code> is a descendant of this node
     * -- if it is this node, one of this node's children, or a descendant of
     * one of this node's children.  Note that a node is considered a
     * descendant of itself.  If <code>anotherNode</code> is null, returns
     * false.  This operation is at worst O(h) where h is the distance from the
     * root to <code>anotherNode</code>.
     *
     * @see	#isNodeAncestor
     * @see	#getSharedAncestor
     * @param	anotherNode	node to test as descendant of this node
     * @return	true if this node is an ancestor of <code>anotherNode</code>
     */
    public boolean isNodeDescendant(DefaultMutableTreeNode anotherNode) {
	if (anotherNode == null)
	    return false;

	return anotherNode.isNodeAncestor(this);
    }

    /**
     * Returns the nearest common ancestor to this node and <code>aNode</code>.
     * Returns null, if no such ancestor exists -- if this node and
     * <code>aNode</code> are in different trees or if <code>aNode</code> is
     * null.  A node is considered an ancestor of itself.
     *
     * @see	#isNodeAncestor
     * @see	#isNodeDescendant
     * @param	aNode	node to find common ancestor with
     * @return	nearest ancestor common to this node and <code>aNode</code>,
     *		or null if none
     */
    public TreeNode getSharedAncestor(DefaultMutableTreeNode aNode) {
	if (aNode == this) {
	    return this;
	} else if (aNode == null) {
	    return null;
	}

	int		level1, level2, diff;
	TreeNode	node1, node2;
	
	level1 = getLevel();
	level2 = aNode.getLevel();
	
	if (level2 > level1) {
	    diff = level2 - level1;
	    node1 = aNode;
	    node2 = this;
	} else {
	    diff = level1 - level2;
	    node1 = this;
	    node2 = aNode;
	}

	// Go up the tree until the nodes are at the same level
	while (diff > 0) {
	    node1 = node1.getParent();
	    diff--;
	}
	
	// Move up the tree until we find a common ancestor.  Since we know
	// that both nodes are at the same level, we won't cross paths
	// unknowingly (if there is a common ancestor, both nodes hit it in
	// the same iteration).
	
	do {
	    if (node1 == node2) {
		return node1;
	    }
	    node1 = node1.getParent();
	    node2 = node2.getParent();
	} while (node1 != null);// only need to check one -- they're at the
	// same level so if one is null, the other is
	
	if (node1 != null || node2 != null) {
	    throw new Error ("nodes should be null");
	}
	
	return null;
    }


    /**
     * Returns true if and only if <code>aNode</code> is in the same tree
     * as this node.  Returns false if <code>aNode</code> is null.
     *
     * @see	#getSharedAncestor
     * @see	#getRoot
     * @return	true if <code>aNode</code> is in the same tree as this node;
     *		false if <code>aNode</code> is null
     */
    public boolean isNodeRelated(DefaultMutableTreeNode aNode) {
	return (aNode != null) && (getRoot() == aNode.getRoot());
    }


    /**
     * Returns the depth of the tree rooted at this node -- the longest
     * distance from this node to a leaf.  If this node has no children,
     * returns 0.  This operation is much more expensive than
     * <code>getLevel()</code> because it must effectively traverse the entire
     * tree rooted at this node.
     *
     * @see	#getLevel
     * @return	the depth of the tree whose root is this node
     */
    public int getDepth() {
	Object	last = null;
	Enumeration	enum = breadthFirstEnumeration();
	
	while (enum.hasMoreElements()) {
	    last = enum.nextElement();
	}
	
	if (last == null) {
	    throw new Error ("nodes should be null");
	}
	
	return ((DefaultMutableTreeNode)last).getLevel() - getLevel();
    }



    /**
     * Returns the number of levels above this node -- the distance from
     * the root to this node.  If this node is the root, returns 0.
     *
     * @see	#getDepth
     * @return	the number of levels above this node
     */
    public int getLevel() {
	TreeNode ancestor;
	int levels = 0;

	ancestor = this;
	while((ancestor = ancestor.getParent()) != null){
	    levels++;
	}

	return levels;
    }


    /**
      * Returns the path from the root, to get to this node.  The last
      * element in the path is this node.
      *
      * @return an array of TreeNode objects giving the path, where the
      *         first element in the path is the root and the last
      *         element is this node.
      */
    public TreeNode[] getPath() {
	return getPathToRoot(this, 0);
    }

    /**
     * Builds the parents of node up to and including the root node,
     * where the original node is the last element in the returned array.
     * The length of the returned array gives the node's depth in the
     * tree.
     * 
     * @param aNode  the TreeNode to get the path for
     * @param depth  an int giving the number of steps already taken towards
     *        the root (on recursive calls), used to size the returned array
     * @return an array of TreeNodes giving the path from the root to the
     *         specified node 
     */
    protected TreeNode[] getPathToRoot(TreeNode aNode, int depth) {
	TreeNode[]              retNodes;

	/* Check for null, in case someone passed in a null node, or
	   they passed in an element that isn't rooted at root. */
	if(aNode == null) {
	    if(depth == 0)
		return null;
	    else
		retNodes = new TreeNode[depth];
	}
	else {
	    depth++;
	    retNodes = getPathToRoot(aNode.getParent(), depth);
	    retNodes[retNodes.length - depth] = aNode;
	}
	return retNodes;
    }

    /**
      * Returns the user object path, from the root, to get to this node.
      * If some of the TreeNodes in the path have null user objects, the
      * returned path will contain nulls.
      */
    public Object[] getUserObjectPath() {
	TreeNode[]          realPath = getPath();
	Object[]            retPath = new Object[realPath.length];

	for(int counter = 0; counter < realPath.length; counter++)
	    retPath[counter] = ((DefaultMutableTreeNode)realPath[counter])
		               .getUserObject();
	return retPath;
    }

    /**
     * Returns the root of the tree that contains this node.  The root is
     * the ancestor with a null parent.
     *
     * @see	#isNodeAncestor
     * @return	the root of the tree that contains this node
     */
    public TreeNode getRoot() {
	TreeNode ancestor = this;
	TreeNode previous;

	do {
	    previous = ancestor;
	    ancestor = ancestor.getParent();
	} while (ancestor != null);

	return previous;
    }


    /**
     * Returns true if this node is the root of the tree.  The root is
     * the only node in the tree with a null parent; every tree has exactly
     * one root.
     *
     * @return	true if this node is the root of its tree
     */
    public boolean isRoot() {
	return getParent() == null;
    }


    /**
     * Returns the node that follows this node in a preorder traversal of this
     * node's tree.  Returns null if this node is the last node of the
     * traversal.  This is an inefficient way to traverse the entire tree; use
     * an enumeration, instead.
     *
     * @see	#preorderEnumeration
     * @return	the node that follows this node in a preorder traversal, or
     *		null if this node is last
     */
    public DefaultMutableTreeNode getNextNode() {
	if (getChildCount() == 0) {
	    // No children, so look for nextSibling
	    DefaultMutableTreeNode nextSibling = getNextSibling();

	    if (nextSibling == null) {
		DefaultMutableTreeNode aNode = (DefaultMutableTreeNode)getParent();

		do {
		    if (aNode == null) {
			return null;
		    }

		    nextSibling = aNode.getNextSibling();
		    if (nextSibling != null) {
			return nextSibling;
		    }

		    aNode = (DefaultMutableTreeNode)aNode.getParent();
		} while(true);
	    } else {
		return nextSibling;
	    }
	} else {
	    return (DefaultMutableTreeNode)getChildAt(0);
	}
    }


    /**
     * Returns the node that precedes this node in a preorder traversal of
     * this node's tree.  Returns null if this node is the first node of the
     * traveral -- the root of the tree.  This is an inefficient way to
     * traverse the entire tree; use an enumeration, instead.
     *
     * @see	#preorderEnumeration
     * @return	the node that precedes this node in a preorder traversal, or
     *		null if this node is the first
     */
    public DefaultMutableTreeNode getPreviousNode() {
	DefaultMutableTreeNode previousSibling;
	DefaultMutableTreeNode myParent = (DefaultMutableTreeNode)getParent();

	if (myParent == null) {
	    return null;
	}

	previousSibling = getPreviousSibling();

	if (previousSibling != null) {
	    if (previousSibling.getChildCount() == 0)
		return previousSibling;
	    else
		return previousSibling.getLastLeaf();
	} else {
	    return myParent;
	}
    }

    /**
     * Creates and returns an enumeration that traverses the subtree rooted at
     * this node in preorder.  The first node returned by the enumeration's
     * <code>nextElement()</code> method is this node.<P>
     *
     * Modifying the tree by inserting, removing, or moving a node invalidates
     * any enumerations created before the modification.
     *
     * @see	#postorderEnumeration
     * @return	an enumeration for traversing the tree in preorder
     */
    public Enumeration preorderEnumeration() {
	return new PreorderEnumeration(this);
    }

    /**
     * Creates and returns an enumeration that traverses the subtree rooted at
     * this node in postorder.  The first node returned by the enumeration's
     * <code>nextElement()</code> method is the leftmost leaf.  This is the
     * same as a depth-first traversal.<P>
     *
     * Modifying the tree by inserting, removing, or moving a node invalidates
     * any enumerations created before the modification.
     *
     * @see	#depthFirstEnumeration
     * @see	#preorderEnumeration
     * @return	an enumeration for traversing the tree in postorder
     */
    public Enumeration postorderEnumeration() {
	return new PostorderEnumeration(this);
    }

    /**
     * Creates and returns an enumeration that traverses the subtree rooted at
     * this node in breadth-first order.  The first node returned by the
     * enumeration's <code>nextElement()</code> method is this node.<P>
     *
     * Modifying the tree by inserting, removing, or moving a node invalidates
     * any enumerations created before the modification.
     *
     * @see	#depthFirstEnumeration
     * @return	an enumeration for traversing the tree in breadth-first order
     */
    public Enumeration breadthFirstEnumeration() {
	return new BreadthFirstEnumeration(this);
    }

    /**
     * Creates and returns an enumeration that traverses the subtree rooted at
     * this node in depth-first order.  The first node returned by the
     * enumeration's <code>nextElement()</code> method is the leftmost leaf.
     * This is the same as a postorder traversal.<P>
     *
     * Modifying the tree by inserting, removing, or moving a node invalidates
     * any enumerations created before the modification.
     *
     * @see	#breadthFirstEnumeration
     * @see	#postorderEnumeration
     * @return	an enumeration for traversing the tree in depth-first order
     */
    public Enumeration depthFirstEnumeration() {
	return postorderEnumeration();
    }

    /**
     * Creates and returns an enumeration that follows the path from
     * <code>ancestor</code> to this node.  The enumeration's
     * <code>nextElement()</code> method first returns <code>ancestor</code>,
     * then the child of <code>ancestor</code> that is an ancestor of this
     * node, and so on, and finally returns this node.  Creation of the
     * enumeration is O(m) where m is the number of nodes between this node
     * and <code>ancestor</code>, inclusive.  Each <code>nextElement()</code>
     * message is O(1).<P>
     *
     * Modifying the tree by inserting, removing, or moving a node invalidates
     * any enumerations created before the modification.
     *
     * @see		#isNodeAncestor
     * @see		#isNodeDescendant
     * @exception	IllegalArgumentException if <code>ancestor</code> is
     *						not an ancestor of this node
     * @return	an enumeration for following the path from an ancestor of
     *		this node to this one
     */
    public Enumeration pathFromAncestorEnumeration(TreeNode ancestor){
	return new PathBetweenNodesEnumeration(ancestor, this);
    }


    //
    //  Child Queries
    //

    /**
     * Returns true if <code>aNode</code> is a child of this node.  If
     * <code>aNode</code> is null, this method returns false.
     *
     * @return	true if <code>aNode</code> is a child of this node; false if 
     *  		<code>aNode</code> is null
     */
    public boolean isNodeChild(TreeNode aNode) {
	boolean retval;

	if (aNode == null) {
	    retval = false;
	} else {
	    if (getChildCount() == 0) {
		retval = false;
	    } else {
		retval = (aNode.getParent() == this);
	    }
	}

	return retval;
    }


    /**
     * Returns this node's first child.  If this node has no children,
     * throws NoSuchElementException.
     *
     * @return	the first child of this node
     * @exception	NoSuchElementException	if this node has no children
     */
    public TreeNode getFirstChild() {
	if (getChildCount() == 0) {
	    throw new NoSuchElementException("node has no children");
	}
	return getChildAt(0);
    }


    /**
     * Returns this node's last child.  If this node has no children,
     * throws NoSuchElementException.
     *
     * @return	the last child of this node
     * @exception	NoSuchElementException	if this node has no children
     */
    public TreeNode getLastChild() {
	if (getChildCount() == 0) {
	    throw new NoSuchElementException("node has no children");
	}
	return getChildAt(getChildCount()-1);
    }


    /**
     * Returns the child in this node's child array that immediately
     * follows <code>aChild</code>, which must be a child of this node.  If
     * <code>aChild</code> is the last child, returns null.  This method
     * performs a linear search of this node's children for
     * <code>aChild</code> and is O(n) where n is the number of children; to
     * traverse the entire array of children, use an enumeration instead.
     *
     * @see		#children
     * @exception	IllegalArgumentException if <code>aChild</code> is
     *					null or is not a child of this node
     * @return	the child of this node that immediately follows
     *		<code>aChild</code>
     */
    public TreeNode getChildAfter(TreeNode aChild) {
	if (aChild == null) {
	    throw new IllegalArgumentException("argument is null");
	}

	int index = getIndex(aChild);		// linear search

	if (index == -1) {
	    throw new IllegalArgumentException("node is not a child");
	}

	if (index < getChildCount() - 1) {
	    return getChildAt(index + 1);
	} else {
	    return null;
	}
    }


    /**
     * Returns the child in this node's child array that immediately
     * precedes <code>aChild</code>, which must be a child of this node.  If
     * <code>aChild</code> is the first child, returns null.  This method
     * performs a linear search of this node's children for <code>aChild</code>
     * and is O(n) where n is the number of children.
     *
     * @exception	IllegalArgumentException if <code>aChild</code> is null
     *						or is not a child of this node
     * @return	the child of this node that immediately precedes
     *		<code>aChild</code>
     */
    public TreeNode getChildBefore(TreeNode aChild) {
	if (aChild == null) {
	    throw new IllegalArgumentException("argument is null");
	}

	int index = getIndex(aChild);		// linear search

	if (index == -1) {
	    throw new IllegalArgumentException("argument is not a child");
	}

	if (index > 0) {
	    return getChildAt(index - 1);
	} else {
	    return null;
	}
    }


    //
    //  Sibling Queries
    //


    /**
     * Returns true if <code>anotherNode</code> is a sibling of (has the
     * same parent as) this node.  A node is its own sibling.  If
     * <code>anotherNode</code> is null, returns false.
     *
     * @param	anotherNode	node to test as sibling of this node
     * @return	true if <code>anotherNode</code> is a sibling of this node
     */
    public boolean isNodeSibling(TreeNode anotherNode) {
	boolean retval;

	if (anotherNode == null) {
	    retval = false;
	} else if (anotherNode == this) {
	    retval = true;
	} else {
	    TreeNode  myParent = getParent();
	    retval = (myParent != null && myParent == anotherNode.getParent());

	    if (retval && !((DefaultMutableTreeNode)getParent())
		           .isNodeChild(anotherNode)) {
		throw new Error("sibling has different parent");
	    }
	}

	return retval;
    }


    /**
     * Returns the number of siblings of this node.  A node is its own sibling
     * (if it has no parent or no siblings, this method returns
     * <code>1</code>).
     *
     * @return	the number of siblings of this node
     */
    public int getSiblingCount() {
	TreeNode myParent = getParent();

	if (myParent == null) {
	    return 1;
	} else {
	    return myParent.getChildCount();
	}
    }


    /**
     * Returns the next sibling of this node in the parent's children array.
     * Returns null if this node has no parent or is the parent's last child.
     * This method performs a linear search that is O(n) where n is the number
     * of children; to traverse the entire array, use the parent's child
     * enumeration instead.
     *
     * @see	#children
     * @return	the sibling of this node that immediately follows this node
     */
    public DefaultMutableTreeNode getNextSibling() {
	DefaultMutableTreeNode retval;

	DefaultMutableTreeNode myParent = (DefaultMutableTreeNode)getParent();

	if (myParent == null) {
	    retval = null;
	} else {
	    retval = (DefaultMutableTreeNode)myParent.getChildAfter(this);	// linear search
	}

	if (retval != null && !isNodeSibling(retval)) {
	    throw new Error("child of parent is not a sibling");
	}

	return retval;
    }


    /**
     * Returns the previous sibling of this node in the parent's children
     * array.  Returns null if this node has no parent or is the parent's
     * first child.  This method performs a linear search that is O(n) where n
     * is the number of children.
     *
     * @return	the sibling of this node that immediately precedes this node
     */
    public DefaultMutableTreeNode getPreviousSibling() {
	DefaultMutableTreeNode retval;

	DefaultMutableTreeNode myParent = (DefaultMutableTreeNode)getParent();

	if (myParent == null) {
	    retval = null;
	} else {
	    retval = (DefaultMutableTreeNode)myParent.getChildBefore(this);	// linear search
	}

	if (retval != null && !isNodeSibling(retval)) {
	    throw new Error("child of parent is not a sibling");
	}

	return retval;
    }



    //
    //  Leaf Queries
    //

    /**
     * Returns true if this node has no children.  To distinguish between
     * nodes that have no children and nodes that <i>cannot</i> have
     * children (e.g. to distinguish files from empty directories), use this
     * method in conjunction with <code>getAllowsChildren</code>
     *
     * @see	#getAllowsChildren
     * @return	true if this node has no children
     */
    public boolean isLeaf() {
	return (getChildCount() == 0);
    }


    /**
     * Finds and returns the first leaf that is a descendant of this node --
     * either this node or its first child's first leaf.
     * Returns this node if it is a leaf.
     *
     * @see	#isLeaf
     * @see	#isNodeDescendant
     * @return	the first leaf in the subtree rooted at this node
     */
    public DefaultMutableTreeNode getFirstLeaf() {
	DefaultMutableTreeNode node = this;

	while (!node.isLeaf()) {
	    node = (DefaultMutableTreeNode)node.getFirstChild();
	}

	return node;
    }


    /**
     * Finds and returns the last leaf that is a descendant of this node --
     * either this node or its last child's last leaf. 
     * Returns this node if it is a leaf.
     *
     * @see	#isLeaf
     * @see	#isNodeDescendant
     * @return	the last leaf in the subtree rooted at this node
     */
    public DefaultMutableTreeNode getLastLeaf() {
	DefaultMutableTreeNode node = this;

	while (!node.isLeaf()) {
	    node = (DefaultMutableTreeNode)node.getLastChild();
	}

	return node;
    }


    /**
     * Returns the leaf after this node or null if this node is the
     * last leaf in the tree.
     * <p>
     * In this implementation of the <code>MutableNode</code> interface,
     * this operation is very inefficient. In order to determine the
     * next node, this method first performs a linear search in the 
     * parent's child-list in order to find the current node. 
     * <p>
     * That implementation makes the operation suitable for short
     * traversals from a known position. But to traverse all of the 
     * leaves in the tree, you should use <code>depthFirstEnumeration</code>
     * to enumerate the nodes in the tree and use <code>isLeaf</code>
     * on each node to determine which are leaves.
     *
     * @see	#depthFirstEnumeration
     * @see	#isLeaf
     * @return	returns the next leaf past this node
     */
    public DefaultMutableTreeNode getNextLeaf() {
	DefaultMutableTreeNode nextSibling;
	DefaultMutableTreeNode myParent = (DefaultMutableTreeNode)getParent();

	if (myParent == null)
	    return null;

	nextSibling = getNextSibling();	// linear search

	if (nextSibling != null)
	    return nextSibling.getFirstLeaf();

	return myParent.getNextLeaf();	// tail recursion
    }


    /**
     * Returns the leaf before this node or null if this node is the
     * first leaf in the tree.
     * <p>
     * In this implementation of the <code>MutableNode</code> interface,
     * this operation is very inefficient. In order to determine the
     * previous node, this method first performs a linear search in the 
     * parent's child-list in order to find the current node. 
     * <p>
     * That implementation makes the operation suitable for short
     * traversals from a known position. But to traverse all of the 
     * leaves in the tree, you should use <code>depthFirstEnumeration</code>
     * to enumerate the nodes in the tree and use <code>isLeaf</code>
     * on each node to determine which are leaves.
     *
     * @see		#depthFirstEnumeration
     * @see		#isLeaf
     * @return	returns the leaf before this node
     */
    public DefaultMutableTreeNode getPreviousLeaf() {
	DefaultMutableTreeNode previousSibling;
	DefaultMutableTreeNode myParent = (DefaultMutableTreeNode)getParent();

	if (myParent == null)
	    return null;

	previousSibling = getPreviousSibling();	// linear search

	if (previousSibling != null)
	    return previousSibling.getLastLeaf();

	return myParent.getPreviousLeaf();		// tail recursion
    }


    /**
     * Returns the total number of leaves that are descendants of this node.
     * If this node is a leaf, returns <code>1</code>.  This method is O(n)
     * where n is the number of descendants of this node.
     *
     * @see	#isNodeAncestor
     * @return	the number of leaves beneath this node
     */
    public int getLeafCount() {
	int count = 0;

	TreeNode node;
	Enumeration enum = breadthFirstEnumeration(); // order matters not

	while (enum.hasMoreElements()) {
	    node = (TreeNode)enum.nextElement();
	    if (node.isLeaf()) {
		count++;
	    }
	}

	if (count < 1) {
	    throw new Error("tree has zero leaves");
	}

	return count;
    }


    //
    //  Overrides
    //

    /**
     * Returns the result of sending <code>toString()</code> to this node's
     * user object, or null if this node has no user object.
     *
     * @see	#getUserObject
     */
    public String toString() {
	if (userObject == null) {
	    return null;
	} else {
	    return userObject.toString();
	}
    }

    /**
     * Overridden to make clone public.  Returns a shallow copy of this node;
     * the new node has no parent or children and has a reference to the same
     * user object, if any.
     *
     * @return	a copy of this node
     */
    public Object clone() {
	DefaultMutableTreeNode newNode = null;

	try {
	    newNode = (DefaultMutableTreeNode)super.clone();

	    // shallow copy -- the new node has no parent or children
	    newNode.children = null;
	    newNode.parent = null;

	} catch (CloneNotSupportedException e) {
	    // Won't happen because we implement Cloneable
	    throw new Error(e.toString());
	}

	return newNode;
    }


    // Serialization support.  
    private void writeObject(ObjectOutputStream s) throws IOException {
	Object[]             tValues;

	s.defaultWriteObject();
	// Save the userObject, if its Serializable.
	if(userObject != null && userObject instanceof Serializable) {
	    tValues = new Object[2];
	    tValues[0] = "userObject";
	    tValues[1] = userObject;
	}
	else
	    tValues = new Object[0];
	s.writeObject(tValues);
    }

    private void readObject(ObjectInputStream s) 
	throws IOException, ClassNotFoundException {
	Object[]      tValues;

	s.defaultReadObject();

	tValues = (Object[])s.readObject();

	if(tValues.length > 0 && tValues[0].equals("userObject"))
	    userObject = tValues[1];
    }

    final class PreorderEnumeration implements Enumeration {
	protected Stack	stack;

	public PreorderEnumeration(TreeNode rootNode) {
	    super();
	    Vector v = new Vector(1);
	    v.addElement(rootNode);	// PENDING: don't really need a vector
	    stack = new Stack();
	    stack.push(v.elements());
	}

	public boolean hasMoreElements() {
	    return (!stack.empty() &&
		    ((Enumeration)stack.peek()).hasMoreElements());
	}

	public Object nextElement() {
	    Enumeration	enumer = (Enumeration)stack.peek();
	    TreeNode	node = (TreeNode)enumer.nextElement();
	    Enumeration	children = node.children();

	    if (!enumer.hasMoreElements()) {
		stack.pop();
	    }
	    if (children.hasMoreElements()) {
		stack.push(children);
	    }
	    return node;
	}

    }  // End of class PreorderEnumeration



    final class PostorderEnumeration implements Enumeration {
	protected TreeNode root;
	protected Enumeration children;
	protected Enumeration subtree;

	public PostorderEnumeration(TreeNode rootNode) {
	    super();
	    root = rootNode;
	    children = root.children();
	    subtree = EMPTY_ENUMERATION;
	}

	public boolean hasMoreElements() {
	    return root != null;
	}

	public Object nextElement() {
	    Object retval;

	    if (subtree.hasMoreElements()) {
		retval = subtree.nextElement();
	    } else if (children.hasMoreElements()) {
		subtree = new PostorderEnumeration(
				(TreeNode)children.nextElement());
		retval = subtree.nextElement();
	    } else {
		retval = root;
		root = null;
	    }

	    return retval;
	}

    }  // End of class PostorderEnumeration



    final class BreadthFirstEnumeration implements Enumeration {
	protected Queue	queue;

	public BreadthFirstEnumeration(TreeNode rootNode) {
	    super();
	    Vector v = new Vector(1);
	    v.addElement(rootNode);	// PENDING: don't really need a vector
	    queue = new Queue();
	    queue.enqueue(v.elements());
	}

	public boolean hasMoreElements() {
	    return (!queue.isEmpty() &&
		    ((Enumeration)queue.firstObject()).hasMoreElements());
	}

	public Object nextElement() {
	    Enumeration	enumer = (Enumeration)queue.firstObject();
	    TreeNode	node = (TreeNode)enumer.nextElement();
	    Enumeration	children = node.children();

	    if (!enumer.hasMoreElements()) {
		queue.dequeue();
	    }
	    if (children.hasMoreElements()) {
		queue.enqueue(children);
	    }
	    return node;
	}


	// A simple queue with a linked list data structure.
	final class Queue {
	    QNode head;	// null if empty
	    QNode tail;

	    final class QNode {
		public Object	object;
		public QNode	next;	// null if end
		public QNode(Object object, QNode next) {
		    this.object = object;
		    this.next = next;
		}
	    }

	    public void enqueue(Object anObject) {
		if (head == null) {
		    head = tail = new QNode(anObject, null);
		} else {
		    tail.next = new QNode(anObject, null);
		    tail = tail.next;
		}
	    }

	    public Object dequeue() {
		if (head == null) {
		    throw new NoSuchElementException("No more elements");
		}

		Object retval = head.object;
		QNode oldHead = head;
		head = head.next;
		if (head == null) {
		    tail = null;
		} else {
		    oldHead.next = null;
		}
		return retval;
	    }

	    public Object firstObject() {
		if (head == null) {
		    throw new NoSuchElementException("No more elements");
		}

		return head.object;
	    }

	    public boolean isEmpty() {
		return head == null;
	    }

	} // End of class Queue

    }  // End of class BreadthFirstEnumeration



    final class PathBetweenNodesEnumeration implements Enumeration {
	protected Stack stack;

	public PathBetweenNodesEnumeration(TreeNode ancestor,
					   TreeNode descendant)
	{
	    super();

	    if (ancestor == null || descendant == null) {
		throw new IllegalArgumentException("argument is null");
	    }

	    TreeNode current;

	    stack = new Stack();
	    stack.push(descendant);

	    current = descendant;
	    while (current != ancestor) {
		current = current.getParent();
		if (current == null && descendant != ancestor) {
		    throw new IllegalArgumentException("node " + ancestor +
				" is not an ancestor of " + descendant);
		}
		stack.push(current);
	    }
	}

	public boolean hasMoreElements() {
	    return stack.size() > 0;
	}

	public Object nextElement() {
	    try {
		return stack.pop();
	    } catch (EmptyStackException e) {
		throw new NoSuchElementException("No more elements");
	    }
	}

    } // End of class PathBetweenNodesEnumeration



} // End of class DefaultMutableTreeNode