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Swing Chapter 17. (Advanced topics) Trees. Easy for reading, Click here!

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Swing Chapter 17. (Advanced topics) Trees. Easy for reading, Click here!

[ Return to Swing (Book) ]

Page: 6/7 

Previous Page Previous Page (5/7) - Next Page (7/7) Next Page
Subpages: 1. JTree 
Basic JTree example 
Directories tree: part I - Dynamic node retrieval 
4. Directories tree: part II - Popup menus and programmatic navigation 
5. Directories tree: part III - Tooltips 
6. JTree and XML documents 
7. Custom editors and renderers 

17.6  JTree and XML documents

Many see the future of the web in XML (Extensible Markup Language). This standard will most likely replace HTML. Unlike HTML, XML allows the definition of custom document tags, allowing the transmition of virtually any type of information over the web. Many sources of information about XML are available, including the standard definition at http://www.w3.org/TR/WD-xml-lang-970331.html.

XML documents have a tree-like structure, and JTree can be very useful for constructing an XML structure browser. In this section we'll show how to build a simple implementation of such a browser. We do not intend to give an introduction to XML, and we will not discuss Sun's API for XML in detail (which contains a fair number of classes and is likely to change significantly in the near future). However, a brief introduction to the XML classes used in this example is appropriate.

Note: Sun's early access XML library is required to run this example. See http://www.javasoft.com/.

17.6.1  XmlDocument

class com.sun.xml.tree.XmlDocument

This class represents a top level XML 1.0 document created with the XML parser built into XmlDocumentBuilder. The getDocumentElement() method retrieves the top-most node in the document (i.e. the root node). All other nodes can be extracted through this node.

17.6.2  XmlDocumentBuilder

class com.sun.xml.tree.XmlDocumentBuilder

This class builds an XML document using an internal parser. Two overloaded methods, createXmlDocument(InputStream in) and createXmlDocument(String documentURL), create new XmlDocument instances. (We will use the latter method in this example.)

17.6.3  DataNode

class com.sun.xml.tree.DataNode

This class represents a node in an XML document tree that encapsulates data and has no child nodes (i.e. a leaf node). Method getData() retrieves the contained data as a String.

17.6.4  ElementNode

class com.sun.xml.tree.ElementNode

This class encapsulates an element in the XML document tree, which is a node with children (i.e. a non-leaf node). The getLength() method retrieves the number of child nodes, and item(int index) returns the node with the given index.

Figure 17.6 An XML document structure tree.

<<file figure17-6.gif>>

The Code: XmlTree.java

see \Chapter17\5

import java.awt.*;

import java.awt.event.*;

import java.util.*;

import java.io.*;

import java.net.*;

import javax.swing.*;

import javax.swing.tree.*;

import javax.swing.event.*;

import com.sun.xml.tree.*;

import com.sun.xml.parser.*;

import org.w3c.dom.*;

public class XmlTree extends JFrame


  protected JTree  m_tree;

  protected DefaultTreeModel m_model;

  protected JTextField m_location;

  public XmlTree() {

    super("XML Tree");

    setSize(400, 300);

    m_location = new JTextField();


    ActionListener lst = new ActionListener() {

      public void actionPerformed(ActionEvent e) {





    getContentPane().add(m_location, BorderLayout.NORTH);

    DefaultMutableTreeNode top = new DefaultMutableTreeNode(


    m_model = new DefaultTreeModel(top);

    m_tree = new JTree(m_model);





    JScrollPane s = new JScrollPane();


    getContentPane().add(s, BorderLayout.CENTER);

    WindowListener wndCloser = new WindowAdapter() {

      public void windowClosing(WindowEvent e) {







  public void readXml(String sUrl) {


    Thread runner = new Thread() {

      public void run () { 

        try {

          URL source;

          try {

            File f = new File(sUrl);

            source = f.toURL();


          catch (Exception ex) {

            source = new URL(sUrl);


          XmlDocument doc =



          ElementNode root =



          DefaultMutableTreeNode top = createTreeNode(root);




        catch (Exception ex) {



            ex.toString(), "Warning",








  protected DefaultMutableTreeNode createTreeNode(ElementNode root) {

    DefaultMutableTreeNode node = new DefaultMutableTreeNode(


    for (int k=0; k<root.getLength(); k++) {

      Node nd = root.item(k);

      if (nd instanceof DataNode) {

        DataNode dn = (DataNode)nd;

        String data = dn.getData().trim();

        if (data.equals("\n") || data.equals("\r\n"))

          data = "";

        if (data.length() > 0)

          node.add(new DefaultMutableTreeNode(data));


      else if (nd instanceof ElementNode) {

        ElementNode en = (ElementNode)nd;




    return node;


  public static void main(String argv[]) {

    new XmlTree();



Understanding the Code

Class XmlTree

Instance variables:

JTree m_tree: used to display an XML document.

DefaultTreeModel m_model: used to store the content of an XML document.

JTextField m_location: used for entry of a file name or URL location of an XML document.

Initially the JTree component receives a single node, "Empty". An ActionListener is added to the m_location text field which calls our readXml() method passing it the current text.

The readXml() method loads an XML document, corresponding to the String passed as parameter, into our tree model. The body of this method is placed in a separate thread because it can be a very expensive procedure, and we want to make sure not to clog up the event dispatching thread (to retain GUI responsiveness). First the given string is treated as a file name. A File instance is created and converted to a URL. If this does not succeed, the string is treated as a URL address:

          URL source;

          try {

            File f = new File(sUrl);

            source = f.toURL();


          catch (Exception ex) {

            source = new URL(sUrl);


The static method XmlDocumentBuilder.createXmlDocument() creates an XmlDocument corresponding to the resulting URL. As soon as this finishes (which may take a while for large documents), the root ElementNode is retrieved from that document:

          XmlDocument doc =



          ElementNode root =



We then transform our XML document into a structure suitable for addition to our Swing tree model. Our createTreeNode() method does this job, returning the top-most node (the root node) as a DefaultMutableTreeNode. Finally that node is set as a root of our tree model, and our tree component is notified that it's content has changed:

          DefaultMutableTreeNode top = createTreeNode(root);



Note: Normally we are expected to avoid calling the treeDidChange() method directly, as it should be called by our UI as needed. However, in this case our JTree will not update correctly without it.

The createTreeNode() method creates a DefaultMutableTreeNode from an ElementNode provided. First it creates a root node corresponding to the given ElementNode, and then all lower level nodes are retrieved and processed in turn to populate the whole tree:

    DefaultMutableTreeNode node = new DefaultMutableTreeNode(


    for (int k=0; k<root.getLength(); k++) {

      Node nd = root.item(k);

Two possibilities are reconciled during this procedure. If a newly processed node is an instance of DataNode, its text is retrieved and is used as the user data object for a new DefaultMutableTreeNode which is then added to the parent node (in this case care should be taken to avoid empty nodes containing only end-of-line symbols). Otherwise, if a newly processed node is an instance of ElementNode, method createTreeNode() is called recursively.

Running the Code

Figure 17.6 shows our XML Tree example displaying the contents of "The Two Gentlemen of Verona" XML document which can be found at ftp://sunsite.unc.edu/pub/sun-info/standards/xml/eg/.

[ Return to Swing (Book) ]

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