Constructing a User-Friendly JTree from a DOM

Now that you know what a DOM looks like internally, you'll be better prepared to modify a DOM or construct one from scratch. Before we go on to that, though, this section presents some modifications to the JTreeModel that let you produce a more user-friendly version of the JTree suitable for use in a GUI.

Note: In this section, we modify the Swing GUI to improve the display, culminating in DomEcho04.java. If you have no interest in the Swing details, you can skip ahead to Creating and Manipulating a DOM and use DomEcho04.java to proceed from there.

Compressing the Tree View

Displaying the DOM in tree form is all very well for experimenting and for learning how a DOM works. But it's not the kind of friendly display that most users want to see in a JTree. However, it turns out that very few modifications are needed to turn the TreeModel adapter into something that presents a user-friendly display. In this section, you'll make those modifications.

Make the Operation Selectable

When you modify the adapter, you're going to compress the view of the DOM, eliminating all but the nodes you really want to display. Start by defining a boolean variable that controls whether you want the compressed or the uncompressed view of the DOM:

public class DomEcho extends JPanel
{
  static Document document; 
   boolean compress = true;
   static final int windowHeight = 460;
   ...

Identify Tree Nodes

The next step is to identify the nodes you want to show up in the tree. To do that, add the following highlighted code:

...
import org.w3c.dom.Document;
import org.w3c.dom.DOMException;
import org.w3c.dom.Node;

public class DomEcho extends JPanel
{
  ...

  public static void makeFrame() {
    ...
  }

  // An array of names for DOM node type
  static final String[] typeName = {
    ...
  };

  static final int ELEMENT_TYPE = Node.ELEMENT_NODE;

  // The list of elements to display in the tree
  static String[] treeElementNames = {
    "slideshow",
    "slide",
    "title",         // For slide show #1
    "slide-title",   // For slide show #10
    "item",
  };

  boolean treeElement(String elementName) {
    for (int i=0; i<treeElementNames.length; i++) {
      if ( elementName.equals(treeElementNames[i]) )
        return true;
    }
    return false;
  }

This code sets up a constant you can use to identify the ELEMENT node type, declares the names of the elements you want in the tree, and creates a method that tells whether or not a given element name is a tree element. Because slideSample01.xml has title elements and because slideSample10.xml has slide-title elements, you set up the contents of this array so that it will work with either data file.

Note: The mechanism you are creating here depends on the fact that structure nodes like slideshow and slide never contain text, whereas text usually does appear in content nodes like item. Although those "content" nodes may contain subelements in slideShow10.xml, the DTD constrains those subelements to be XHTML nodes. Because they are XHTML nodes (an XML version of HTML that is constrained to be well formed), the entire substructure under an item node can be combined into a single string and displayed in the htmlPane that makes up the other half of the application window. In the second part of this section, you'll do that concatenation, displaying the text and XHTML as content in the htmlPane.

Although you could simply reference the node types defined in the class org.w3c.dom.Node, defining the ELEMENT_TYPE constant keeps the code a little more readable. Each node in the DOM has a name, a type, and (potentially) a list of subnodes. The functions that return these values are getNodeName(), getNodeType, and getChildNodes(). Defining our own constants will let us write code like this:

Node node = nodeList.item(i);
int type = node.getNodeType();
if (type == ELEMENT_TYPE) {
  ....

As a stylistic choice, the extra constants help us keep the reader (and ourselves!) clear about what we're doing. Here, it is fairly clear when we are dealing with a node object, and when we are dealing with a type constant. Otherwise, it would be tempting to code something like if (node == ELEMENT_NODE), which of course would not work at all.

Control Node Visibility

The next step is to modify the AdapterNode's childCount function so that it counts only tree element nodes--nodes that are designated as displayable in the JTree. Make the following highlighted modifications to do that:

public class DomEcho extends JPanel
{
  ...
  public class AdapterNode 
  { 
    ...
    public AdapterNode child(int searchIndex) {
      ... 
    }
    public int childCount() {
      if (!compress) {
        // Indent this
        return domNode.getChildNodes().getLength(); 
      } 
      int count = 0;
      for (int i=0;
        i<domNode.getChildNodes().getLength(); i++) 
      {
        org.w3c.dom.Node node =
          domNode.getChildNodes().item(i); 
        if (node.getNodeType() == ELEMENT_TYPE
        &&  treeElement( node.getNodeName() )) 
        {
          ++count;
        }
      }
      return count;
    }
  } // AdapterNode

The only tricky part about this code is checking to make sure that the node is an element node before comparing the node. The DocType node makes that necessary, because it has the same name (slideshow) as the slideshow element.

Control Child Access

Finally, you need to modify the AdapterNode's child function to return the Nth item from the list of displayable nodes, rather than the Nth item from all nodes in the list. Add the following highlighted code to do that:

public class DomEcho extends JPanel
{
  ...
  public class AdapterNode 
  { 
    ...
    public int index(AdapterNode child) {
      ...
    }
    public AdapterNode child(int searchIndex) {
    //Note: JTree index is zero-based. 
    org.w3c.dom.Node node =
      domNode.getChildNodes()Item(searchIndex);
    if (compress) {
      // Return Nth displayable node
      int elementNodeIndex = 0;
      for (int i=0;
        i<domNode.getChildNodes().getLength(); i++) 
      {
        node = domNode.getChildNodes()Item(i);
        if (node.getNodeType() == ELEMENT_TYPE 
        && treeElement( node.getNodeName() )
        && elementNodeIndex++ == searchIndex) {
          break; 
        }
      }
    }
    return new AdapterNode(node); 
  } // child
}  // AdapterNode

There's nothing special going on here. It's a slightly modified version of the same logic you used when returning the child count.

Check the Results

When you compile and run this version of the application on slideSample01.xml and then expand the nodes in the tree, you see the results shown in Figure 6-8. The only nodes remaining in the tree are the high-level "structure" nodes.

Extra Credit

The way the application stands now, the information that tells the application how to compress the tree for display is hardcoded. Here are some ways you can consider extending the app:

Use a command-line argument: Whether you compress or don't compress the tree could be determined by a command-line argument rather than being a hardcoded Boolean variable. On the other hand, the list of elements that goes into the tree is still hardcoded, so maybe that option doesn't make much sense, unless...

Read the treeElement list from a file: If you read the list of elements to include in the tree from an external file, that would make the whole application command-driven. That would be good. But wouldn't it be really nice to derive that information from the DTD or schema instead? So you might want to consider...

Automatically build the list: Watch out, though! As things stand right now, there are no standard DTD parsers! If you use a DTD, then, you'll need to write your parser to make sense out of its somewhat arcane syntax. You'll probably have better luck if you use a schema instead of a DTD. The nice thing about schemas is that they use XML syntax, so you can use an XML parser to read the schema in the same way you use it to read any other XML file.

As you analyze the schema, note that the JTree-displayable structure nodes are those that have no text, whereas the content nodes may contain text and, optionally, XHTML subnodes. That distinction works for this example and will likely work for a large body of real world applications. It's easy to construct cases that will create a problem, though, so you'll have to be on the lookout for schema/DTD specifications that embed non-XHTML elements in text-capable nodes, and take the appropriate action.

Acting on Tree Selections

Now that the tree is being displayed properly, the next step is to concatenate the subtrees under selected nodes to display them in the htmlPane. While you're at it, you'll use the concatenated text to put node-identifying information back in the JTree.

Identify Node Types

When you concatenate the subnodes under an element, the processing you do depends on the type of node. So the first thing to do is to define constants for the remaining node types. Add the following highlighted code:

public class DomEcho extends JPanel
{
  ...
  // An array of names for DOM node types
  static final String[] typeName = {
    ...
  };
  static final int ELEMENT_TYPE =   1;
  static final int ATTR_TYPE = Node.ATTRIBUTE_NODE;
  static final int TEXT_TYPE = Node.TEXT_NODE;
  static final int CDATA_TYPE = Node.CDATA_SECTION_NODE;
  static final int ENTITYREF_TYPE =
             Node.ENTITY_REFERENCE_NODE;
  static final int ENTITY_TYPE = Node.ENTITY_NODE;
  static final int PROCINSTR_TYPE =
             Node.PROCESSING_INSTRUCTION_NODE;
  static final int COMMENT_TYPE = Node.COMMENT_NODE;
  static final int DOCUMENT_TYPE = Node.DOCUMENT_NODE;
  static final int DOCTYPE_TYPE = Node.DOCUMENT_TYPE_NODE;
  static final int DOCFRAG_TYPE = Node.DOCUMENT_FRAGMENT_NODE;
  static final int NOTATION_TYPE = Node.NOTATION_NODE;

Concatenate Subnodes to Define Element Content

Next, you define the method that concatenates the text and subnodes for an element and returns it as the element's content. To define the content method, you'll add the following big chunk of highlighted code, but this is the last big chunk of code in the DOM tutorial.

public class DomEcho extends JPanel
{
  ...
  public class AdapterNode 
  { 
    ...
    public String toString() {
    ...
    }
    public String content() {
      String s = "";
      org.w3c.dom.NodeList nodeList =
        domNode.getChildNodes();
      for (int i=0; i<nodeList.getLength(); i++) {
        org.w3c.dom.Node node = nodeList.item(i);
        int type = node.getNodeType();
        AdapterNode adpNode = new AdapterNode(node);
        if (type == ELEMENT_TYPE) { 
          if ( treeElement(node.getNodeName()) )
            continue;
          s += "<" + node.getNodeName() + ">";
          s += adpNode.content();
          s += "</" + node.getNodeName() + ">";
        } else if (type == TEXT_TYPE) {
          s += node.getNodeValue();
        } else if (type == ENTITYREF_TYPE) {
          // The content is in the TEXT node under it
          s += adpNode.content();
        } else if (type == CDATA_TYPE) { 
          StringBuffer sb = new StringBuffer(
            node.getNodeValue() );
          for (int j=0; j<sb.length(); j++) {
            if (sb.charAt(j) == '<') {
              sb.setCharAt(j, '&');
              sb.insert(j+1, "lt;");
              j += 3;
            } else if (sb.charAt(j) == '&') {
              sb.setCharAt(j, '&');
              sb.insert(j+1, "amp;");
              j += 4;
            }
          }
          s += "<pre>" + sb + "</pre>";
        }
      }
      return s;
    }
    ...
} // AdapterNode

Note: This code collapses EntityRef nodes, as inserted by the JAXP 1.1 parser that is included in the Java 1.4 platform. With JAXP 1.2, that portion of the code is not necessary because entity references are converted to text nodes by the parser. Other parsers may insert such nodes, however, so including this code future proofs your application, should you use a different parser in the future.

Although this code is not the most efficient that anyone ever wrote, it works and will do fine for our purposes. In this code, you are recognizing and dealing with the following data types:

For elements with names such as the XHTML em node, you return the node's content sandwiched between the appropriate <em> and </em> tags. However, when processing the content for the slideshow element, for example, you don't include tags for the slide elements it contains, so when returning a node's content, you skip any subelements that are themselves displayed in the tree.

Unlike CDATA nodes, entity references can contain multiple subelements. So the strategy here is to return the concatenation of those subelements.

As with a text node, you return the node's value. However, because the text in this case may contain angle brackets and ampersands, you need to convert them to a form that displays properly in an HTML pane. Unlike the XML CDATA tag, the HTML <pre> tag does not prevent the parsing of character-format tags, break tags, and the like. So you must convert left angle brackets (<) and ampersands (&) to get them to display properly.

On the other hand, there are quite a few node types you are not processing with the preceding code. It's worth a moment to examine them and understand why:

These nodes do not appear in the DOM but are obtained by invoking getAttributes on element nodes.

These nodes also do not appear in the DOM. They are obtained by invoking getEntities on DocType nodes.

The DocType node contains the DTD specification, with or without external pointers. It appears only under the root node and has no data to display in the tree.

This node is equivalent to a document node. It's a root node that the DOM specification intends for holding intermediate results during operations such as cut-and-paste. As with a document node, there's no data to display.

Display the Content in the JTree

With the content concatenation out of the way, only a few small programming steps remain. The first is to modify toString so that it uses the first line of the node's content for identifying information. Add the following highlighted code:

public class DomEcho extends JPanel
{
  ...
  public class AdapterNode 
  { 
    ...
    public String toString() {
      ...
      if (! nodeName.startsWith("#")) {
        s += ": " + nodeName;
      }
      if (compress) {
        String t = content().trim();
        int x = t.indexOf("\n");
        if (x >= 0) t = t.substring(0, x);
        s += " " + t;
        return s;
      }
      if (domNode.getNodeValue() != null) {
        ...
      }
      return s;
    }

Wire the JTree to the JEditorPane

Returning now to the app's constructor, create a tree selection listener and use it to wire the JTree to the JEditorPane:

public class DomEcho extends JPanel
{
  ...
  public DomEcho()
  {
    ...
    // Build right-side view
    JEditorPane htmlPane = new JEditorPane("text/html","");
    htmlPane.setEditable(false);
    JScrollPane htmlView = new JScrollPane(htmlPane);
    htmlView.setPreferredSize( 
      new Dimension( rightWidth, windowHeight ));

      tree.addTreeSelectionListener(
        new TreeSelectionListener() {
          public void valueChanged(TreeSelectionEvent e)
          {
            TreePath p = e.getNewLeadSelectionPath();
            if (p != null) {
              AdapterNode adpNode = 
                (AdapterNode)
                  p.getLastPathComponent();
              htmlPane.setText(adpNode.content());
            }
          }
        }
      );

Now, when a JTree node is selected, its contents are delivered to the htmlPane.

Note: The TreeSelectionListener in this example is created using an anonymous inner-class adapter. If you are programming for the 1.1 version of the platform, you'll need to define an external class for this purpose.

If you compile this version of the app, you'll discover immediately that the htmlPane needs to be specified as final to be referenced in an inner class, so add the following highlighted keyword:

public DomEcho04()
{
  ...
  // Build right-side view
  final JEditorPane htmlPane = new
    JEditorPane("text/html","");
  htmlPane.setEditable(false);
  JScrollPane htmlView = new JScrollPane(htmlPane);
  htmlView.setPreferredSize( 
    new Dimension( rightWidth, windowHeight ));

Run the App

When you compile the application and run it on slideSample10.xml (the browsable version is slideSample10-xml.html), you get a display like that shown in Figure 6-9. Expanding the hierarchy shows that the JTree now includes identifying text for a node whenever possible.

Selecting an item that includes XHTML subelements produces a display like that shown in Figure 6-10:

Selecting a node that contains an entity reference causes the entity text to be included, as shown in Figure 6-11:

Finally, selecting a node that includes a CDATA section produces results like those shown in Figure 6-12:

Extra Credit

Now that you have the application working, here are some ways you might think about extending it in the future:

Use title text to identify slides: Special case the slide element so that the contents of the title node are used as the identifying text. When selected, convert the title node's contents to a centered H1 tag, and ignore the title element when constructing the tree.

Convert item elements to lists: Remove item elements from the JTree and convert them to HTML lists using <ul>, <li>, and </ul> tags, including them in the slide's content when the slide is selected.

Handling Modifications

A full discussion of the mechanisms for modifying the JTree's underlying data model is beyond the scope of this tutorial. However, a few words on the subject are in order.

Most importantly, note that if you allow the user to modify the structure by manipulating the JTree, you must take the compression into account when you figure out where to apply the change. For example, if you are displaying text in the tree and the user modifies that, the changes would have to be applied to text subelements and perhaps would require a rearrangement of the XHTML subtree.

When you make those changes, you'll need to understand more about the interactions between a JTree, its TreeModel, and an underlying data model. That subject is covered in depth in the Swing Connection article, "Understanding the TreeModel" at http://java.sun.com/products/jfc/tsc/articles/jtree/index.html.

Finishing Up

You now understand what there is to know about the structure of a DOM, and you know how to adapt a DOM to create a user-friendly display in a JTree. It has taken quite a bit of coding, but in return you have obtained valuable tools for exposing a DOM's structure and a template for GUI applications. In the next section, you'll make a couple of minor modifications to the code that turn the application into a vehicle for experimentation, and then you'll experiment with building and manipulating a DOM.