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> ## 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 going 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.

> ### Compressing the Tree View

> Displaying the DOM in tree form is all very well for experimenting and to learn 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 will present a user-friendly display. In this section, you'll make those modifications.

> Note: The code discussed in this section is in `DomEcho03.java`. The file it operates on is `slideSample01.xml`. (The browsable version is `slideSample01-xml.html`.)

> #### 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 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 code highlighted below:

>     ...
>     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 slideshow #1
>         "slide-title",   // For slideshow #10
>         "item",
>       };

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

> With this code, you set up a constant you can use to identify the `ELEMENT` node type, declared the names of the elements you want in the tree, and created a method tells whether or not a given element name is a "tree element". Since `slideSample01.xml` has `title` elements and `slideSample10.xml` has `slide-title` elements, you set up the contents of this arrays so it would 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, while 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 fairly 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 only counts "tree element" nodes--nodes which are designated as displayable in the `JTree`. Make the modifications highlighted below 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 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 code highlighted below 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 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 8-8. The only nodes remaining in the tree are the high-level "structure" nodes.

> Figure 8-8 Tree View with a Collapsed Hierarchy

> #### Extra Credit

> The way the application stands now, the information that tells the application how to compress the tree for display is "hard-coded". Here are some ways you could 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 hard-coded boolean variable. On the other hand, the list the list of elements that goes into the tree is still hard coded, 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 use XML syntax, so you can use an XML parser to read the schema the same way you use any other file.

> As you analyze the schema, note that the `JTree`-displayable structure nodes are those that have no text, while 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 pretty 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`.

> Note: The code discussed in this section is in `DomEcho04.java`.

> #### Identify Node Types

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

>     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 need to define add 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 need to add the big chunk of code highlighted below, 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 ins included in the 1.4 Java 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 well 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 it will do fine for our purposes. In this code, you are recognizing and dealing with the following data types:

> Element

> For elements with names like 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.

> Text

> No surprise here. For a text node, you simply return the node's `value`.

> Entity Reference

> Unlike `CDATA` nodes, Entity References can contain multiple subelements. So the strategy here is to return the concatenation of those subelements.

> CDATA

> Like a text node, you return the node's `value`. However, since 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 have to 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 code above. It's worth a moment to examine them and understand why:

> Attribute

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

> Entity

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

> Processing Instruction

> These nodes don't contain displayable data.

> Comment

> Ditto. Nothing you want to display here.

> Document

> This is the root node for the DOM. There's no data to display for that.

> DocType

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

> Document Fragment

> This node is equivalent to a document node. It's a root node that the DOM specification intends for holding intermediate results during cut/paste operations, for example. Like a document node, there's no data to display.

> Notation

> We're just flat out ignoring this one. These nodes are used to include binary data in the DOM. As discussed earlier in Choosing your Parser Implementation and Using the DTDHandler and EntityResolver, the MIME types (in conjunction with namespaces) make a better mechanism for that.

> #### 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 node's content for identifying information. Add the code highlighted below to do that:

>     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(");
>             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 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, it's 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 keyword highlighted below:

>     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 8-9. Expanding the hierarchy shows that the `JTree` now includes identifying text for a node whenever possible.

> Figure 8-9 Collapsed Hierarchy Showing Text in Nodes

> Selecting an item that includes XHTML subelements produces a display like that shown in Figure 8-10:

> Figure 8-10 Node with `<em>` Tag Selected

> Selecting a node that contains an entity reference causes the entity text to be included, as shown in Figure 8-11:

> Figure 8-11 Node with Entity Reference Selected

> Finally, selecting a node that includes a `CDATA` section produces results like those shown in Figure 8-12:

> Figure 8-12 Node with `CDATA` Component Selected

> #### 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 is 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>`, `</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 modifying the structure by manipulating the `JTree`, you have 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 require a rearrangement of the XHTML subtree.

> When you make those changes, you'll need to understand more about the interactions between a `JTree`, it's `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 pretty much what there is 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 apps. 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 experiment with building and manipulating a DOM.

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