Excerpts from How to Use Trees
http://java.sun.com/docs/books/tutorial/uiswing/components/tree.html

Creates a JSplitPane Creates a JTree, and places it in a JScrollPane Places the JScrollPane in the top half of the JSplitPane Displays Descriptive text, from html files, in the bottom half

---

public class TreeDemo extends JPanel implements TreeSelectionListener {
    private JEditorPane htmlPane;
    private JTree tree;
    private URL helpURL;
    private static boolean DEBUG = false;

    public TreeDemo() {
        super(new GridLayout(1,0));

        //Create the nodes.
        DefaultMutableTreeNode top = new DefaultMutableTreeNode("The Java Series");
        createNodes(top);  // call method to create nodes under root node

        //Create a tree that allows one selection at a time.
        tree = new JTree(top);
        tree.getSelectionModel().setSelectionMode(TreeSelectionModel.SINGLE_TREE_SELECTION);

        //Listen for when the selection changes.
        tree.addTreeSelectionListener(this);

        //Create the scroll pane and add the tree to it. 
        JScrollPane treeView = new JScrollPane(tree);

        //Create the HTML viewing pane.
        htmlPane = new JEditorPane();
        htmlPane.setEditable(false);
        initHelp();
        JScrollPane htmlView = new JScrollPane(htmlPane);

        //Add the scroll panes to a split pane.
        JSplitPane splitPane = new JSplitPane(JSplitPane.VERTICAL_SPLIT);
        splitPane.setTopComponent(treeView);
        splitPane.setBottomComponent(htmlView);

        Dimension minimumSize = new Dimension(100, 50);
        htmlView.setMinimumSize(minimumSize);
        treeView.setMinimumSize(minimumSize);
        splitPane.setDividerLocation(100); //XXX: ignored in some releases
                                           //of Swing. bug 4101306
        //workaround for bug 4101306:
        //treeView.setPreferredSize(new Dimension(100, 100)); 

        splitPane.setPreferredSize(new Dimension(500, 300));

        //Add the split pane to this panel.
        add(splitPane);
    }

User Object: the argument to the DefaultMutableTreeNode constructor

• An object that contains or points to the data associated with the tree node
• Can be a string
• Can be a custom object. If custom, implement its toString method so a string can be displayed for that node.

• Example: BookInfo class a custom class that holds two pieces of data
  • the name of a book
  • the URL for an HTML file describing the book.
  The toString method is implemented to return the book name.

    private class BookInfo {
        public String bookName;
        public URL bookURL;

        public BookInfo(String book, String filename) {
            bookName = book;
            bookURL = TreeDemo.class.getResource(filename);
            if (bookURL == null) {
                System.err.println("Couldn't find file: "
                                   + filename);
            }
        }

        public String toString() {
            return bookName;
        }
    }

Note: getResource is a method of java.lang.class, a subclass of java.lang.object.
From the API:

public URL getResource(String name)
 
Finds a resource with a given name. This method returns null if no resource with this name is found. The rules for searching resources associated with a given class are implemented by the * defining class loader of the class. 

This method delegates the call to its class loader, after making these changes to the resource name: if the resource name starts with "/", it is unchanged; otherwise, the package name is prepended to the resource name after converting "." to "/". If this object was loaded by the bootstrap loader, the call is delegated to ClassLoader.getSystemResource.

Parameters:
name - name of the desired resource
Returns:
a java.net.URL object.
Since: 
JDK1.1
See Also: 
ClassLoader

TreeSelectionListener

• Used to respond to tree node selections. E.g.:
  • Get default TreeSelectionModel for the tree
  • Set TreeSelectionModel so at most one tree node at a time can be selected.
  • Register an event handler on the tree:
    • an object that implements the TreeSelectionListener interface.
    • determines which node is selected by invoking the tree's getLastSelectedPathComponent method.
    • Uses getUserObject method to get the data associated with the node.

    /** Required by TreeSelectionListener interface. */
    public void valueChanged(TreeSelectionEvent e) {
        DefaultMutableTreeNode node = (DefaultMutableTreeNode)
                           tree.getLastSelectedPathComponent();  // this method belongs to class JTree

        if (node == null) return;

        Object nodeInfo = node.getUserObject();
        if (node.isLeaf()) {
            BookInfo book = (BookInfo)nodeInfo;
            displayURL(book.bookURL);
            if (DEBUG) {
                System.out.print(book.bookURL + ":  \n    ");
            }
        } else {
            displayURL(helpURL); 
        }
        if (DEBUG) {
            System.out.println(nodeInfo.toString());
        }
    }

    private void displayURL(URL url) {
        try {
            if (url != null) {
                htmlPane.setPage(url);  //a JEditorPane can diplay text/plain, text/html, text/rtf MIME types
            } else { //null url
		htmlPane.setText("File Not Found");
                if (DEBUG) {
                    System.out.println("Attempted to display a null URL.");
                }
            }
        } catch (IOException e) {
            System.err.println("Attempted to read a bad URL: " + url);
        }
    }

where setPage() method belongs to the JEditorPane class. From the API:

public void setPage(URL page)
             throws IOException
 
Sets the current URL being displayed. The content type of the pane is set, and if the editor kit for the pane is non-null, then a new default document is created and the URL is read into it. 

Creating the Nodes

This method is passed the root node, an instance of DefaultMutableTreeNode

    private void createNodes(DefaultMutableTreeNode top) {
        DefaultMutableTreeNode category = null;
        DefaultMutableTreeNode book = null;

        category = new DefaultMutableTreeNode("Books for Java Programmers");
        top.add(category);

        //original Tutorial
        book = new DefaultMutableTreeNode(new BookInfo
            ("The Java Tutorial: A Short Course on the Basics",
            "tutorial.html"));
        category.add(book);

        //Tutorial Continued
        book = new DefaultMutableTreeNode(new BookInfo
            ("The Java Tutorial Continued: The Rest of the JDK",
            "tutorialcont.html"));
        category.add(book);

        //JFC Swing Tutorial
        book = new DefaultMutableTreeNode(new BookInfo
            ("The JFC Swing Tutorial: A Guide to Constructing GUIs",
            "swingtutorial.html"));
        category.add(book);

        //Bloch
        book = new DefaultMutableTreeNode(new BookInfo
            ("Effective Java Programming Language Guide",
	     "bloch.html"));
        category.add(book);

        //Arnold/Gosling
        book = new DefaultMutableTreeNode(new BookInfo
            ("The Java Programming Language", "arnold.html"));
        category.add(book);

        //Chan
        book = new DefaultMutableTreeNode(new BookInfo
            ("The Java Developers Almanac",
             "chan.html"));
        category.add(book);

        category = new DefaultMutableTreeNode("Books for Java Implementers");
        top.add(category);

        //VM
        book = new DefaultMutableTreeNode(new BookInfo
            ("The Java Virtual Machine Specification",
             "vm.html"));
        category.add(book);

        //Language Spec
        book = new DefaultMutableTreeNode(new BookInfo
            ("The Java Language Specification",
             "jls.html"));
        category.add(book);
    }
        

Changing the Icon in a JTree

• icon displayed by a node is determined by whether
  • the node is a leaf
  • whether the node is expanded.
• The specific icon depends on the look and feel implementation, but can be overridden by your program:
  • first create an instance of DefaultTreeCellRenderer
  • Next, specify the icons to use by invoking methods on the renderer:
    • setLeafIcon (for leaf nodes)
    • setOpenIcon (for expanded branch nodes)
    • setClosedIcon (for collapsed branch nodes).
  • Then use the tree's setCellRenderer method to specify that the DefaultTreeCellRenderer paint its nodes.

public class TreeIconDemo extends JPanel implements TreeSelectionListener {
    private JEditorPane htmlPane;
    private JTree tree;
    private URL helpURL;
    private static boolean DEBUG = false;

    public TreeIconDemo() {
        super(new GridLayout(1,0));

        //Create the nodes.
        DefaultMutableTreeNode top =
            new DefaultMutableTreeNode("The Java Series");
        createNodes(top);

        //Create a tree that allows one selection at a time.
        tree = new JTree(top);
        tree.getSelectionModel().setSelectionMode
                (TreeSelectionModel.SINGLE_TREE_SELECTION);

        //Set the icon for leaf nodes.
        ImageIcon leafIcon = createImageIcon("images/dukeWaveRed.gif");
        if (leafIcon != null) {
            DefaultTreeCellRenderer renderer = new DefaultTreeCellRenderer();
            renderer.setLeafIcon(leafIcon);
            tree.setCellRenderer(renderer);
        } else {
            System.err.println("Leaf icon missing; using default.");
        }

// The rest of the code is the same as TreeDemo, except:

   /** Returns an ImageIcon, or null if the path was invalid. */
    protected static ImageIcon createImageIcon(String path) {
        java.net.URL imgURL = TreeIconDemo2.class.getResource(path);
        if (imgURL != null) {
            return new ImageIcon(imgURL);
        } else {
            System.err.println("Couldn't find file: " + path);
            return null;
        }
    }

Varying Node Icon

• Another example: TreeIconDemo2
• creates cell renderer that varies the leaf icon depending on whether the word "Tutorial" is in the node's text data.
• the renderer also specifies tool-tip text

public class TreeIconDemo2 extends JPanel 
                           implements TreeSelectionListener {
    private JEditorPane htmlPane;
    private JTree tree;
    private URL helpURL;
    private static boolean DEBUG = false;

    public TreeIconDemo2() {
        super(new GridLayout(1,0));

        //Create the nodes.
        DefaultMutableTreeNode top =
            new DefaultMutableTreeNode("The Java Series");
        createNodes(top);

        //Create a tree that allows one selection at a time.
        tree = new JTree(top);
        tree.getSelectionModel().setSelectionMode
                (TreeSelectionModel.SINGLE_TREE_SELECTION);

        //Enable tool tips. This lets us call setToolTipText() below.
        ToolTipManager.sharedInstance().registerComponent(tree);

        //Set the icon for leaf nodes.
        ImageIcon tutorialIcon = createImageIcon("images/middle.gif");
        if (tutorialIcon != null) {
            tree.setCellRenderer(new MyRenderer(tutorialIcon));
        } else {
            System.err.println("Tutorial icon missing; using default.");
        }

etc., as in TreeDemo.
Re: ToolTipManager, From the API:

public class ToolTipManager
extends MouseAdapter
implements MouseMotionListener
 

Finally:

    private class MyRenderer extends DefaultTreeCellRenderer {
        Icon tutorialIcon;

        public MyRenderer(Icon icon) {
            tutorialIcon = icon;
        }
	// override this method from DefaultTreeCellRenderer:
        public Component getTreeCellRendererComponent(  
                            JTree tree,
                            Object value,
                            boolean sel,
                            boolean expanded,
                            boolean leaf,
                            int row,
                            boolean hasFocus) {

            super.getTreeCellRendererComponent(
                            tree, value, sel,
                            expanded, leaf, row,
                            hasFocus);
            if (leaf && isTutorialBook(value)) {
                setIcon(tutorialIcon);
                setToolTipText("This book is in the Tutorial series.");
            } else {
                setToolTipText(null); //no tool tip
            }

            return this;
        }

        protected boolean isTutorialBook(Object value) {
            DefaultMutableTreeNode node =
                    (DefaultMutableTreeNode)value;
            BookInfo nodeInfo = 
                    (BookInfo)(node.getUserObject());
            String title = nodeInfo.bookName;
            if (title.indexOf("Tutorial") >= 0) {
                return true;
            } 

            return false;
        }