JTree Usage example , Tree nodes

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import com.sun.java.swing.JTree;
//Why do I have to import stuff from the tree package?
//Why Mutable?  I don't want to change nodes!  Why not DefaultTreeNode?
import com.sun.java.swing.tree.DefaultMutableTreeNode;
import com.sun.java.swing.JScrollPane;
import com.sun.java.swing.JPanel;
import com.sun.java.swing.JFrame;
import java.awt.GridLayout;
import java.awt.event.WindowListener;
import java.awt.event.WindowAdapter;
import java.awt.event.WindowEvent;
import java.util.Enumeration;

// It would be cool to modify this so that nodes are links.
public class TreeDemo extends JPanel {
    public TreeDemo() {
	//Create the nodes.
	DefaultMutableTreeNode top = new DefaultMutableTreeNode("The Java Series");
	DefaultMutableTreeNode category;
	DefaultMutableTreeNode book;

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

	//Tutorial
	book = new DefaultMutableTreeNode("The Java Tutorial: Object-Oriented Programming for the Internet");
        book.add(new DefaultMutableTreeNode("Mary Campione"));
        book.add(new DefaultMutableTreeNode("Kathy Walrath"));
	category.add(book);

	//Arnold/Gosling
	book = new DefaultMutableTreeNode("The Java Programming Language");
        book.add(new DefaultMutableTreeNode("Ken Arnold"));
        book.add(new DefaultMutableTreeNode("James Gosling"));
	category.add(book);

	//FAQ
	book = new DefaultMutableTreeNode("The Java FAQ");
        book.add(new DefaultMutableTreeNode("Jonni Kanerva"));
	category.add(book);

	//Chan/Lee
	book = new DefaultMutableTreeNode("The Java Class Libraries: An Annotated Reference");
        book.add(new DefaultMutableTreeNode("Patrick Chan"));
        book.add(new DefaultMutableTreeNode("Rosanna Lee"));
	category.add(book);

	//Threads
	book = new DefaultMutableTreeNode("Concurrent Programming in Java: Design Principles and Patterns");
        book.add(new DefaultMutableTreeNode("Doug Lea"));
	category.add(book);

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

	//VM
	book = new DefaultMutableTreeNode("The Java Virtual Machine Specification");
        book.add(new DefaultMutableTreeNode("Tim Lindholm"));
        book.add(new DefaultMutableTreeNode("Frank Yellin"));
	category.add(book);

	//Language Spec
	book = new DefaultMutableTreeNode("The Java Language Specification");
        book.add(new DefaultMutableTreeNode("James Gosling"));
        book.add(new DefaultMutableTreeNode("Bill Joy"));
        book.add(new DefaultMutableTreeNode("Guy Steele"));
	category.add(book);

	JTree tree = new JTree(top);

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

	//Add the scroll pane to this panel.
	setLayout(new GridLayout(1, 0)); 
        add(scrollPane);
    }

    public static void main(String[] args) {
	/*
	 * Create a window.  Use JFrame since this window will include 
	 * lightweight components.
	 */
	JFrame frame = new JFrame("TreeDemo");

	WindowListener l = new WindowAdapter() {
	    public void windowClosing(WindowEvent e) {System.exit(0);}
	};
	frame.addWindowListener(l);

	frame.getContentPane().add("Center", new TreeDemo());
	frame.setSize(500, 200);
	frame.show();
    }
}