GUI Changes: The AWT Grows Up

How to Use Trees

With the JTree class, you can display hierarchical data. JTree doesn't actually contain your data; it's simply a view of the data. Here's a picture of a tree:

As the preceding figure shows, JTree displays its data vertically. Each row contains exactly one item of data (called a node). Every tree has a root node (called Root in the above figure) from which all nodes descend. Nodes that can't have children are called leaf nodes. In the above figure, the look-and-feel marks leaf nodes with a circle.

Non-leaf nodes can have any number of children, or even no children. In the above figure, the look-and-feel marks non-leaf nodes by drawing a folder. Typically, the user can expand and unexpand non-leaf nodes -- making their children visible or invisible -- by clicking them. By default, non-leaf nodes start out unexpanded.

When you initialize a tree, you create a TreeNode instance for each node in the tree, including the root. To make a node a leaf, you invoke setAllowsChildren(false) on it.

When data values change -- for example, nodes are added -- you [the data model?] can let the JTree[Model?] know there's new data by invoking one of the TreeModelListener methods. The JTree then displays the new data. [CHECK ALL THIS!]

Here is a picture of an application that displays a tree in a scroll pane:

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Try this:

1. Compile and run the application. The source file is TreeDemo.java.
   See Getting Started with Swing if you need help.
2. Expand a node.
   If you're using the Basic look and feel, you do this by clicking the + sign to the left of the item.
3. Select a node.
   If you're using the Basic look and feel, you do this by clicking the text for the node or the icon just to the left.

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Below is the code from TreeDemo.java that implements the tree in the previous example.

//In initialization code in a JPanel subclass:
    //Create the nodes.
    TreeNode top = new TreeNode("The Java Series");
    TreeNode category;
    TreeNode book;

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

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

    ...//Do the same for each book in the category...

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

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

    ...//Do the same for each book in the category...

    //Make all child-free nodes leaves.
    makeLeaves(top);

    JTree tree = new JTree(top);
. . .
//This method will be added to the TreeNode API in a future release.
protected void makeLeaves(TreeNode top) {
    Enumeration       childEnum = top.preorderEnumeration();
    TreeNode          descendant;

    while(childEnum.hasMoreElements()) {
        descendant = (TreeNode)childEnum.nextElement();
        if(descendant.getChildCount() == 0)
            descendant.setAllowsChildren(false);
    }       
}

[API discussion to follow. Discuss renderer options.]

GUI Changes: The AWT Grows Up