Trees - The Definitive Guide to Java Swing - page 652

When JTree gets a Vector as its constructor argument, the tree creates a
DefaultMutableTreeNode for the root node, and then creates another one for each element in
the Vector , making each element node a child of the root node. The data for the root node
unfortunately is not the "Root" you specify, but rather root , and isn't shown.
If, instead, you wanted to use DefaultMutableTreeNode to manually create the nodes of a
tree, or if you wanted to display the root node, a few more lines would be necessary, as follows:
DefaultMutableTreeNode root = new DefaultMutableTreeNode("Root");
DefaultMutableTreeNode mercury = new DefaultMutableTreeNode("Mercury");
root.insert(mercury, 0);
DefaultMutableTreeNode venus = new DefaultMutableTreeNode("Venus");
root.insert(venus, 1);
DefaultMutableTreeNode mars = new DefaultMutableTreeNode("Mars");
root.insert(mars, 2);
JTree tree = new JTree(root);
Besides using the insert() method from MutableTreeNode to associate a child with a parent,
DefaultMutableTreeNode has an add() method that automatically adds a child node at the end,
without providing an index.
DefaultMutableTreeNode root = new DefaultMutableTreeNode("Root");
DefaultMutableTreeNode mercury = new DefaultMutableTreeNode("Mercury");
root.add(mercury);
DefaultMutableTreeNode venus = new DefaultMutableTreeNode("Venus");
root.add(venus);
DefaultMutableTreeNode mars = new DefaultMutableTreeNode("Mars");
root.add(mars);
JTree tree = new JTree(root);
Both of the previous blocks of source create a tree like the one shown in Figure 17-17.
Figure 17-17. Using DefaultMutableTreeNode
If you don't need a root node and want the same behavior as if you had used NamedVector
at the root of a tree, you could do the following, too:
String elements[] = { "Mercury", "Venus", "Mars"} ;
JTree tree = new JTree(elements);