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How to Create a JTree Component in Java

By Doug Lowe

In Java, you use the JTree class to create a tree component that displays the nodes of a tree. In order to use it correctly, check out the key constructors and methods of this class.

Constructor Description
void JTree() Creates an empty tree (not very useful, if you ask me).
void JTree(TreeNode root) Creates a tree that displays the tree that starts at the
specified node.
Method Description
void
addTreeSelectionListener(TreeSelectionListener
listener)
Adds the specified listener to listen for tree-selection
events.
Object
getLastSelectedPathComponent()
Gets the node that is currently selected.
TreeSelectionModel
getSelectionModel()
Gets the selection model for the tree. See this section for
what you can do with the TreeSelectionModel object.
void setVisibleRowCount(int
count)
Sets the number of rows visible in the display.

The first step in creating a JTree component is declaring a JTree variable as a class instance variable so that you can access the constructor and other methods, as follows:

JTree tree1;

Then, in the frame constructor, you call the JTree constructor to create the tree component, passing the root node of the tree you want it to display as a parameter:

tree1 = new JTree(root);

By default, the user is allowed to select multiple nodes from the tree. To restrict the user to a single selection, use this strange incantation:

tree1.getSelectionModel().setSelectionMode(
 TreeSelectionModel.SINGLE_TREE_SELECTION);

Here the getSelectionModel method is called to get a TreeSelectionModel object that determines how the user can select nodes in the tree. This class provides a method named setSelectionMode that lets you set the selection mode. To limit the tree to a single node selection, you must pass this method the TreeSelectionModel.SINGLE_TREE_SELECTION field.

You can control the size of the tree component by calling the setVisibleRowCount method, as in this example:

tree1.setVisibleRowCount(12);

Here the tree is just large enough to show 12 rows at a time.

Finally, you add the tree component to a scroll pane so that the user can scroll the tree if it doesn’t fit in the space provided. Then you should add the scroll pane to a panel that is in turn added to the frame, like this:

JScrollPane scroll = new JScrollPane(tree1);
panel1.add(scroll);

That’s it!