Trail: Creating a GUI With JFC/Swing
Lesson: Writing Event Listeners
Section: Implementing Listeners for Commonly Handled Events

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How to Write a Tree Selection Listener

To detect when the user selects a node in a tree, you need to register a tree selection listener. Here is an example, taken from the TreeDemo example discussed in Responding to Node Selection, of detecting node selection in a tree that can have at most one node selected at a time:

tree.addTreeSelectionListener(new TreeSelectionListener() {
    public void valueChanged(TreeSelectionEvent e) {
        DefaultMutableTreeNode node = (DefaultMutableTreeNode)
                           tree.getLastSelectedPathComponent();

    /* if nothing is selected */ 
        if (node == null) return;

    /* retrieve the node that was selected */ 
        Object nodeInfo = node.getUserObject();
        ...
    /* React to the node selection. */
        ...
    }
});

To specify that the tree should support single selection, the program uses this code:

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

The TreeSelectionModel interface defines three values for the selection mode:

DISCONTIGUOUS_TREE_SELECTION: This is the default mode for the default tree selection model. With this mode, any combination of nodes can be selected.
SINGLE_TREE_SELECTION: This is the mode used by the preceding example. At most one node can be selected at a time.
CONTIGUOUS_TREE_SELECTION: With this mode, only nodes in adjoining rows can be selected.

The Tree Selection Listener API

The TreeSelectionListener Interface

Because TreeSelectionListener has only one method, it has no corresponding adapter class.

Method	Purpose
valueChanged(TreeSelectionEvent)	Called whenever the selection changes.

The TreeSelectionEvent API

Method	Purpose
Object getSource() (in `java.util.EventObject`)	Return the object that fired the event.
TreePath getNewLeadSelectionPath()	Return the current lead path.
TreePath getOldLeadSelectionPath()	Return the path that was previously the lead path.
TreePath getPath()	Return the first path element.
TreePath[] getPaths()	Return the paths that have been added or removed from the selection.
boolean isAddedPath()	Return true if the first path element has been added to the selection. Returns false if the first path has been removed from the selection.
boolean isAddedPath(int)	Return true if the path specified by the index was added to the selection.
boolean isAddedPath(TreePath)	Return true if the specified path was added to the selection.
Object getLastSelectedPathComponent()	Return the last path component in the first node of the current selection.
TreePath getLeadSelectionPath() (in `JTree`)	Return the current lead path.

Examples that Use Tree Selection Listeners

The following table lists the examples that use tree selection listeners.

Example	Where Described	Notes
`TreeDemo`	How to Use Trees	The tree listener responds to node clicks by showing the appropriate HTML document.

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