How to Write a Tree Expansion Listener

Sometimes when using a tree, you might need to react when a branch becomes expanded or collapsed. For example, you might need to load or save data.

Two kinds of listeners report expansion and collapse occurrences: tree expansion listeners and tree-will-expand listeners. This page discusses tree expansion listeners. See How to Write a Tree-Will-Expand Listener for a description of Tree-Will-Expand listeners.

A tree expansion listener detects when an expansion or collapse has already occured. In general, you should implement a tree expansion listener unless you need to prevent an expansion or collapse from ocurring .

This example demonstrates a simple tree expansion listener. The text area at the bottom of the window displays a message every time a tree expansion event occurs. It's a straightforward, simple demo. To see a more interesting version that can veto expansions, see How to Write a Tree-Will-Expand Listener.


Try this:

  • Click the Launch button to run TreeExpandEventDemo using Java™ Web Start (download JDK 7 or later). Alternatively, to compile and run the example yourself, consult the example index.

  • Expand a node. A tree-expanded event is fired.

  • Collapse the node. A tree-collapsed event is fired.

The following code shows how the program handles expansion events. You can find the source code for this example in

public class TreeExpandEventDemo ... {
    void saySomething(String eventDescription, TreeExpansionEvent e) {
        textArea.append(eventDescription + "; "
                        + "path = " + e.getPath()
                        + newline);

    class DemoArea ... implements TreeExpansionListener {
        public DemoArea() {
        // Required by TreeExpansionListener interface.
        public void treeExpanded(TreeExpansionEvent e) {
            saySomething("Tree-expanded event detected", e);

        // Required by TreeExpansionListener interface.
        public void treeCollapsed(TreeExpansionEvent e) {
            saySomething("Tree-collapsed event detected", e);

The Tree Expansion Listener API

The TreeExpansionListener Interface

TreeExpansionListener has no adapter class.

treeCollapsed(TreeExpansionEvent)Called just after a tree node collapses.
treeExpanded(TreeExpansionEvent)Called just after a tree node expands.

The TreeExpansionEvent API

Object getSource()Return the object that fired the event.
TreePath getPath()Returns a TreePath object that identifies each node from the root of the tree to the collapsed/expanded node, inclusive.

Examples that Use Tree Expansion Listeners

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

ExampleWhere DescribedNotes
TreeExpandEventDemoThis sectionDisplays a message whenever a tree expansion event occurs.
TreeExpandEventDemo2How to Write a Tree-Will-Expand ListenerAdds a tree-will-expand listener to TreeExpandEventDemo .