How to Write a Tree-Will-Expand Listener

The tree-will-expand listener prevents a tree node from expanding or collapsing. To be notified just after an expansion or collapse occurs, you should use a tree expansion listener instead.

This demo adds a tree-will-expand listener to the TreeExpandEventDemo example discussed in How to Write a Tree Expansion Listener. The code added here demonstrates that tree-will-expand listeners prevent node expansions and collapses: The listener will prompt you for confirmation each time you try to expand a node.


Try this:

  • Click the Launch button to run TreeExpandEventDemo2 using Java™ Web Start (download JDK 7 or later). Alternatively, to compile and run the example yourself, consult the example index.

  • Click the graphic to the left of the Potrero Hill node. This tells the tree that you want to expand the node.
    A dialog appears asking you whether you really want to expand the node.

  • Click "Expand" or dismiss the dialog.
    Messages in the text area tell you that both a tree-will-expand event and a tree-expanded event have occurred. At the end of each message is the path to the expanded node.

  • Try to expand another node, but this time press the "Cancel Expansion" button in the dialog.
    The node does not expand. Messages in the text area tell you that a tree-will-expand event occurred, and that you cancelled a tree expansion.

  • Collapse the Potrero Hill node.
    The node collapses without a dialog appearing, because the event handler's treeWillCollapse method lets the collapse occur, uncontested.

The following snippet shows the code that this program adds to TreeExpandEventDemo . The bold line prevents the tree expansion from happening. You can find all the demo's source code in

public class TreeExpandEventDemo2 ... {
    class DemoArea ... implements ... TreeWillExpandListener {
        public DemoArea() {
        //Required by TreeWillExpandListener interface.
        public void treeWillExpand(TreeExpansionEvent e) 
                    throws ExpandVetoException {
            saySomething("Tree-will-expand event detected", e);
            // a dialog...
            if (/* user said to cancel the expansion */) {
                //Cancel expansion.
                saySomething("Tree expansion cancelled", e);
                throw new ExpandVetoException(e);

        //Required by TreeWillExpandListener interface.
        public void treeWillCollapse(TreeExpansionEvent e) {
            saySomething("Tree-will-collapse event detected", e);

The Tree-Will-Expand Listener API

The TreeWillExpandListener Interface

TreeWillExpandListener has no adapter class.

treeWillCollapse(TreeExpansionEvent)Called just before a tree node collapses. To prevent the collapse from occurring, your implementation of this method should throw a ExpandVetoException event.
treeWillExpand(TreeExpansionEvent)Called just before a tree node expands. To prevent the expansion from occurring, your implementation of this method should throw a ExpandVetoException event.

See The Tree Expansion Event API for information about the TreeExpansionEvent argument for the preceding methods.

Examples that Use Tree-Will-Expand Listeners

TreeExpandEventDemo2, featured in this section, is our only example that uses a tree-will-expand listener.