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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. Or you might need to prevent the user from expanding a particular node.

Two kinds of listeners report expansion and collapse occurrences: tree expansion listeners and tree-will-expand listeners. This page discusses the former. A tree expansion listener detects when an expansion or collapse has happened. In general, you should implement a tree expansion listener unless you might need to prevent an expansion or collapse from happening.

Tree-will-expand listeners are discussed in How to Write a Tree-Will-Expand Listener. A tree-will-expand listener detects when an expansion or collapse is about to happen.

The following 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.

[PENDING: updated screenshot with sample output forthcoming.]

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Try this: 

1. Run TreeExpandEventDemo using Java^TM Web Start. Or, to compile and run the example yourself, consult the example index.
2. Expand a node. A tree-expanded event is fired.
3. Collapse the node. A tree-collapsed event is fired.

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The following code shows how the program handles expansion events. You can find all the example's source code in TreeExpandEventDemo.java.

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

    class DemoArea ... implements TreeExpansionListener {
        ...
        public DemoArea() {
            ...
            tree.addTreeExpansionListener(this);
            ...
        }
        ...
        // 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.

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

The TreeExpansionEvent API

Method	Purpose
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.

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

Example	Where Described	Notes
TreeExpandEventDemo	This section	Displays a message whenever a tree expansion event occurs.
TreeExpandEventDemo2	How to Write a Tree-Will-Expand Listener	Adds a tree-will-expand listener to TreeExpandEventDemo.

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