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 occurred. In general, you should implement a tree expansion listener unless you need to prevent an expansion or collapse from occurring .

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.

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

The TreeExpansionEvent API

Examples that Use Tree Expansion Listeners

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