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Trail: Creating a GUI with JFC/Swing
Lesson: Writing Event Listeners
Section: Implementing Listeners for Commonly Handled Events
Writing Event Listeners
Introduction to Event Listeners
General Information about Writing Event Listeners
Listeners Supported by Swing Components
Implementing Listeners for Commonly Handled Events
How to Write an Action Listener
How to Write a Caret Listener
How to Write a Change Listener
How to Write a Component Listener
How to Write a Container Listener
How to Write a Document Listener
How to Write a Focus Listener
How to Write an Internal Frame Listener
How to Write an Item Listener
How to Write a Key Listener
How to Write a List Data Listener
How to Write a List Selection Listener
How to Write a Mouse Listener
How to Write a Mouse-Motion Listener
How to Write a Mouse-Wheel Listener
How to Write a Property Change Listener
How to Write a Table Model Listener
How to Write a Tree Expansion Listener
How to Write a Tree Model Listener
How to Write a Tree Selection Listener
How to Write a Tree-Will-Expand Listener
How to Write an Undoable Edit Listener
How to Write Window Listeners
Listener API Table
Solving Common Event-Handling Problems
Home Page > Creating a GUI with JFC/Swing > Writing Event Listeners
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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.

> > Try this:
> > 1. Run TreeExpandEventDemo using JavaTM 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.

> 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.

### Examples that Use Tree Expansion Listeners

> 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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