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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-Will-Expand Listener

> As explained in How to Write a Tree Expansion Listener, you can use a tree-will-expand listener to prevent 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.

> The following demo adds a tree-will-expand listener to the `TreeExpandEventDemo` example discussed in How to Write a Tree Expansion Listener. The new code demonstrates the ability of tree-will-expand listeners to veto node expansions and collapses: it asks for confirmation each time you try to expand a node.

> > Try this:
> > 1. Run TreeExpandEventDemo2 using JavaTM Web Start. Or, to compile and run the example yourself, consult the example index.
> > 2. 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.
> > 3. 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.
> > 4. 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.
> > 5. 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 `TreeExpandEventDemo2.java`.

> >     public class TreeExpandEventDemo2 ... {
> >         ...
> >         class DemoArea ... implements ... TreeWillExpandListener {
> >             ...
> >             public DemoArea() {
> >                 ...
> >                 tree.addTreeWillExpandListener(this);
> >                 ...
> >             }
> >             ...
> >             //Required by TreeWillExpandListener interface.
> >             public void treeWillExpand(TreeExpansionEvent e)
> >                         throws ExpandVetoException {
> >                 saySomething("Tree-will-expand event detected", e);
> >                 //...show 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.

> Method | Purpose
> --- | ---
> 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.

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