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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.]
Try this:
- Run TreeExpandEventDemo using JavaTM Web Start. Or, to compile and run the example yourself, consult the example index.
- Expand a node. A tree-expanded event is fired.
- 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 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.
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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