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<TITLE>Listening for Expansion and Collapse Events in a JTree Component (Java Developers Almanac Example)
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        <B><A HREF="../javax.swing.tree/pkg.html#Events">Events</A></B><font color="#666666" class="xsmall-font">
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  <h3>e1032. Listening for Expansion and Collapse Events in a JTree Component</h3>

The tree component allows you to get events before and after a node
has been expanded or collapsed.  The events that are fired before a
node is expanded or collapsed can be vetoed, thereby preventing the
operation.


<pre>    // Create tree
    JTree tree = new JTree();
    
    // Add pre-expansion event listener
    tree.addTreeWillExpandListener(new MyTreeWillExpandListener());
    
    // Add post-expansion event listener
    tree.addTreeExpansionListener(new MyTreeExpansionListener());
    
    // Pre-expansion/collapse event listener
    public class MyTreeWillExpandListener implements TreeWillExpandListener {
        public void treeWillExpand(TreeExpansionEvent evt) throws ExpandVetoException {
            JTree tree = (JTree)evt.getSource();
    
            // Get the path that will be expanded
            TreePath path = evt.getPath();
    
            // Cancel the operation if desired
            boolean veto = <font color="#0066ff"><i>false</i></font>;
            if (veto) {
                throw new ExpandVetoException(evt);
            }
        }
    
        public void treeWillCollapse(TreeExpansionEvent evt) throws ExpandVetoException {
            JTree tree = (JTree)evt.getSource();
    
            // Get the path that will be collapsed
            TreePath path = evt.getPath();
    
            // Cancel the operation if desired
            boolean veto = <font color="#0066ff"><i>false</i></font>;
            if (veto) {
                throw new ExpandVetoException(evt);
            }
        }
    }
    
    // Post-expansion/collapse event listener
    public class MyTreeExpansionListener implements TreeExpansionListener {
        public void treeExpanded(TreeExpansionEvent evt) {
            JTree tree = (JTree)evt.getSource();
    
            // Get the path that was expanded
            TreePath path = evt.getPath();
        }
    
        public void treeCollapsed(TreeExpansionEvent evt) {
            JTree tree = (JTree)evt.getSource();
    
            // Get the path that was collapsed
            TreePath path = evt.getPath();
        }
    }
</pre>
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            &nbsp;<b>Related Examples</b></font></td>
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e1033. <a class="eglink" href="SelectTree.html?l=rel">
    Listening for Selection Events in a JTree Component
</a>
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<br>
        See also: 
<a class="eglink" href="/egs/javax.swing.tree/pkg.html?l=rel#Layout">
    Layout
</a>&nbsp;&nbsp;

<a class="eglink" href="/egs/javax.swing.tree/pkg.html?l=rel#Node%20Expansion">
    Node Expansion
</a>&nbsp;&nbsp;

<a class="eglink" href="/egs/javax.swing.tree/pkg.html?l=rel#Nodes">
    Nodes
</a>&nbsp;&nbsp;

<a class="eglink" href="/egs/javax.swing.tree/pkg.html?l=rel#Selections">
    Selections
</a>&nbsp;&nbsp;

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