Java by API/javax.swing.event/TreeWillExpandListener

implements TreeWillExpandListener

   <source lang="java">

import java.awt.BorderLayout; import javax.swing.JFrame; import javax.swing.JScrollPane; import javax.swing.JTree; import javax.swing.event.TreeExpansionEvent; import javax.swing.event.TreeWillExpandListener; import javax.swing.tree.DefaultMutableTreeNode; import javax.swing.tree.ExpandVetoException; import javax.swing.tree.TreePath; public class Main {

 public static void main(String args[]) {
   JFrame frame = new JFrame();
   frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
   DefaultMutableTreeNode root = new DefaultMutableTreeNode("Root");
   root.add(new DefaultMutableTreeNode("A"));
   root.add(new DefaultMutableTreeNode("B"));
   root.add(new DefaultMutableTreeNode("C"));
   JTree tree = new JTree(root);
   TreeWillExpandListener treeWillExpandListener = new TreeWillExpandListener() {
     public void treeWillCollapse(TreeExpansionEvent treeExpansionEvent)
         throws ExpandVetoException {
       TreePath path = treeExpansionEvent.getPath();
       DefaultMutableTreeNode node = (DefaultMutableTreeNode) path.getLastPathComponent();
       String data = node.getUserObject().toString();
       System.out.println("WillCollapse: " + data);
     }
     public void treeWillExpand(TreeExpansionEvent treeExpansionEvent) throws ExpandVetoException {
       TreePath path = treeExpansionEvent.getPath();
       DefaultMutableTreeNode node = (DefaultMutableTreeNode) path.getLastPathComponent();
       String data = node.getUserObject().toString();
       System.out.println("WillExpand: " + data);
     }
   };
   tree.addTreeWillExpandListener(treeWillExpandListener);
   JScrollPane scrollPane = new JScrollPane(tree);
   frame.add(scrollPane, BorderLayout.CENTER);
   frame.setSize(300, 150);
   frame.setVisible(true);
 }

}

 </source>