For example, I want to expand all children of root Node (only the first
level children)

What code can achieve this goal?

Thanks!

expandAll2Last (silly name?) should read

// assumes 'tree' has a DefaultTreeModel!

public void expandAll2Last(JTree tree)
{
   DefaultTreeModel data = (DefaultTreeModel).getModel();

   DefaultMutableTreeNode root = (DefaultMutableTreeNode)data.getRoot();

   if (root != null)
       tree.scrollPathToVisible(new TreePath(data.getPathToRoot(root.getLastLeaf())));
}

or more generally (untested):

public void expandAll2Last(JTree tree)
{
   TreeModel data = tree.getModel();

   Object node = data.getRoot();

   if (node == null)
       return;

   TreePath p = new TreePath(node);

   while (true)
   {
        int count = data.getChildCount(node);

        if (count == 0)
            break;

        node = data.getChild(node, count - 1);
        p = p.pathByAddingChild(node);
   }

   tree.scrollPathToVisible(p);
}

Also, it is not clear why this method should also scroll, and that it does
not expand the last component (even if not a leaf).

Christian