import javax.swing.tree.*;

// dummy text you can use as a temporary child for a lazy-filled node
sS jDynamicTree_v1_dummy = "<dummy>";

// updateChildren: func(DefaultMutableTreeNode) -> bool
//   (true=there was a change)
static JTree jDynamicTree_v1(final DefaultMutableTreeNode rootNode, final F1<DefaultMutableTreeNode, Bool> updateChildren) {
  ret swing(func -> JTree {
    final JTree tree = new JTree(rootNode);
    tree.setShowsRootHandles(true);
    tree.collapseRow(0);
    
    tree.addTreeWillExpandListener(new TreeWillExpandListener {
      public void treeWillCollapse(TreeExpansionEvent e) {}
      public void treeWillExpand(TreeExpansionEvent e) {
        DefaultMutableTreeNode node = cast e.getPath().getLastPathComponent();
        if (isTrue(pcallF(updateChildren, node)))
          ((DefaultTreeModel) tree.getModel()).nodeStructureChanged(node);
      }
    });
    
    ret tree;
  });
}

Began life as a copy of #1008647