int index = mod.getIndexOfChild(parent, node) + 1;
int index = treeModel.getIndexOfChild(parent, node) + 1;
  Object getNextSibling(Object node)  {    TreeModel mod = tree.getModel();    Object parent = getParent(mod.getRoot(), node);    if (parent == null)      return null;    int index = mod.getIndexOfChild(parent, node) + 1;        int size = 0;    if (!mod.isLeaf(parent))      size = mod.getChildCount(parent);    if (index == 0 || index >= size)      return null;    return mod.getChild(parent, index);  }