Extending Protocol Buffers in Java

I'm having trouble accessing extended protocol buffer members. Here is the scenario:

Error while working with List<? extends DefaultMutableTreeNodel>

public void findClassNodesMatching(String lowerCaseSearchText, List<? extends DefaultMutableTreeNode> foundNodes) {

    findClassNodesMatching(lowerCaseSearchText, (DefaultMutableTreeNode) getRoot(), foundNodes);

}



private void findClassNodesMatching(String lowerCaseSearchText, DefaultMutableTreeNode node, List<? extends DefaultMutableTreeNode> foundNodes) {

    String nodeLabel = node.toString().toLowerCase();

    if (nodeLabel.indexOf(lowerCaseSearchText) >= 0) {

        foundNodes.add(node);

    }

}