The default comparator in TreeTableView is a Comparator<TreeItem<S>>, not a Comparator<S> as indicated by the signature of the getComparator method. This causes the code below to fail with a ClassCastException:

    TreeTableView<Item> table = new TreeTableView<>(getRoot());
    table.comparatorProperty().addListener((ObservableValue<? extends Comparator<Item>> observable, Comparator<Item> oldValue, Comparator<Item> newValue) ->
        newValue.compare(new Item(0), new Item(1))
    );

Workaround: getting rid of generics and explicitly casting to Comparator<TreeItem<Item>>.