1. Create your own List implementation including iterators and make sure it works with ObservableList as a wrapper.
2. Make the add* methods not actually add anything.
3. Create a new instance of the list and wrap it in ObservableList, store it in "list" variable.
4. Wrap the ObservableList in SortedList.
5. Perform list.add(...).

I use my own List+Set implementation (ArrayHashList, LinkedHashList), whose instance is a list with no duplicates in it. Very useful thing. It however does not work with the SortedList wrapper because if I do this, I get the IndexOutOfBoundsException:

final ObservableList<String> list = FXCollections.observableList(new ArrayHashList<String>());
final ObservableList<String> sortedList = new SortedList<String>(list, String.CASE_INSENSITIVE_ORDER);
list.add("a");
list.add("a"); // Does not change the list