Until the collections library has been fully implemented, subclassing conventions from collections implementation classes are still in flux. In general, these classes will be among the most complex uses of multiple classes selected to implement an independent object type. (See library definition conventions for a summary of the distinction between types and classes.) New methods are being developed to simplify subclassing from such implementations. In the meantime, if you need to use to a collection within the implementation of your own class, just put an instance variable in your class and put the collection in that, and pass through the messages of the collection you want to have available on your class to this variable. In many if not most cases, this is better design anyway, because you control all use of the underlying structure.