In the project I work I have found that it would be efficient (at a small cost
of memory) to use filtered maps, lists, and sets instead of creating a new copy
of the decorated entities that contains only the filtered elements.
To begin with I have implemented a FilteredMap and plan to implement
FilteredList and FilteredSet. I will post the code for these in a couple of
days and it would be great if Collections library could accept these contributions.