Why no return a collection when possible?

[Jose]

There are many cases in which the entry point of a pilpeline is a collection
and its size has not changed at the other end (no filters invoked). 
In this case it would be good to get a Collection. Currently I doing this by
hand at the end of each pipeline using something like this:

public static <T> Collection<T> newCollection(final Iterable<T> iterable,
final int size) {
     return new AbstractCollection<T>() {
           public Iterator<T> iterator() {
                return iterable.iterator();
            }
            public int size() {
                return size;
            }
      };
}

The point is to preserve compatibility with the existing code, refractoring
collections to Iterables would be a major task.