Yes James, for the DISTINCT count the summing up of counts is not needed. That is the reason why I have not done that and gone with a Set as of now..  I started checking the Percentile case.. Ya there it is very much needed..  Base class + derived classes was my idea too :)  Any way for all I need one server aggregator only. That is why now itself from the server class I was returning the Map with counts.. (If u see infact for the distinct count we dont even need this couters.. Keys are enough)

Let me see with using ImmutableBytesPtr as Key and then I can go with HashMap..   Any way at client side we would need sorted map for percentile.. But as u said we can do at once at client side ...Let me see..