@Override public <K,A,D>CompletableFuture<Map<K,D>> groupBy(final Function<? super T,? extends K> classifier,final Collector<? super T,A,D> downstream){
  return CompletableFuture.supplyAsync(null,exec);
}

@ Override public < K , A , D > CompletableFuture < Map < K , D > > groupBy ( final Function < ? super T , ? extends K > classifier , final Collector < ? super T , A , D > downstream ) { return CompletableFuture . supplyAsync ( null , exec ) ; }
perform an asyncrhonous groupby operation
