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

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