A DESCRIPTION OF THE PROBLEM :
https://docs.oracle.com/javase/8/docs/api/java/util/stream/Collector.html

Interface Collector<T,A,R>

API Note:
    Performing a reduction operation with a Collector should produce a result equivalent to:


         R container = collector.supplier().get();
         for (T t : data)
             collector.accumulator().accept(container, t);
         return collector.finisher().apply(container);
     
MUST BE:
       A container = collector.supplier().get();


FREQUENCY : always