Returns a concurrent
Collector implementing a "group by"
operation on input elements of type
T, grouping elements
according to a classification function.
This is a
Collector.Characteristics#CONCURRENT and
Collector.Characteristics#UNORDERED Collector.
The classification function maps elements to some key type
K.
The collector produces a
ConcurrentMap> whose keys are the
values resulting from applying the classification function to the input
elements, and whose corresponding values are
Lists containing the
input elements which map to the associated key under the classification
function.
There are no guarantees on the type, mutability, or serializability
of the
ConcurrentMap or
List objects returned, or of the
thread-safety of the
List objects returned.
Implementation Requirements:
This produces a result similar to:
groupingByConcurrent(classifier, toList())