Returns a Collector which partitions the input elements according to a Predicate, reduces the values in each partition according to another Collector, and organizes them into a Map<Boolean, D> whose values are the result of the downstream reduction.

The returned Map always contains mappings for both false and true keys. There are no guarantees on the type, mutability, serializability, or thread-safety of the Map returned.

Parameters:

<T>		the type of the input elements
<A>		the intermediate accumulation type of the downstream collector
<D>		the result type of the downstream reduction
predicate		a predicate used for classifying input elements
downstream		a Collector implementing the downstream reduction

Returns:  a Collector implementing the cascaded partitioning operation

See also:
partitioningBy(Predicate)

@apiNote If a partition has no elements, its value in the result Map will be obtained by calling the downstream collector's supplier function and then applying the finisher function.