MapReduce is deprecated and will be removed in 4.0.
For map aggregations, you can use Aggregator on IMap.
For general data processing, it is superseded by Hazelcast Jet.
public interface Collator<ValueIn,ValueOut>
This interface can be implemented to define a Collator which is executed after calculation
of the MapReduce algorithm on remote cluster nodes but before returning the final result.
Collator can, for example, be used to sum up a final value.