This is the base of all mapping phase implementations. It's responsibility is to test keys prior to mapping the special value. This is used for preselected key-sets and given {@link com.hazelcast.mapreduce.KeyPredicate} implementations.
@param < KeyIn> type of the input key
@param < ValueIn> type of the input value
@param < KeyOut> type of the emitted key
@param < ValueOut> type of the emitted value
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