Have a look at this class [https://github.com/stratosphere/stratosphere/blob/master/stratosphere-java/src/main/java/eu/stratosphere/api/java/operators/translation/PlanUnwrappingReduceGroupOperator.java](https://github.com/stratosphere/stratosphere/blob/master/stratosphere-java/src/main/java/eu/stratosphere/api/java/operators/translation/PlanUnwrappingReduceGroupOperator.java) (esp. TupleUnwrappingCombinableGroupReducer). This is very similar to what you need to do for the Hadoop Reduce Wrapper. However, we need to pay attention to the Key that is extracted since this is a mutable object which might change its value while the iterator is consumed.