Thank you for expanding on the process - it was unclear from the word "last". What you meant was "largest", given the sorting involved. What you are looking to do is only emit the largest (by value) key out, i.e. a MAX(…) behaviour in SQL for example. This is simple to perform: 1. In the Mapper's setup call, initialise a zero-valued string (lowest ascii value) as the base key, along with a zeroed counter. 2. Across all map(…) calls keep track of if the current probable key is greater than the previous encountered key (beginning with the base key set above). Don't emit anything just yet - just keep reassigning the base key if its greater than the existing one (and reset the counter to 1). If its found equal, increment its counter. 3. In the cleanup(…) method, emit just the base key. 4. Given a MAX-like operation, configure a single reducer, and perform the very same max-tracking/final-emit within the setup(…), reduce(…) and cleanup(…) of the Reducer implementation, but take care to do the count aggregations before the compare, so you get the real count.
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