title: Different keys in Mapper & Combiners
body: <p>In Map-Reduce, is it possible to have different type of keys propagated between Mappers, Combiners and Reducers. </p>
  
  <p>For example, if I have a mapper (implemented in Java) which outputs Text,IntWritable as key/value pairs.
  Then, in combiner I consolidate all the output as single key and want to output that as NullWritable, Text. 
  and Then in Reducer, I want to output Text, IntWritable. </p>
  
  <p>Is it possible to do something like above ? If not, why ? </p>