code: @SuppressWarnings({"unchecked"})
    public OutputCollector getReducerCollector(OutputCollector output,
                                               Reporter reporter) {
      return new ChainOutputCollector(reducerKeySerialization,
                                      reducerValueSerialization, output,
                                      reporter);
    }
docs: Returns the OutputCollector to be used by a Mapper instance in the chain.
  
  @param output   the original OutputCollector of the task.
  @param reporter the reporter of the task.
  @return the OutputCollector to be used in the chain.