code: @SuppressWarnings({"unchecked"})
    public OutputCollector getMapperCollector(int mapperIndex,
                                              OutputCollector output,
                                              Reporter reporter) {
      Serialization keySerialization = mappersKeySerialization.get(mapperIndex);
      Serialization valueSerialization =
        mappersValueSerialization.get(mapperIndex);
      return new ChainOutputCollector(mapperIndex, keySerialization,
                                      valueSerialization, output, reporter);
    }
docs: Returns the OutputCollector to be used by a Mapper instance in the chain.
  
  @param mapperIndex index of the Mapper instance to get the OutputCollector.
  @param output      the original OutputCollector of the task.
  @param reporter    the reporter of the task.
  @return the OutputCollector to be used in the chain.