code: @SuppressWarnings({"unchecked"})
    public void map(Object key, Object value, OutputCollector output,
                    Reporter reporter) throws IOException {
      Mapper mapper = chain.getFirstMap();
      if (mapper != null) {
        mapper.map(key, value, chain.getMapperCollector(0, output, reporter),
                   reporter);
      }
    }
docs: Chains the <code>map(...)</code> methods of the Mappers in the chain.