code: @Override
    public GoogleCloudStorageFileSet finish(Collection<? extends OutputWriter<ByteBuffer>> writers) {
      List<String> out = Lists.newArrayList();
      for (OutputWriter<ByteBuffer> w : writers) {
        GoogleCloudStorageFileOutputWriter writer = (GoogleCloudStorageFileOutputWriter) w;
        out.add(writer.getFile().getObjectName());
      }
      return new GoogleCloudStorageFileSet(bucket, out);
    }
docs: Returns a list of GcsFilename that has one element for each reduce shard.