code: public static Optional<WatermarkInterval> getCopyableFileWatermark(CopyableFile copyableFile,
        Optional<CopyableFileWatermarkGenerator> watermarkGenerator)
        throws IOException {
      if (!watermarkGenerator.isPresent()) {
        return Optional.absent();
      }
      return watermarkGenerator.get().generateWatermarkIntervalForCopyableFile(copyableFile);
    }
docs: Return Optional {@link WatermarkInterval} for {@link CopyableFile} using {@link CopyableFileWatermarkGenerator}.