code: public CompleteMultipartUploadResult onCompletion(List<PartETag> partETags) {
          return s3.completeMultipartUpload(
              new CompleteMultipartUploadRequest(
                      req.getBucketName(), req.getKey(), uploadId, partETags));
      }
docs: <!--- Notified from
  {@link AmazonS3EncryptionClient#uploadObject(UploadObjectRequest)} when
  all parts have been successfully uploaded to S3.---> This method is
  responsible for finishing off the upload by making a complete multi-part
  upload request to S3 with the given list of etags.
  
  @param partETags
  all the etags returned from S3 for the previous part uploads.
  
  @return the completed multi-part upload result