code: public static BlobId of(String bucket, String name, Long generation) {
      return new BlobId(checkNotNull(bucket), checkNotNull(name), generation);
    }
docs: Creates a {@code BlobId} object.
  
  @param bucket name of the containing bucket
  @param name blob's name
  @param generation blob's data generation, used for versioning. If {@code null} the identifier
  refers to the latest blob's generation