code: private BufferedWriter createWriter(Optional<String> prefixExtension) throws IOException {
              if (prefix.isPresent()) {
                  String suffix = getSuffix();
                  return Files.newBufferedWriter(
                          prefixExtension.isPresent() ?
                                  Paths.get(prefix.get(), prefixExtension.get() + suffix) :
                                  Paths.get(prefix.get() + suffix),
                          Charset.forName("UTF-8")
                  );
              }
              return new BufferedWriter(new OutputStreamWriter(System.out, "UTF-8"));
          }
docs: We need a direct access to the writer to close it (cannot be done via the RDFHandler)