code: @SuppressWarnings("all")
    private static void zip(File zipDirectory, File relativeDirectory, ZipOutputStream outputStream) {
  
      Assert.isTrue(FileUtils.isDirectory(relativeDirectory),
        "[%s] is not a valid directory", relativeDirectory);
  
      stream(relativeDirectory.listFiles()).forEach(file -> {
  
        if (file.isDirectory()) {
          zip(zipDirectory, file, outputStream);
        }
        else {
          zipEntry(zipDirectory, file, outputStream);
        }
      });
    }
docs: Zips the contents of the specified {@link File directory} to the supplied {@link ZipOutputStream}.
  
  @param zipDirectory {@link File directory} being zipped.
  @param relativeDirectory the current {@link File directory} with contents to zip.
  @param outputStream {@link ZipOutputStream} used to zip the contents of the relative {@link File directory}.
  @throws IllegalArgumentException if relative {@link File directory} is not a valid directory.
  @throws SystemException if a file system entry could not be added to the {@link File ZIP file}.
  @see #zipEntry(File, File, ZipOutputStream)
  @see java.util.zip.ZipOutputStream
  @see java.io.File