private static void listDirectoriesOf(File f, String prefix){
        String[] content = f.list();

        File[] directories = Arrays.stream(content)
                .map(child -> new File(f + File.separator + child))
                .filter(File::isDirectory).toArray(File[]::new);

        if(directories.length == 0){
            return;
        }

        for (File directory : directories){
            System.out.println(prefix + directory);
            listDirectoriesOf(directory, "    " + prefix);
        }
    }