@Test
 public void walk_root() throws URISyntaxException, IOException {
   String authority = "local.walkroot";
   String uri = SCHEME + "://" + authority;
   Map<String, Object> env = new HashMap<>();
   env.put(FileSystemIO.BASE_DIRECTORY, BASE_FILE.toString() + "/" + authority);
   FileSystems.newFileSystem(URI.create(uri), env);
   Path root = Paths.get(URI.create(uri));
   createFiles(root, "root", 50);
   walk(authority, root, "root", 50);
 }
 @BeforeClass
 public static void setUpClass() throws IOException {
   Map<String, Object> env = new HashMap<>();
   env.put(FileSystemIO.BASE_DIRECTORY, BASE_FILE.toString() + "/" + AUTHORITY);
   FileSystems.newFileSystem(URI.create(URI_PREFIX), env);
 }