docs: public static void walkFileTree(File directory, FileTreeVisitor visitor) {
      visitor.preVisitDirectory(directory);
      File[] files = directory.listFiles();
      if (files != null) {
        for (File file: files) {
          if (file.isDirectory()) {
            walkFileTree(file, visitor);
          } else {
            visitor.visitFile(file);
          }
        }
      }
      visitor.postVisitDirectory(directory);
    }