build tree 12h1

  try {
      List<File> file_dir = Files.list(getPath()).map(file -> {
        try {
          File files = FileUtil.toFileRepresentation(file.toAbsolutePath());
          if (files.isRegularFile()) {
            return file;
          } else {
            Files.list(files.getPath()).filter(Files::isRegularFile).map(f -> {
              return new RegularFile(f.toAbsolutePath());
            });
          }
        } catch (IOException ex) {
          System.err.println(file.toAbsolutePath() + " is invalid: " + ex.getMessage());
          return;
        }
      }).collect(Collectors.toList());
      Iterator<File> res = file_dir.iterator();
      return res;
    } catch (IOException ex) {
      ex.printStackTrace();
      System.err.println(getPath() + " is invalid: " + ex.getMessage());
      return null;
    }