public static void main(String[] args) {
        File root = new File("/Users/user/IdeaProjects/MyProject");
        streamFilesOrSelf(root)
                .map(File::getName)
                .sorted()
                .forEach(System.out::println);
    }

    public static Stream<File> streamFilesOrSelf(File file) {
        return file.isDirectory()
                ? Arrays.stream(file.listFiles()).flatMap(Test::streamFilesOrSelf)
                : Stream.of(file);
    }