public Book get(long bookId) throws BookNotFoundException {
    if (!collection.containsKey(bookId)) {
      throw new BookNotFoundException("Not found book with id: " + bookId);
    }

    // return copy of the book
    return new Book(collection.get(bookId));
  }
@Test
  void testDefaultVersionRemainsZeroAfterAdd() throws BookNotFoundException {
    var book = bookRepository.get(bookId);
    assertEquals(0, book.getVersion());
  }