@Override
  public GetBookResult execute(GetBookCommand command) {
    validateCommand(command);
    Book book = jpacrudOperationDAO.getById(Book.class, command.getBookId());

    if (book == null || !book.getBookId().equals(command.getBookId())) {
      throw new RuntimeException("Book id not valid");
    }

    return new GetBookResult(book);
  }
 private void validateCommand(GetBookCommand command) {
   checkNotNull(command, "GetBookCommand must not be null");
   checkNotNull(command.getBookId(), "Book id must not be null");
 }