### Question:
InMemoryBookService implements BookService { @Override public Optional<Book> find(String isbn) { return Optional.ofNullable(books.get(isbn)); }  @Override Iterable<Book> findAll(); @Override Book create(Book book); @Override Optional<Book> find(String isbn);  }### Answer:
@Test public void shouldReturnEmptyOptionalWhenNotFound() { assertThat(service.find("1234")).isEmpty(); }