// 1

    private Map<String, List<Book>> createCategoryToBooksListMap(List<Book> allBooks) {
        return allBooks
                .map(toCategoriesAndBooksList())
                .flatMap(fromListOfList())
                .groupBy(category())
                .mapValues(toBooksList());
    }

    private Function<Book, List<Tuple2<String, Book>>> toCategoriesAndBooksList() {
        return book -> List.of(book.getCategories()).map(category -> Tuple.of(category, book));
    }

    private Function<List<Tuple2<String, Book>>, Iterable<? extends Tuple2<String, Book>>> fromListOfList() {
        return list->list;
    }

    private Function<Tuple2<String, Book>, String> category() {
        return t -> t._1;
    }

    private Function<List<Tuple2<String, Book>>, List<Book>> toBooksList() {
        return v->v.map(t->t._2);
    }


// 2
    private Map<String, List<Book>> createCategoryToBooksListMap(List<Book> allBooks) {
        return allBooks
                .map(this::toCategoriesAndBooksList)
                .flatMap(this::fromListOfLists)
                .groupBy(this::category)
                .mapValues(this::toBooksList);
    }

    private List<Tuple2<String, Book>> toCategoriesAndBooksList(Book book) {
        return List.of(book.getCategories()).map(category -> Tuple.of(category, book));
    }

    private List<Tuple2<String, Book>> fromListOfLists(List<Tuple2<String, Book>> list) {
        return list;
    }

    private String category(Tuple2<String, Book> t) {
        return t._1;
    }

    private List<Book> toBooksList(List<Tuple2<String, Book>> v) {
        return v.map(t->t._2);
    }