Find the difference between two collections in Java 8?

Question

I am trying to make a List of all of the books in one Collection that are not present in another. My problem is that I need to compare based on book ID

Answer 1

List books1 = ...;
List books2 = ...;
Set ids = books2.stream()
        .map(Book::getId)
        .collect(Collectors.toSet());
List parentBooks = books1.stream()
        .filter(book -> !ids.contains(book.getId()))
        .collect(Collectors.toList());