192. Filtering nested collections with Streams

This is a classical problem in interviews that usually starts from a model, as follows (we assume that the collection is a List):

public class Author {
  private final String name;
  private final List<Book> books;
  ...
}
public class Book {
    
  private final String title;
  private final LocalDate published;
  ...
}

Having List<Author> denoted as authors, write a stream pipeline that returns the List<Book> published in 2002. You already should recognize this as a typical problem for flatMap(), so without further details, we can write this:

List<Book> book2002fm = authors.stream()
  .flatMap(author -> author.getBooks().stream())
  .filter(book -> book.getPublished().getYear() == 2002)
  .collect(Collectors.toList());

From Problem 185, we know that wherever flatMap() is useful, we should also consider the JDK 16’s mapMulti(). Before checking the following snippet of code, challenge...