Agregate nested list with Stream api

Question

Let's say I have a Shelf class and each Shelf has multiple Books.

public class Shelf{
   private String shelfCode;
   private ArrayList<Book> books; //add getters, setters etc.
}

public class Book{
  private String title;
}

Now, let's say from some method I have a List of Shelfs, each containing some books. How do I use stream to collect all the books to this list?

List<Shelf> shelves = new ArrayList<Shelf>();

Shelf s1 = new Shelf();
s1.add(new Book("book1"));
s1.add(new Book("book2"));

Shelf s2 = new Shelf();
s1.add(new Book("book3"));
s1.add(new Book("book4"));

shelves.add(s1);
shelves.add(s2);

List<Book> booksInLibrary = //??

I'm thinking something like

List<Book> booksInLibrary = 
      shelves.stream()
             .map(s -> s.getBooks())
             .forEach(booksInLibrary.addall(books));

but it doesn't seem to work, throwing a compilation error.

Answer 1

You can use flatMap for this

shelves.stream()
       .flatMap(s -> s.getBooks().stream())
       .collect(Collectors.toList());

The streaming process is quite simple : s -> s.getBooks().stream() makes a stream for each book on each shelf, flatMap flattens everything, and collect(Collectors.toList()) stores the result in a list.