Iteratorパターン - Qiita

public class BookShelf implements Aggregate { private List<Book> _books = new ArrayList<Book>(); public void addBook(Book book) { _books.add(book); } public Book getBookAt(int idx) throws IndexOutOfBoundsException { return _books.get(idx); } public void removeBookAt(int idx) throws IndexOutOfBoundsException { _books.remove(idx); } public int getNumberOfBooks() { return _books.size(); } @Override p