Iterator Method

It provides a way to access the elements of an aggregate object sequentially without exposing its underlying representation. This pattern allows for traversing elements of a collection (like lists, sets, etc.) without needing to understand the details of the collection’s implementation.

Structure

• Iterator Interface: Defines the methods for traversing elements.
• Concrete Iterator: Implements the iterator interface to iterate over the collection.
• Aggregate Interface: Defines a method to create an iterator.
• Concrete Aggregate: Implements the aggregate interface to provide the appropriate iterator.
• Client: Uses the iterator to traverse the collection.

Example

// Iterator Interface
interface Iterator {
    boolean hasNext();
    Object next();
}

// Aggregate Interface
interface BookCollection {
    Iterator createIterator();
}

// Concrete Iterator for BookCollection
class BookIterator implements Iterator {
    private Book[] books;
    private int position;

    public BookIterator(Book[] books) {
        this.books = books;
        this.position = 0;
    }

    @Override
    public boolean hasNext() {
        return position < books.length && books[position] != null;
    }

    @Override
    public Object next() {
        return hasNext() ? books[position++] : null;
    }
}

// Concrete Aggregate
class Library implements BookCollection {
    private Book[] books;
    private int count;

    public Library(int size) {
        books = new Book[size];
        count = 0;
    }

    public void addBook(Book book) {
        if (count < books.length) {
            books[count++] = book;
        } else {
            System.out.println("Library is full. Cannot add more books.");
        }
    }

    @Override
    public Iterator createIterator() {
        return new BookIterator(books);
    }
}

// Book class (the element)
class Book {
    private String title;

    public Book(String title) {
        this.title = title;
    }

    public String getTitle() {
        return title;
    }
}

// Client code
public class Main {
    public static void main(String[] args) {
        Library library = new Library(5);
        library.addBook(new Book("1984"));
        library.addBook(new Book("To Kill a Mockingbird"));
        library.addBook(new Book("The Great Gatsby"));

        // Create an iterator for the library
        Iterator iterator = library.createIterator();

        // Traverse the collection
        while (iterator.hasNext()) {
            Book book = (Book) iterator.next();
            System.out.println("Book Title: " + book.getTitle());
        }
    }
}