Adding an Iterator to a Collection

Question

Below is my learning objective. I got it started, but I don't really know where to go from here to implement the program in main. I would appreciate any help!

Objective:

• Add an Iterator object to the card collection
• Iterators are added to collections by creating a private inner class.
• You may use any of the inner class types that are appropriate
• Enumerators and Iterators use a large number to determine when the collection changes.

• Implement the correct methods, interfaces, and extend the appropriate classes for a class consistent with the Java API.

  public class CardCollection {
  
  private ArrayList<Card> cards;
  private ArrayList<Note> notes;
  
  public CardCollection() { //constructor initializes the two arraylists
    cards = new ArrayList<Card>();
    notes = new ArrayList<Note>();
   }
  
  private class Card implements Iterable<Card> { //create the inner class
  
      public Iterator<Card> iterator() { //create the Iterator for Card
          return cards.iterator();
      }
  }
  
  private class Note implements Iterable<Note> { //create the inner class
  
      public Iterator<Note> iterator() { //create the Iterator for Note
          return notes.iterator();
      }
  
  }
  
  public Card cards() {
      return new Card();
   }
  
   public Note notes() {
       return new Note();
   }
  
   public void add(Card card) {
       cards.add(card);
   }
  
   public void add(Note note) {
       notes.add(note);
   }
  
  }
  

Answer 1

You have two concepts here that I think you may be mixing up. An object if Iterable if you can iterate over some internal elements.

So if I have a shopping cart with items in it, I can iterate over my groceries.

public class ShoppingCart implements Iterable<GroceryItem>
{
   public Iterator<GroceryItem> iterator()
   {
      // return an iterator
   }
}

So in order to use this functionality, I need to provide an Iterator. In your code example, you are reusing the iterators from ArrayList. From your exercise description, I believe you need to implement one yourself. For example:

public class GroceryIterator implements Iterator<GroceryItem>
{
  private GroceryItem[] items;
  private int currentElement = 0;

  public GroceryIterator(GroceryItem[] items)
  {
    this.items = items;
  }

  public GroceryItem next() // implement this
  public void remove() // implement this
  public boolean hasNext() // implement this
}

So I sorta gave you a hint with the constructor/member variables. After you make this class, your Iterable class (my ShoppingCart) will return my new iterator.

The assignment recommends using a private inner class for your custom Iterator.

Good luck

Answer 2

• Iterable objects are usually collections. That suits better with CardCollection than with Card
• Public methods cards() and notes() are returning types Card and Note which are private. I think those are meant to be public.
• I think methods cards() and notes() are meant to return the iterator.