public class ArrayIndexedList<T> implements IndexedList<T> {

  private T[] data;

  /* other fields/methods not shown */

  @Override
  public Iterator<T> iterator() {
    return new ArrayIndexedListIterator();
  }

  private class ArrayIndexedListIterator implements Iterator<T> {
    private int nextIndex;

    private ArrayIndexedListIterator() {
      nextIndex = 0;
    }

    @Override
    public boolean hasNext() {
      // you can directly access the private member 'data'
      return nextIndex < data.length;
    }

    @Override
    public T next() {
      if (!hasNext()) {
        throw new NoSuchElementException();
      }
      T t = data[nextIndex];
      nextIndex += 1;
      return t;
    }
  }
}

public class LinkedIndexedList<T> implements IndexedList<T> {

  private Node<T> head;

  /* other fields/methods not shown */

  @Override
  public Iterator<T> iterator() {
    return new LinkedIndexListIterator();
  }

  // Node does not have access to members of LinkedIndexedList
  // because it is static
  private static class Node<T> {
    T data;
    Node<T> next;
  }

  // An iterator to traverse the linked list from front (head) to back.
  private class LinkedIndexListIterator implements Iterator<T> {
    private Node<T> current;

    LinkedIndexListIterator() {
      current = head;
    }

    @Override
    public boolean hasNext() {
      return current != null;
    }

    @Override
    public T next() {
      if (!hasNext()) {
        throw new NoSuchElementException();
      }
      T t = current.data;
      current = current.next;
      return t;
    }
  }
}