/**
  @author Adriano Luchetta
  @version 3-Dic-2004
  @version 21-Nov-2005
*/

import java.util.NoSuchElementException;

public class ListaSuCatena implements List
{
  // variabili di istanza
  private ListNode head, tail;

  // costruttore
  public ListaSuCatena()
  {
    makeEmpty();
  }

  public void makeEmpty()
  {
    head = tail = new ListNode();
  }

  public boolean isEmpty()
  {
    return (head == tail);
  }
  
  public int size()
  {
    int count = 0;
    ListNode tmp = head;

    while (tmp != tail)
    {
      count++;
      tmp = tmp.next;
    }
    
    return count;
  }

  public ListIterator getIterator()
  {
    return new LinkedListIterator(head);
  }

  private class LinkedListIterator implements ListIterator
  {

    private ListNode current; // nodo che precede la posizione attuale (non è mai null)
    private ListNode previous;

    public LinkedListIterator(ListNode h)
    {
      current = h;
      previous = null;
    }

    public boolean hasNext()
    {
      previous = null;
      return current.getNext() != null;
    }

    public Object next() throws NoSuchElementException
    {
      if (!hasNext())
        throw new NoSuchElementException();

      previous = current;
      current = current.getNext();
      return current.getElement();
    }

    public void add(Object obj)
    {
      ListNode n = new ListNode(obj, current.getNext());
      current.setNext(n);
      previous = null;
    }

    public void remove() throws IllegalStateException
    {
      if (previous == null)
        throw new IllegalStateException();

      previous.setNext(current.getNext());
      current = previous;
      previous = null;
    }
  }

  private class ListNode
  {
    private Object element;
    private ListNode next;

    public ListNode(Object e, ListNode n)
    {
      element = e;
      next = n;
    }

    public ListNode()
    {
      element = null; next = null;
    }

    public Object getElement()
    {
        return element;
    }

    public ListNode getNext()
    {
        return next;
    }

    public void setElement(Object e)
    {
        element = e;
    }

    public void setNext(ListNode n)
    {
        next = n;
    }
  }
}