/**
 * ListOfObjects  class, patterned after ListOfInts
 *
 *  You do not need to modify or turn in this file
 *
 * @author Robert A. Amar, Ron K. Cytron
 * @version 2.0
 */

public class ListOfObjects {
   ListItem marker; // the current item is the one after the marker
   ListItem head;   // always refers to the dummy
   ListItem rear;   // always refers to the last item in the list
 
   public ListOfObjects() {
      head = rear = marker = new ListItem(null, null);  // create dummy
   }

   public boolean isEmpty() {
      return head == rear;
   }

   public ListOfObjects prepend(Object n) {
      ListItem newHead = new ListItem(n, head.getNext());
      head.setNext(newHead);
      if (isEmpty())  // check if this is also the last item
         rear = newHead;
      return this;
   }

   public ListOfObjects append(Object n) {
      ListItem newRear = new ListItem(n, null);
      rear.setNext(newRear);
      rear = newRear;
      return this;
   }

   public boolean atEnd() {
      return marker.getNext() == null;
   }

   public void reset() {
      marker = head;
   }

   public ListOfObjects next() {
      if (!atEnd())
         marker = marker.getNext();
      else 
         throw new Error("Attempt to advance beyond end of list.");
      return this;
   }

   public Object getItem() {  // gets current item
      if (isEmpty()) 
         throw new Error("Attempt to get an item from an empty list.");
      else if (atEnd())
         throw new Error("Attempt to get an item beyond end of list.");
      else return marker.getNext().getItem();
   }

   public void setItem(Object n) {  // replaces value of current item
      if (isEmpty()) 
         throw new Error("Attempt to set an item in an empty list.");
      else if (atEnd())
         throw new Error("Attempt to set an item beyond end of list.");
      else marker.getNext().setItem(n);
   }

   public void insert(Object n) {  // inserts n just before the current item, and
                                 // the new item n becomes the current item
      if (atEnd())
         append(n);
      else 
         marker.setNext(new ListItem(n, marker.getNext()));
   }

   public Object delete() {  // deletes the current item
      if (atEnd()) {
         throw new Error("Tried to delete item past end of list!");
      }
      else {
         Object result = getItem();
         if (marker.getNext() == rear)
           rear = marker;
         marker.setNext(marker.getNext().getNext());
         return result;
      }
   }

   public String toString() {
      if (isEmpty())
         return "( )";
      else {
         ListItem first = head.getNext();
         return "(" + first.toString() + " )";
      }
   }
}