import java.io.*;

public class LinkedList implements Serializable
{
  // Default constructor - creates an empty list
  public LinkedList() 
  {}

  // Constructor to create a list containing one object
  public LinkedList(Object item)
  {
    if(item != null)
      current = end = start = new ListItem(item);    // item is the start and end
  }

  // Construct a linked list from an array of objects
  public LinkedList(Object[] items)
  {
    if(items != null)
    {
      // Add the items to the list
      for(int i = 0; i < items.length; i++)
        addItem(items[i]);
      current = start;
    }
  }

  // Add an item object to the list
  public void addItem(Object item)
  {
    ListItem newEnd = new ListItem(item);   // Create a new ListItem
    if(start == null)                       // Is the list empty?
      start = end = newEnd;                 // Yes, so new element is start and end
    else
    {                                       // No, so append new element
      end.next = newEnd;                    // Set next variable for old end
      end = newEnd;                         // Store new item as end 
    }
  }

  // Get the first object in the list
  public Object getFirst()
  {
    current = start;
    return start == null ? null : start.item;
  }

  // Get the next object in the list
  public Object getNext()
  {
    if(current != null)
      current = current.next;        // Get the reference to the next item
    return current == null ? null : current.item;
  }

  private ListItem start = null;         // First ListIem in the list
  private ListItem end = null;           // Last ListIem in the list
  private ListItem current = null;       // The current item for iterating

  private class ListItem implements Serializable
  {
    // Constructor 
    public ListItem(Object item)
    {
      this.item = item;        // Store the item
      next = null;             // Set next as end point
    }

    // Return class name & object
    public String toString()
    {
      return "ListItem " + item ;
    }

    ListItem next;             // Refers to next item in the list
    Object item;  
  }
}