import java.io.Serializable;

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 ListItem in the list
  private ListItem end = null;                                         // Last ListItem in the list
  private ListItem current = null;                                     // The current item for iterating
  private static final long serialVersionUID = 7001L;

  // ListItem as a nested class
  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
    @Override
    public String toString() {
      return "ListItem " + item ;
    }

    ListItem next;                                                     // Refers to next item in the list
    Object item;                                                       // The item for this ListItem
    private static final long serialVersionUID = 7001L;
  }
}