package list;
/**
 * Represents the empty list.
 * Implemented as a singleton.
 * 
 * @author Dung X. Nguyen
 * @version 1.0
 * @since 09/10/00
 * @Custom Copyright 2000 -All rights reserved
 */

class EmptyList extends AList
{
    public final static EmptyList Singleton = new EmptyList ();

    /**
     * NOTE: The constructor is private so that no client can
     * instantiate an EmptyList.  I.e., there is one "true" empty list,
     * Singleton, and every list uses it.
     */
    private EmptyList()
    {
    }
   
    /**
     * Throws an IllegalArgumentException.
     * @return does not return.
     */
    public Object getFirst()
    {
	throw new java.util.NoSuchElementException ("Empty has no element.");
    }

    /**
     * Throws an IllegalArgumentException.
     * @return does not return.
     */
    public AList getRest()
    {
	throw new java.util.NoSuchElementException ("Empty has no tail.");
    }

    /**
     * @param input
     */
    public Object execute(IListAlgo algo, Object input)
    {
	return algo.forEmpty (this, input);
    }
}