package edu.rice.cs.dxn.QFList;

/**
 * null list node. singleton pattern.
*/
class NullNode implements IState
{
   final static NullNode singleton = new NullNode();
   private NullNode()
   {
   }

   /**
	 * @param owner
	 * @return null
	 */
	public QFList getRest(QFList owner)
	{
	   throw new java.util.NoSuchElementException ("Empty list has no first.");
   }

	/**
	 * @param owner
	 * @return null
	 */
	public Object getFirst(QFList owner)
	{
	   throw new java.util.NoSuchElementException ("Empty list has no first.");
   }

	/**
	 * @param tail
	 * @param owner
	 */
	public void setRest(QFList tail, QFList owner)
	{
	   throw new java.util.NoSuchElementException ("Empty list has no tail.");
   }

	/**
	 * @param dat
	 * @param owner
	 */
	public void setFirst(Object dat, QFList owner)
	{
	   throw new java.util.NoSuchElementException ("Empty list has no first.");
   }

	/**
	 * @param dat
	 * @param owner
	 */
	public void insertFront(Object dat, QFList owner)
	{
	   owner.setHead(new NonNullNode(dat));
   }

	/**
	 * @param owner
    * @return
   */
	public Object removeFront(QFList owner)
	{
	   throw new java.util.NoSuchElementException ("Empty list has no front.");
   }

   public Object execute(IAlgo visitor, Object param, QFList owner)
   {
      return visitor.nullCase(owner, param);
   }
}