package lrs;

/**
 * Represents the empty state. Singleton pattern.
 * @author Dung X. Nguyen  Copyright 2000 - All rights reserved.
 * @author Alan L. Cox
 * @since 02/13/01
 */
class EmptyNode extends ANode
{
    /**
     * Singleton Pattern.
     */
    final static EmptyNode singleton = new EmptyNode();

    private EmptyNode()
    {
    }

    /**
     * @throw java.util.NoSuchElementException.
     */
    Object getFirst(LRStruct owner)
    {
	throw new java.util.NoSuchElementException("Empty list has no first.");
    }

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

    /**
     * @throw java.util.NoSuchElementException.
     */
    LRStruct getRest(LRStruct owner)
    {
	throw new java.util.NoSuchElementException("Empty list has no first.");
    }

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

    /**
     * Inserts a data object at the front of the LRStruct owner.
     *
     * @param data the object to be inserted at the front.
     * @param owner the LRS referencing this EmptyNode.
     */
    void insertFront(Object data, LRStruct owner)
    {
	owner.setHead(new NonEmptyNode(data));
    }

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

    /**
     * Executes the visitor's empty case and returns the output.
     *
     * @param visitor the algorithm to be executed.
     * @param input the input needed by the algorithm.
     * @param owner the LRStruct referencing this EmptyNode.
     */
    Object execute(IAlgo visitor, Object input, LRStruct owner)
    {
	return visitor.emptyCase(owner, input);
    }
}