package lrs;

/**
 * Represents the empty state. Singleton pattern.
 * @author Dung X. Nguyen  Copyright 2000 - All rights reserved.
 * @since 02/15/00
*/
class NullNode extends ANode
{
    /**
     * Singleton Pattern.
     */
    final static NullNode singleton = new NullNode();

    private NullNode()
    {
    }

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

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

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

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

    void insertFront(Object dat, LRStruct owner)
    {
	owner.setHead(new NonNullNode(dat));
    }

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

    /**
     * Calls the visitor's null case.
     */
    Object execute(IAlgo visitor, Object input, LRStruct owner)
    {
	return visitor.nullCase(owner, input);
    }
}