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
{
    /** @SBGen Variable (singleton pattern,,,0)   */
    public final static EmptyList Singleton = new EmptyList ();

    /*
     * NOTE: the constructor is private so that no client can
     * instantiate an EmptyList.
     */
    private EmptyList ()
    {
    }
   
    /**
     * Throws an IllegalArgumentException.
     * @return does not return.
     */
    public Object getFirst ()
    {
	throw new IllegalArgumentException ("Empty List has no data.");
    }

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

    public String toString()
    {
	return "()";
    }

    /**
     * Terminates the String representation of the original AList
     * by returning ")".
     */
    protected String toStringHelp()
    {
	return ")"; 
    }

    /**
     * Returns 0.
     */
    public int length()
    {
	return 0;
    }

    /**
     * Returns 0.
     */
    public int getLength()
    {
	return 0; 
    }

    /**
     * Returns the accumulated length, since this is the end of the list.
     */
    protected int getLengthHelp(int acc)
    {
	return acc; 
    }

    /**
     * Returns the empty AList itself since the clone of the empty list
     * is the empty list.
     */
    public AList makeClone()
    {
	return this;
    }

    /**
     * Throws an IllegalArgumentException.
     */
    public Object nthElement(int n)
    {
	throw new IllegalArgumentException
	    ("nthElement: Index out of range.");
    }

    /**
     * Throws an IllegalArgumentException.
     */
    public Object lastElement()
    {
	throw new IllegalArgumentException
	    ("lastElement: Empty list has no element.");
    }

    /**
     * Returns the input element that is being passed down,
     * since this is the end of the original list.
     * @return previous
     */
    protected Object lastElementHelp(Object previous)
    {
	return previous;
    }

    /**
     * Returns the empty list itself since this is the longest list
     * we can return.
     */
    public AList firstElements(int num_elts)
    {
	if (num_elts < 0) {
	    throw new IllegalArgumentException
		("firstElements: Meaningless to ask for a negative number (" +
		 num_elts +
		 ") of elements.");
	}
	if (num_elts > 0) {
	    throw new IllegalArgumentException
		("firstElements: Empty list has no elements.");
	}
	return this;
    }

    /**
     * Returns the empty AList itself since the reverse of the empty list
     * is the empty list.
     */
    public AList reverse()
    {
	return this;
    }

    /**
     * Returns the reversed list so far, since this is the end of the
     * original list.
     * @return stack
     */
    protected AList reverseHelp(AList stack)
    {
	return stack;
    }

    /**
     * Returns the list to be concatenated with.
     */
    public AList append(AList list2)
    {
	return list2;
    }
}