package list;

/**
 * Manufactures AList objects.
 * Implemented as a singleton.
 * 
 * @author Dung X. Nguyen
 * @version 1.0
 * @since 09/17/00
 * @Custom Copyright 2000 -All rights reserved
 */
public class ListFactory
{
    public final static ListFactory Singleton = new ListFactory ();

    private ListFactory()
    {
    }
    
    public AList makeEmptyList()
    {
	return EmptyList.Singleton;
    }

    /**
     * Creates a non-empty list with a given non-null first element and
     * a non-null rest.
     * Throws an IllegalArgumentException if any of the parameters is null.
     * 
     * @param first != null, the first data element of the non-empty list.
     * @param rest != null, the rest of this non-empty list.
     */
    public AList makeNEList(Object first, AList rest)
    {
	if (first == null || rest == null) {
	    throw new IllegalArgumentException ("first and rest must be non-null.");
	}
	return new NEList (first, rest);
    }
}