package schemeFW;

/**
 * Represents the empty list.
 * Implemented as a singleton, using the singleton pattern.
 * @author Dung X. Nguyen
 * @author Stephen B. Wong
 * @version 1.1
 * @since 09/10/01
 * @Custom Copyright 2001 -All rights reserved
 */
public class EmptyList implements IEmptyList {
    public final static EmptyList Singleton = new EmptyList ();

    private EmptyList() {
        // NOTE: the constructor is private so that no external client can
        // instantiate an EmptyList.
    }

    /**
     * Calls the <code>emptyCase()</code> method of the supplied IListAlgo
     * with the supplied input parameter.
     * @param algo The visitor algorithm to be executed.
     * @param inp input to be handed to the algo's <code>emptyCase()</code>.
     * @return <code>Object</code> output of algo's <code>emptyCase()</code>.
     */
    public Object execute(IListAlgo algo, Object inp) {
        return algo.emptyCase(this, inp);
    }
}