/**
 * Represents the empty list.  Uses the Singleton pattern to model the
 * uniqueness of the empty list.
 * @author Dung X. Nguyen
 * @version 1.1
 * @since 01/25/02
 * @Custom Copyright 2002 -All rights reserved
 */
public class EmptyList extends AList {

    /**
     * Singleton pattern.
     */
    public final static EmptyList Singleton = new EmptyList();

    /**
     * Disables public instantiation of EmptyList.
     */
    private EmptyList() {
    }

    /**
     * Throws an IllegalArgumentException.
     * @return does not return.
     * @exception IllegalArgumentException.
     */
    public Object getFirst() {
        throw new IllegalArgumentException ("EmptyList has no first!");
    }

    /**
     * Throws an IllegalArgumentException
     * @return does not return.
     * @exception IllegalArgumentException.
     */
    public AList getRest() {
        throw new IllegalArgumentException ("EmptyList has no rest!");
    }

    /**
     * Computes a String representation of this EmptyList.
     * @return "()".
     */
    public String toString() {
        return "()";
    }

    /**
    * Computes the length of this EmptyList.
    * @return 0
    */
    public int getLength() {
        return 0;
    }
}