/*
Wotonomy: OpenStep design patterns for pure Java applications.
Copyright (C) 2001 Intersect Software Corporation

This library is free software; you can redistribute it and/or
modify it under the terms of the GNU Lesser General Public
License as published by the Free Software Foundation; either
version 2.1 of the License, or (at your option) any later version.

This library is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
Lesser General Public License for more details.

You should have received a copy of the GNU Lesser General Public
License along with this library; if not, see http://www.gnu.org
*/

package net.wotonomy.control;

import java.io.Serializable;

import net.wotonomy.foundation.NSNull;

/**
 * EONullValue is used to represent null in Collections classes because List and
 * Map do not specify whether null values are allowed and because NSArray and
 * NSDictionary explicitly do not allow null values. <br>
 * <br>
 *
 * Use of the static singleton method nullValue() is required by this
 * implementation because Java cannot return a singleton instance from a
 * constructor. <br>
 * <br>
 *
 * This implementation duplicates NSNull, but the singleton instances are of
 * course different. Be careful. I have no idea why this class was even created,
 * given that NSNull exists.
 *
 * @author michael@mpowers.net
 * @author $Author: cgruber $
 * @version $Revision: 894 $
 */
public class EONullValue implements Serializable {
	private static final EONullValue instance = new EONullValue();

	/**
	 * Create a new instance of EONullValue.
	 */
	private EONullValue() {
	}

	/**
	 * Constructor specifying name and object.
	 */
	public static EONullValue nullValue() {
		return instance;
	}

	/**
	 * Returns a human-readable string representation.
	 */
	public String toString() {
		return "[null]";
	}

	/**
	 * Hashcode of all instances is zero.
	 */
	public int hashCode() {
		return 0;
	}

	/**
	 * Implemented to return true for any instance of EONullValue and for any
	 * instance of NSNull.
	 */
	public boolean equals(Object anObject) {
		if (anObject instanceof EONullValue)
			return true;
		if (anObject instanceof NSNull)
			return true;
		return false;
	}
}

/*
 * $Log$ Revision 1.2 2006/02/16 16:47:14 cgruber Move some classes in to
 * "internal" packages and re-work imports, etc.
 *
 * Also use UnsupportedOperationExceptions where appropriate, instead of
 * WotonomyExceptions.
 *
 * Revision 1.1 2006/02/16 13:19:57 cgruber Check in all sources in
 * eclipse-friendly maven-enabled packages.
 *
 * Revision 1.3 2003/08/06 23:07:52 chochos general code cleanup (mostly,
 * removing unused imports)
 *
 * Revision 1.2 2001/03/01 20:35:38 mpowers Implemented equals and hashCode.
 *
 * Revision 1.1 2001/02/26 22:41:51 mpowers Implemented null placeholder
 * classes. Duplicator now uses NSNull. No longer catching base exception class.
 *
 *
 */