/*
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.foundation;

import java.io.Serializable;

/**
 * NSNull 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. Even then, more than one instance may exist in the application
 * due to object serialization. Be sure to compare with equals().
 *
 * @author michael@mpowers.net
 * @author $Author: cgruber $
 * @version $Revision: 892 $
 */
public class NSNull implements Serializable {
	private static final long serialVersionUID = 6516669267906970248L;
	private static final NSNull instance = new NSNull();

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

	/**
	 * Returns the static instance of nullValue. Note that serialization may mean
	 * that more than one instance of NSNull exists, so be sure to compare with
	 * equals().
	 */
	public static NSNull nullValue() {
		return instance;
	}

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

	/**
	 * Hash code of all instances is zero.
	 */
	@Override
	public int hashCode() {
		return 0;
	}

	/**
	 * Implemented to return true for any instance of NSNull.
	 */
	@Override
	public boolean equals(Object anObject) {
		return (anObject instanceof NSNull);
	}
}

/*
 * $Log$ Revision 1.1 2006/02/16 12:47:16 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 20:36:09 mpowers Implemented equals, hashcode, and
 * serializable.
 *
 * Revision 1.1 2001/02/26 22:41:51 mpowers Implemented null placeholder
 * classes. Duplicator now uses NSNull. No longer catching base exception class.
 *
 *
 */