/*
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 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.
    */
    public String toString()
    {
        return "[null]";
    }
    
    /**
    * Hashcode of all instances is zero.
    */
    public int hashCode()
    {
        return 0;   
    }
    
    /**
    * Implemented to return true for any instance of NSNull.
    */
    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.
 *
 *
 */