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/*
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.
*
*
*/
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