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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 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.
*
*
*/
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