1 files changed, 104 insertions, 0 deletions
diff --git a/projects/net.wotonomy.foundation/src/main/java/net/wotonomy/foundation/NSNull.java b/projects/net.wotonomy.foundation/src/main/java/net/wotonomy/foundation/NSNull.java
new file mode 100644
index 0000000..db1f216
--- /dev/null
+++ b/projects/net.wotonomy.foundation/src/main/java/net/wotonomy/foundation/NSNull.java
@@ -0,0 +1,104 @@
+/*
+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.
+ *
+ *
+ */
+