org.apache.commons.lang.ObjectUtils.Null

 public static class ObjectUtils.Null implements java.io.Serializable 

Class used as a null placeholder where null has another meaning.

For example, in a HashMap the HashMap.get(java.lang.Object) method returns null if the Map contains null or if there is no matching key. The Null placeholder can be used to distinguish between these two cases.

Another example is Hashtable, where null cannot be stored.

See Also: Serialized Form