public static class ObjectUtils.Null extends Object implements 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.
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