Null interface is another name for marker interface. (I personally prefer marker)
Such an interface is empty, i.e. without any method declarations. Serializable interface is a classic example.
When any object implements Serializable, the JVM knows that it is supposed to treat it in a special way.
Google() for more information. This is a much discussed topic.
null interface is an interface that does not contain any method but is used to perform a special task. For e.g. Serializable interface, which does not have any method. But if we wish to serialize any class it must implement Serializable interface.