What is the use of adding a null key or value to a HashMap in Java?

Question

HashMap allows one null key and any number of null values. What is the use of it?

Answer 1

The answers so far only consider the worth of have a null key, but the question also asks about any number of null values.

The benefit of storing the value null against a key in a HashMap is the same as in databases, etc - you can record a distinction between having a value that is empty (e.g. string ""), and not having a value at all (null).