title: Hashcode for NULL key in HashMap
body: <p>I was just reading about the difference between HashMap and HashTable class in java. There I found a difference that former allow null key and later doesn't privileges for the same.
  As far as the working of HashMap is concern I know that, it calls hashcode method on key for finding the bucket in which that key value pair is to be placed. Here comes my question:
  How hashcode for a null value is computed or Is there any default value for hashcode of null key (if so please specify the value)?</p>