Associates the specified value with the specified key in this map. If the map previously contained a mapping for this key, the old value is replaced.

Parameters:

key		the key with which the specified value is to be associated
value		the value to be associated with the specified key

Returns:  the previous value associated with specified key, or null if there was no mapping for key. (A null return can also indicate that the map previously associated null with the specified key.)

Exceptions:

NullPointerException

if the specified key is null