replace
ported from java.util.Map.replace()
Replaces the entry for the specified key only if it is currently mapped to some value.
Return
the previous value associated with the specified key, or null if there was no mapping for the key. (A null return can also indicate that the map previously associated null with the key, if the implementation supports null values.)
Parameters
key
key with which the specified value is associated
value
value to be associated with the specified key