putFirst
Inserts the given mapping into the map if it is not already present, or replaces the value of a mapping if it is already present (optional operation). After this operation completes normally, the given mapping will be present in this map, and it will be the first mapping in this map's encounter order.
Return
the value previously associated with k, or null if none
Parameters
k
the key
v
the value
Throws
if this collection implementation does not support this operation