Returns the value for the given key or null if key is not in the map.
Returns the value for the given key or null if key is not in the map.
Some maps allows keys to have null
as a value,
For those maps, a lookup using this operator does cannot be used to
distinguish between a key not being in the map, and the key having a null
value.
Methods like containsKey or putIfAbsent can be use if the distinction
is important.