If the specified key is not already associated with a value in this map, attempts to compute its value using the given mapping function and enters it into this map unless null.
map from which to get the key's value or compute and associate
key for which to get the value or with which the computed value is to be associated
function to compute a value for key
If the specified key is not already associated with a value in this map, attempts to compute its value using the given mapping function and enters it into this map unless null.