In Java8 functional style, how can i map the values to already existing key value pair

Question

I have a map:

Map> dataMap;

Now i want to add new key value pairs to the map like below:

i         


        

Answer 1

you can use

dataMap.compute(key,(k,v)->v!=null?v:new ArrayList<>()).add(someNewObject)

or

dataMap.merge(key,new ArrayList<>(),(v1,v2)->v1!=null?v1:v2).add(someNewObject)