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 also use compute method.

dataMap.compute(key, (k, v) -> {
                              if(v == null)
                                  return new ArrayList<>();
                              else {
                                   v.add(someNewObject);
                                   return v;
                              }
                         });