How to add VALUE value to HASHMAP if it is represented as ArrayList?

Question

Java

Good afternoon, there is a Hashmap>. If the key is contained in the hashmap, you need to add another collection to the current collection. I know only one AddAll method, but since it's boolean, doesn't work.

Answer 1

You need to use the computeIfPresent
method And inside the lambda already calculate a new list using addAll, for example.
And if you also need to add an element, in the case when there is no key, just use compute