I’m trying to turn Map<String, A> into Map<String, B> with a stream. I have a function that turns A into B, namely A.fromB().

This code is not working:

Map<String, A> aMap = some map

final Map<String, B> bMap =
    aMap.keySet().stream()
        .collect(Collectors.toMap(aKey -> aKey,
            aKey -> aMap.get(aKey).stream()
                                  .map(B::fromA)
                                  .collect(Collectors.toList())));

I just got the following error from my IDE:

Required type: Map<String,B>
Provided: Map<Object,Object>
no instance(s) of type variable(s) T exist so that List conforms to B inference variable U has incompatible bounds: equality constraints: B lower bounds: List<T4192099>

What is the correct way to do this?