Java streams: transforming map's values

Question

Having an instance of a Map<A, Set<B>> where B is a POJO with property Integer price (among others), how do I elegantly transform this map into a Map<A, Integer> where Integer denotes the sum of prices of a given set, for each key A?

Answer 1

You can use stream like this:

Map<A, Integer> result = map.entrySet().stream()
        .collect(Collectors.toMap(
                Map.Entry::getKey, 
                b -> b.getValue().stream()
                        .mapToInt(B::getPrice)
                        .sum()
                ));