How create a new map from the values in an existing map

Question

Having the next original map:

G1=[7,8,45,6,9]
G2=[3,9,34,2,1,65]
G3=[6,5,9,1,67,5]

Where G1, G2 and G3 are groups of people's ages, How can I create a new map like this:

45=[7,8,45,6,9]
65=[3,9,34,2,1,65]
67=[6,5,9,1,67,5]

Where the new keys are the max people's age in each group.

I have tried this:

Map<Integer, List<Integer>> newMap = originalMap.entrySet().stream()
                .collect(Collectors.toMap(Collections.max(x -> x.getValue()), x -> x.getValue()));

But the compiler say me: "The target type of this expression must be a functional interface" in this fragment of code:

Collections.max(x -> x.getValue())

Any help with this will be appreciated.

Answer 1

toMap consumes function for it's keyMapper and valueMapper. You're doing this correctly for the valueMapper in your code but not for the keyMapper thus you need to include the keyMapper function as follows:

originalMap.entrySet()
           .stream()
           .collect(toMap(e -> Collections.max(e.getValue()), Map.Entry::getValue));

note the e -> Collections.max(e.getValue()).

Further, since you're not working with the map keys, you can avoid having to call entrySet() and instead work on the map values:

originalMap.values()
           .stream()
           .collect(Collectors.toMap(Collections::max, Function.identity()));