How to group a stream to a map by using a specific key and value?

Question

I have a Stream<Pair<String, String>> myStream; and I'd like to aggregate it into a Map<String, Set<String>> result;

I managed to get to the following:

Map<String, Set<Pair<String, String>>> result = myStream
  .collect(Collectors.groupingBy(Pair::getKey, Collectors.toSet()));

This fails with "Non-static method cannot be referenced from a static contex":

Map<String, Set<String>> result = myStream
  .collect(Collectors.groupingBy(Pair::getKey, Pair::getValue, Collectors.toSet()));

What am I doing wrong?

Answer 1

Correct code is:

Map<String, Set<String>> result = myStream
  .collect(Collectors.groupingBy(Pair::getKey,
              Collectors.mapping(Pair::getValue, Collectors.toSet())));

If you use import static, it is shortened to:

Map<String, Set<String>> result = myStream
  .collect(groupingBy(Pair::getKey, mapping(Pair::getValue, toSet())));