Collectors.groupingBy doesn’t accept null keys

I had the same kind of problem.
This failed, because groupingBy performs Objects.requireNonNull on the value returned from the classifier:

    Map<Long, List<ClaimEvent>> map = events.stream()
      .filter(event -> eventTypeIds.contains(event.getClaimEventTypeId()))
      .collect(groupingBy(ClaimEvent::getSubprocessId));

Using Optional, this works:

    Map<Optional<Long>, List<ClaimEvent>> map = events.stream()
      .filter(event -> eventTypeIds.contains(event.getClaimEventTypeId()))
      .collect(groupingBy(event -> Optional.ofNullable(event.getSubprocessId())));