Collectors.groupingBy doesn't accept null keys

Question

In Java 8, this works:

Stream stream = Stream.of(ArrayList.class);
HashMap> map = (HashMap)stream.collect(Collect         


        

Answer 1

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

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

Using Optional, this works:

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