java - Collectors.groupingBy doesn't accept null keys

Question

In Java 8, this works:

Stream<Class> stream = Stream.of(ArrayList.class);
HashMap<Class, List<Class>> map = (HashMap)stream.collect(Collectors.groupingBy(Class::getSuperclass));

But this doesn't:

Stream<Class> stream = Stream.of(List.class);
HashMap<Class, List<Class>> map = (HashMap)stream.collect(Collectors.groupingBy(Class::getSuperclass));

Maps allows a null key, and List.class.getSuperclass() returns null. But Collectors.groupingBy emits a NPE, at Collectors.java, line 907:

K key = Objects.requireNonNull(classifier.apply(t), "element cannot be mapped to a null key"); 

It works if I create my own collector, with this line changed to:

K key = classifier.apply(t);  

My questions are:

1) The Javadoc of Collectors.groupingBy doesn't say it shouldn't map a null key. Is this behavior necessary for some reason?

2) Is there another, easier way, to accept a null key, without having to create my own collector?

Answer 1

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())));