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())));

For the first question, I agree with skiwi that it shouldn't be throwing a NPE. I hope they will change that (or else at least add it to the javadoc). Meanwhile, to answer the second question I decided to use Collectors.toMap instead of Collectors.groupingBy:

Stream<Class<?>> stream = Stream.of(ArrayList.class);

Map<Class<?>, List<Class<?>>> map = stream.collect(
    Collectors.toMap(
        Class::getSuperclass,
        Collections::singletonList,
        (List<Class<?>> oldList, List<Class<?>> newEl) -> {
        List<Class<?>> newList = new ArrayList<>(oldList.size() + 1);
        newList.addAll(oldList);
        newList.addAll(newEl);
        return newList;
        }));

Or, encapsulating it:

/** Like Collectors.groupingBy, but accepts null keys. */
public static <T, A> Collector<T, ?, Map<A, List<T>>>
groupingBy_WithNullKeys(Function<? super T, ? extends A> classifier) {
    return Collectors.toMap(
        classifier,
        Collections::singletonList,
        (List<T> oldList, List<T> newEl) -> {
            List<T> newList = new ArrayList<>(oldList.size() + 1);
            newList.addAll(oldList);
            newList.addAll(newEl);
            return newList;
            });
    }

And use it like this:

Stream<Class<?>> stream = Stream.of(ArrayList.class);
Map<Class<?>, List<Class<?>>> map = stream.collect(groupingBy_WithNullKeys(Class::getSuperclass));

Please note rolfl gave another, more complicated answer, which allows you to specify your own Map and List supplier. I haven't tested it.

Use filter before groupingBy##

Filter out the null instances before groupingBy.

MyObjectlist.stream()
  .filter(p -> p.getSomeInstance() != null)
  .collect(Collectors.groupingBy(MyObject::getSomeInstance));

To your 1st question, from the docs:

There are no guarantees on the type, mutability, serializability, or thread-safety of the Map or List objects returned.

Because not all Map implementations allow null keys they probably added this to reduce to the most common allowable definition of a map to get maximum flexibility when choosing a type.

To your 2nd question, you just need a supplier, wouldn't a lambda work? I'm still getting acquainted with Java 8, maybe a smarter person can add a better answer.