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

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> stream = Stream.of(ArrayList.class);

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

And use it like this:

Stream> stream = Stream.of(ArrayList.class);
Map, List>> 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.