Collectors.groupingBy doesn't accept null keys
collectors, hashmap, java, java-8, java-stream
Solution
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.
Problem
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?