问题描述

在 Java 8 中，这是可行的:

In Java 8, this works:

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

但这不是:

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

Maps 允许使用 null 键，并且 List.class.getSuperclass() 返回 null.但是 Collectors.groupingBy 在 Collectors.java 第 907 行发出 NPE:

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"); 

如果我创建自己的收集器，它会起作用，将这一行更改为:

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

K key = classifier.apply(t);  

我的问题是:

1) Collectors.groupingBy 的 Javadoc 并没有说它不应该映射空键.出于某种原因，这种行为是否必要?

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

2) 是否有另一种更简单的方法来接受空键，而不必创建自己的收集器?

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

推荐答案

对于第一个问题，我同意skiwi 的观点，它不应该抛出NPE.我希望他们会改变这一点(或者至少将它添加到 javadoc 中).同时，为了回答第二个问题，我决定使用 Collectors.toMap 而不是 Collectors.groupingBy:

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;
        }));

或者，封装它:

/** 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;
            });
    }

并像这样使用它:

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

请注意 rolfl 给出了另一个更复杂的答案，它允许您指定自己的地图和列表供应商.我没有测试过.

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

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