Collectors.groupingBy不接受空键 [英] Collectors.groupingBy doesn't accept null keys

问题描述

在Java 8中，这是可行的：

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

但这不是：

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

地图允许空键，而List.class.getSuperclass（）返回空。但Collectors.groupingBy在Collectors.java第907行发出NPE：

  K key = Objects.requireNonNull（classifier.apply （t），元素不能映射到空键）; 
  

如果我创建了自己的收藏夹，此行更改为：

  K key = classifier.apply（t）; 
  

我的问题是：

1） Collectors.groupingBy的Javadoc没有说它不应该映射一个空键。这种行为是否有必要出于某种原因？

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藏夹？

解决方案

对于第一个问题，我同意skiwi，它不应该抛出 NPE 。我希望他们会改变这种情况（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 ：

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

或者封装它：

  / **与Collectors.groupingBy类似，但接受空键。 * / 
 public static< T，A>收集器< T，？，Map< A，List< T>>> 
 groupingBy_WithNullKeys（函数< ;? super T，？extends A>分类器）{
返回Collectors.toMap（
分类符，
 Collections :: singletonList，
（List< （oldList），oldList（newList），newList（newList），newListList（newListList）。 
 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，它允许您指定您自己的地图和列表供应商。我没有测试过它。

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?

解决方案

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.

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