[英]Java stream collect to map with multiple keys
I thought I was getting pretty good at Java 8 streams, but then...我以为我已经很擅长 Java 8 流了,但后来……
I have a Foo
interface:我有一个Foo
接口:
public interface Foo {
String getKey();
Stream<Bar> bars();
}
I know I can collect a Stream<Foo>
into a Map<String, Foo>
using the key of each:我知道我可以使用每个键将Stream<Foo>
收集到Map<String, Foo>
中:
Map<String, Foo> foosByKey = fooStream.collect(
Collectors.toMap(Foo::getKey, Function.identity()));
But what if I want to collect them into a Map<Bar, Foo>
?但是如果我想将它们收集到Map<Bar, Foo>
怎么办? In other words, for each Foo
in the steam, I want to put that Foo
in the map keyed to every one of the Bar
instances returned by Foo.bars()
.换句话说,对于 Steam 中的每个Foo
,我想将该Foo
放入映射到每个由Foo.bars()
返回的Bar
实例的Foo.bars()
。 Where do I start?我从哪里开始?
As suggested here , you'll want extract the Bar
values from each Foo
and create pairs of them.正如此处所建议的,您需要从每个Foo
提取Bar
值并创建它们对。 Once you have the pairs, you can collect them into a Map
.获得对后,您可以将它们收集到Map
。 For example,例如,
Map<Bar, Foo> map = fooStream.flatMap(foo -> foo.bars().map(bar -> new SimpleEntry<>(bar, foo)))
.collect(Collectors.toMap(Entry::getKey, Entry::getValue));
We use SimpleEntry
here because it's available (Java doesn't have a simpler Pair
type).我们在这里使用SimpleEntry
是因为它可用(Java 没有更简单的Pair
类型)。 You could write your own to be more specific.你可以自己写一个更具体的。
You could define a new collector for that.您可以为此定义一个新的收集器。 One simple implementation (that always creates HashMap's of ArrayList's; no downstream support) could be:一个简单的实现(总是创建 ArrayList 的 HashMap;没有下游支持)可以是:
public static <T, K>
Collector<T, ?, Map<K, List<T>>> multiGroupingBy(
Function<? super T, Collection<? extends K>> multiClassifier) {
return Collector.of(
HashMap::new,
(map, entry) -> {
multiClassifier.apply(entry)
.forEach(
key -> map
.computeIfAbsent(key,
__ -> new ArrayList<>())
.add(entry));
},
(map1, map2) -> {
map2.forEach(
(key, list) -> map1
.computeIfAbsent(key,
__ -> new ArrayList<>())
.addAll(list));
return map1;
});
}
Then you could call:然后你可以调用:
fooStream.collect(multiGroupingBy(Foo::bars));
声明:本站的技术帖子网页,遵循CC BY-SA 4.0协议,如果您需要转载,请注明本站网址或者原文地址。任何问题请咨询:yoyou2525@163.com.