将集合流合并为一个集合 - Java 8
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Combine stream of Collections into one Collection - Java 8
提问by Andrew Mairose
So I have a Stream<Collection<Long>>that I obtain by doing a series of transformations on another stream.
所以我有一个Stream<Collection<Long>>我通过在另一个流上进行一系列转换而获得的。
What I need to do is collect the Stream<Collection<Long>>into one Collection<Long>.
我需要做的是将其收集Stream<Collection<Long>>到一个Collection<Long>.
I could collect them all into a list like this:
我可以将它们全部收集到这样的列表中：
<Stream<Collection<Long>> streamOfCollections = /* get the stream */;
List<Collection<Long>> listOfCollections = streamOfCollections.collect(Collectors.toList());
And then I could iterate through that list of collections to combine them into one.
然后我可以遍历该集合列表以将它们合并为一个。
However, I imagine there must be a simple way to combine the stream of collections into one Collection<Long>using a .map()or .collect(). I just can't think of how to do it. Any ideas?
但是，我想一定有一种简单的方法可以Collection<Long>使用 a .map()or将集合流合并为一个.collect()。我只是想不出该怎么做。有任何想法吗？
回答by rgettman
This functionality can be achieved with a call to the flatMapmethodon the stream, which takes a Functionthat maps the Streamitem to another Streamon which you can collect.
可以通过调用流上的flatMap方法来实现此功能，该方法采用Function将Stream项目映射到Stream您可以收集的另一个项目的。
Here, the flatMapmethod converts the Stream<Collection<Long>>to a Stream<Long>, and collectcollects them into a Collection<Long>.
在这里，该flatMap方法将 转换Stream<Collection<Long>>为 a Stream<Long>，并将collect它们收集到 a 中Collection<Long>。
Collection<Long> longs = streamOfCollections
.flatMap( coll -> coll.stream())
.collect(Collectors.toList());
回答by Vivin Paliath
回答by Helmut Provost
You don't need to specify classes when not needed. A better solution is:
不需要时不需要指定类。更好的解决方案是：
Collection<Long> longs = streamOfCollections.collect(
ArrayList::new,
Collection::addAll,
Collection::addAll
);
Say, you don't need an ArrayList but need a HashSet, then you also need to edit only one line.
比如说，你不需要一个 ArrayList 但需要一个 HashSet，那么你也只需要编辑一行。
