如何获取列表<E>从 HashMap<String,List<E>>

2022-01-22 00:00:00 collections java-8 java java-stream collectors

我想从 Map 中提取一个 List<E> (E 是随机的类)使用 stream().

I want to extract a List<E> from a Map<String, List<E>> (E is a random Class) using stream().

我想要一个使用 java 8 的流的简单单行方法.

I want a simple one-line method using java 8's stream.

到目前为止我所尝试的:

What I have tried until now :

HashMap<String,List<E>> map = new HashMap<>();
List<E> list = map.values(); // does not compile
list = map.values().stream().collect(Collectors.toList()); // does not compile

推荐答案

map.values() 返回一个 Collection> 而不是 >List<E>，如果你想要后者，那么你需要将嵌套的 List<E> 展平为单个 List<E> 为如下:

map.values() returns a Collection<List<E>> not a List<E>, if you want the latter then you're required to flatten the nested List<E> into a single List<E> as follows:

List<E> result = map.values()
                    .stream()
                    .flatMap(List::stream)
                    .collect(Collectors.toList());

相关文章