如何从HashMap <String，List <E >>获取List <E>

Question

我想List<E>从使用Map<String, List<E>>（E是一个随机类）中提取一个stream()。

我想要一个使用Java 8流的简单的单行方法。

到目前为止，我一直在尝试：

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

Answer 1

map.values()返回一个Collection<List<E>>not List<E>，如果需要后者，则需要按以下步骤将嵌套拼合List<E>为单个List<E>：List<E> result = map.values()&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; .stream()&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; .flatMap(List::stream)&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; .collect(Collectors.toList());

Answer 2

或使用&nbsp;forEach&nbsp;map.forEach((k,v)->list.addAll(v));或如Aomine所评论使用的map.values().forEach(list::addAll);

Answer 3

这是使用Java-9及更高版本的替代方法：List<E> result = map.values()&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; .stream()&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; .collect(Collectors.flatMapping(List::stream, Collectors.toList()));