Java 流 - 将列表排序为列表的 HashMap
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Java stream - Sort a List to a HashMap of Lists
提问by Bick
Let's say I have a Dogclass.
假设我有一Dog堂课。
Inside it I have a Map<String,String>and one of the values is Breed.
在它里面我有一个Map<String,String>，其中一个值是Breed.
public class Dog {
String id;
...
public Map<String,String>
}
I want to get a Mapof Lists:
我想获得Map的ListS：
HashMap<String, List<Dog>> // breed to a List<Dog>
I'd prefer to use a Streamrather than iterating it.
我更喜欢使用 aStream而不是迭代它。
How can I do it?
我该怎么做？
采纳答案by Eran
You can do it with groupingBy.
你可以用groupingBy.
Assuming that your input is a List<Dog>, the Mapmember inside the Dogclass is called map, and the Breed is stored for the "Breed" key :
假设您的输入是 a List<Dog>，则类中的Map成员Dog称为map，并且 Breed 存储为“Breed”键：
List<Dog> dogs = ...
HashMap<String, List<Dog>> map = dogs.stream()
.collect (Collectors.groupingBy(d -> d.map.get("Breed")));
回答by Nestor Milyaev
The great answer above can further be improved by using functional programming notation:
通过使用函数式编程符号可以进一步改进上面的好答案：
List<Dog> dogs = ...
Map<String, List<Dog>> map = dogs.stream()
.collect(Collectors.groupingBy(Dog::getBreed));
