Java流-将列表排序到列表的HashMap [英] Java stream - Sort a List to a HashMap of Lists

问题描述

比方说我有一个Dog类.

在其中，我有一个Map<String,String>，其中一个值是Breed.

Inside it I have a Map<String,String> and one of the values is Breed.

public class Dog {
    String id;
    ...
    public Map<String,String>
}

我想获得List的Map:

HashMap<String, List<Dog>> // breed to a List<Dog>

我宁愿使用Stream而不是对其进行迭代.

I'd prefer to use a Stream rather than iterating it.

我该怎么办?

推荐答案

您可以使用groupingBy来实现.

假设您的输入是List<Dog>，在Dog类内的Map成员称为map，并且将品种存储为品种".密钥:

Assuming that your input is a List<Dog>, the Map member inside the Dog class is called map, and the Breed is stored for the "Breed" key :

List<Dog> dogs = ...
Map<String, List<Dog>> map = dogs.stream()
     .collect(Collectors.groupingBy(d -> d.map.get("Breed")));

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