Java stream - Sort a List to a HashMap of Lists

问题

Let's say I have a Dog class.

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

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

I want to get a Map of Lists:

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

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

How can I do it?

回答1:

You can do it with groupingBy.

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

回答2:

The great answer above can further be improved by using functional programming notation:

List<Dog> dogs = ...
HashMap<String, List<Dog>> map = dogs.stream()
     .collect(Collectors.groupingBy(Dog::getBreed)); 

来源：https://stackoverflow.com/questions/28064263/java-stream-sort-a-list-to-a-hashmap-of-lists