Java stream - Sort a List to a HashMap of Lists

Question

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?

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

Answer 2

The great answer above can further be improved by method reference notation:

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