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 and one of the values is Breed.

public         


        

Answer 1

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)); 

Answer 2

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")));