Java stream – Sort a List to a HashMap of Lists

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")));