List of objects to map in java 8

Question

I have a class like this

ObjectA
id 
type

where id is unique but the type can be duplicate so if I have a list of ObjectA like this

(id, type) = (1, "A") , (2, "B"), (3, "C"), (4, "A")

then I want Map<type, List<id>> where map is

< "A",[1,4],
  "B",[2],
  "C",[3] >

It is working with Java 7 but can't find a way to do it in Java 8. In Java 8 I can create key, value pair but not able to create a list if a type is same.

Answer 1

yourTypes.stream()
         .collect(Collectors.groupingBy(
               ObjectA::getType,
               Collectors.mapping(
                    ObjectA::getId, Collectors.toList()
)))

Answer 2

A way to do it without streams:

Map<String, List<Integer>> map = new HashMap<>();
list.forEach(object -> 
    map.computeIfAbsent(object.getType(), k -> new ArrayList<>())
        .add(object.getId()));

This uses Iterable.forEach and Map.computeIfAbsent to iterate the list and group its elements (here named object) by its type attribute, creating a new ArrayList if there was no entry for that type in the map yet. Then, the id of the object is added to the list.