How to collect data to Map using Java Stream API?

Question

I have a question and hope someone would help me improve my knowledge of Java Stream API.

My service receives data from another service via RabbitMQ as array like this:

[ {"deviceId":1,"userId":1},
{"deviceId":2,"userId":1},
{"deviceId":3,"userId":2},
{"deviceId":4,"userId":2},
{"deviceId":5,"userId":1},
{"deviceId":6,"userId":2},
... ]

What I need to do with this data is to collect it into List where User has userId and List of deviceId's. So from the data above it should be:

User {userId: 1, List<deviceId>: 1,2,5} & User {userId: 2, List<deviceId>: 3,4,6}

I do have a working solution but it is clumsy & I believe it all could be done way better with Java Stream API.

The solution is:

    private List<User> parseDeviceData(JsonNode jsonNode) {
    List<JsonNode> list = StreamSupport.stream(jsonNode.spliterator(), false)
            .filter(this::validDeviceRecord) // checking for presence of deviceId & userId
            .collect(Collectors.toList());

    Map<Integer, ArrayList<Integer>> map = new HashMap<>();
    for (JsonNode j : list) {
        Integer id = j.get("deviceId").asInt();
        Integer userId = j.get("userId").asInt();
        ArrayList<Integer> l;
        if (map.containsKey(userId)) {
            l = map.get(userId);
        } else {
            l = new ArrayList<>();
        }
        l.add(id);
        map.put(userId, l);
    }

    return map.entrySet().parallelStream()
            .map(entry -> new User(entry.getKey(), entry.getValue()))
            .collect(Collectors.toList());
}

How could I do the same with streams? Thanks!

(the second answer seems to be what you are looking for) – Hulk Jan 22 '21 at 12:28 — Hulk, Jan 22 '21 at 12:28

score 1 · Accepted Answer · answered Jan 22 '21 at 12:36

You can make below code as sample.

List<MyObject> list = new ArrayList<>();
        list.add(new MyObject("1", "1"));
        list.add(new MyObject("2", "2"));
        list.add(new MyObject("3", "1"));
        list.add(new MyObject("4", "2"));
        list.add(new MyObject("5", "3"));
        list.add(new MyObject("6", "1"));
        Map<String, List<String>> items = list.stream().collect(Collectors.groupingBy(MyObject::getUserId, Collectors.mapping(MyObject::getDeviceId,Collectors.toList())));
        System.out.println(items);

How to collect data to Map using Java Stream API?

1 Answers1