count each element inside jsonObject using java stream

Question

I have a json Structure like this

[{
                "startAt": 1617605301292,
                "endAt": 1617605317095,
                "duration": 15803,
                "selection": {
                    "selected.Speed": "0",
                    "selected.Low": "65535",
                    "selected.Fast": "7173",
                    "selected.medium": "5"
                },
                "details": {
                    "phase": [{
                        "value": "2",
                        "timestamp": 1617605301316
                    }]
                }
            },....]

I need the count of every item inside selection object.

For Example

0 occurred 4 times
65535 occurred 2 times
7173 occurred 3 times
5 occurred 1 times

this is what I have done so far

Map<String, Long> counted = jObj.stream()
                        .filter(y -> ((Map<String, Object>) y.get("selection")).containsKey("selected.Speed"))
                        .map(x -> {
                            String Speed = String.valueOf(((Map<String, Object>) x.get("details"))
                                    .get("selected.Speed"));
                            return Speed;
                        }).collect(Collectors.toList()).stream()
                        .collect(Collectors.groupingBy(Function.identity(), Collectors.counting()));

so from this I am able to get the count of a single key but not the every key inside json Object. Pardon if this is a very basic question, I am new to the stream.

TIA

You are reading values from another entry than the one expected to hold the values x.get("details") which should be x.get("selection"). Is that the actual JSON or is there any intermediate processing step? And are you looking for values i.e. 3 times, 1 time... mapped to each property key? (i.e. 0 for selected.Speed occurred 4 times) — tmarwen
– tmarwen, Commented Apr 10, 2021 at 21:06
@tmarouane yes you are right I am looking for the same like 0 selected.Speed occurred 4 times 65535 selected.Low occurred 2 time etc actually based on that I need to prepare a json which will be like this [{ "selected.Low ":65535, "count":2 },{ "selected.Low ":0, "count":4 } ] — Doctor Who
– Doctor Who, Commented Apr 10, 2021 at 21:14

Yassin Hajaj · Accepted Answer · 2021-04-10 21:08:19Z

`Stream#mapMulti`

If you're using java-16, you can already make use of Stream#mapMulti for this use case. Here is how to solve it with this. If you're not using java-16 yet, scroll further to find the traditional solution to the problem

Map<String, Long> counted = jObj.stream()
        .map(x -> (Map<String, Object>) x.get("details"))
        .filter(x -> x.containsKey("selected.Speed"))
        .map(Map::values)
        .mapMulti((Collection<Object> collection, Consumer<String> consumer) -> {
            collection.forEach(element -> consumer.accept(String.valueOf(element)));
        })
        .collect(Collectors.groupingBy(Function.identity(), Collectors.counting()));

With this solution, you don't have the overhead of an extra Stream creation

Traditional approach

Normally this should solve it for you using Stream#flatMap, Map#values then List#stream and mapping every entry to a String

Map<String, Long> counted = jObj.stream()
                        .filter(y -> ((Map<String, Object>) y.get("details")).containsKey("selected.Speed"))
                        .flatMap(x -> ((Map<String, Object>)x.get("details"))
                                                          .values()
                                                          .stream()
                                                          .map(String::valueOf))
                        .collect(Collectors.groupingBy(Function.identity(), Collectors.counting()));

You can even use an extra map intermediate operation to avoid casting x.get("details") to a Map<String, Object> twice

Map<String, Long> counted = jObj.stream()
                        .map(x -> (Map<String, Object>) x.get("details"))
                        .filter(x -> x.containsKey("selected.Speed"))
                        .map(Map::values)
                        .flatMap(Collection::stream)
                        .map(String::valueOf)
                        .collect(Collectors.groupingBy(Function.identity(), Collectors.counting()));

Here is the full test I've written for this

List<Map<String, Object>> jObj = new ArrayList<>();
jObj.add(Map.of("details", Map.of("selected.Speed", "0", "selected.Low", "65535", "selected.Fast", "8000", "selected.medium", "5")));
jObj.add(Map.of("details", Map.of("selected.Speed", "1", "selected.Low", "32", "selected.Fast", "8000", "selected.medium", "5")));
jObj.add(Map.of("details", Map.of("selected.Speed", "2", "selected.Low", "32", "selected.Fast", "8000", "selected.medium", "5")));
jObj.add(Map.of("details", Map.of("selected.Speed", "0", "selected.Low", "65535", "selected.Fast", "8000", "selected.medium", "5")));


Map<String, Long> counted = jObj.stream()
        .map(x -> (Map<String, Object>) x.get("details"))
        .filter(x -> x.containsKey("selected.Speed"))
        .map(Map::values)
        .flatMap(Collection::stream)
        .map(String::valueOf)
        .collect(Collectors.groupingBy(Function.identity(), Collectors.counting()));

System.out.println("counted = " + counted);

Which prints the expected result

counted = {0=2, 1=1, 2=1, 8000=4, 5=4, 65535=2, 32=2}

Thanks, is there any way I can get key as well with the output. actually based on that I need to prepare a json which will be like this [{ "selected.Low ":65535, "count":2 },{ "selected.Low ":0, "count":4 } ]
Ok I guess this might be a second question @DoctorWho because the solution will differ quite much. Can I ask only one question per post ?

tmarwen · Accepted Answer · 2021-04-10 22:06:08Z

Here is how you should iterate through your JSON entries and process its fields:

jObj.stream()
    .map(x -> (Map<String, Object>) x.get("selection"))
    .filter(x -> x.containsKey("selected.Speed"))
    .flatMap(values -> values.entrySet().stream()) // flat-mapping entries to group them 
    .collect(Collectors.groupingBy(Map.Entry::getKey)) // group entries by their keys: `selected.Low`, `selected.Fast`...
    .entrySet()
    .stream() // streaming through grouped entries
    .map(e -> new AbstractMap.SimpleEntry<>(
            e.getKey(),
            e.getValue().stream().map(Map.Entry::getValue).collect(Collectors.groupingBy(Function.identity(), Collectors.counting())))
    ) // grouping entries values counts
    .collect(Collectors.toList());

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count each element inside jsonObject using java stream

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