Create a nested map from Json using streams

Question

I have the below Json that I'm reading into nested POJOs with the same structure.

{
  "employees": [
    {
      "name": "John",
      "age": "30",
      "proData": [
        {
          "year": "1",
          "idList": [
            "234342",
            "532542",
            "325424",
            "234234"
          ]
        },
        {
          "year": "2",
          "idList": [
            "234342",
            "532542",
            "325424",
            "234234"
          ]
        },
        {
          "year": "3",
          "idList": [
            "234342",
            "532542",
            "325424",
            "234234"
          ]
        }
      ]
    },
    {
      "name": "Scott",
      "age": "32",
      "proData": [
        {
          "year": "1",
          "idList": [
            "234342",
            "532542",
            "325424",
            "234234"
          ]
        },
        {
          "year": "2",
          "idList": [
            "234342",
            "532542",
            "325424",
            "234234"
          ]
        },
        {
          "year": "3",
          "idList": [
            "234342",
            "532542",
            "325424",
            "234234"
          ]
        }
      ]
    }
  ]
}

Now I wanted to map this to a structure like below, the ProData can be initialized using each of the string in the idList.

Map<String,Map<String,List<ProData>>> finalMap

I have written something like the below and it works.

        Map<String,Map<String,List<ProData>>> finalMap = new HashMap<>();

        for(Employee employee:root.getEmployees()){
            Map<String,List<ProData>> proDataMap = new HashMap<>();
            for(ProData proData: employee.getProData()){
                List<ProData> finalObjs = new ArrayList<>();
                for(String id:proData.getIdList()){
                   finalObjs.add(new ProData(id));
                }

                proDataMap.put(proData.getYear(),finalObjs);
            }
            finalMap.put(employee.getName(),proDataMap);
        }

I wanted to make a better version using the stream API.

this link has done something like your need, give it a visit : stackoverflow.com/questions/49887953/… — arash yousefi
– arash yousefi, Commented Jan 26, 2021 at 6:30
and again, stop treating StackOverflow as your coder conversion application! Share what you have tried, attempt something with streams to fail, and then ak. — Naman
– Naman, Commented Jan 26, 2021 at 8:32

Sweeper · Accepted Answer · 2021-01-26 06:31:16Z

2

The end result is a map, so use the toMap collector. The maps' keys are the employee names (assuming no duplicates), and the map values require a little bit more work.

root.getEmployees().stream().collect(
    Collectors.toMap(
        Employee::getName,
        Employee::getProDataMap
    )
}

Now let's try writing getProDataMap in Employee. Again we use the toMap collector. The keys are the years (assuming no duplicates), and the values are the id lists mapped to ProData using the constructor.

public Map<String, List<ProData>> getProDataMap() {
    return this.getProData().stream().collect(
        Collectors.toMap(
            ProData::getYear,
            proData -> proData.getIdList().stream()
                .map(ProData::new)
                .collect(Collectors.toList())
        )
    )
}

answered Jan 26, 2021 at 6:31

Sweeper

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1 Comment

Jeeppp Over a year ago

Thank you for the detailed response. Its what I was expecting.

Hard Worker · Accepted Answer · 2021-01-26 06:36:06Z

0

should be something similar to,

root.stream().forEach(employee -> {
                Map<String,List<ProData>> proDataMap = new HashMap<>();
                employee.getProdData().stream().forEach(data -> {
                    List<ProData> finalObjs = new ArrayList<>();
                    data.getIdList().stream().forEach(data -> {
                        finalObjs.add(new ProData(id));
                    });
                });
            });

but regardless you can also use Gson to parse it https://mkyong.com/java/how-to-parse-json-with-gson/

answered Jan 26, 2021 at 6:36

Hard Worker

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1 Comment

Nikolas Over a year ago

What is the advantage of using collection.stream().forEach(..) over collection.forEach(..)? This is not the way Stream API should be used.

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