Json structured file Parsing with Java

Question

I have this Json structured file, which validates as a Json file.

I need to parse this Json data using Java code. I have tried several libraries like json.org and simple Jason but I can't quite get the values from the Json file error like invalid s

{
    "capacity": "30",
    "participants": {
        "858113": {
            "studentNum": "R1506D858113",
            "module_id": "300",
            "offer_id": "4254",
            "grade": null,
            "code": "WA"
        },
        "1007938": {
            "studentNum": "R1509D1007938",
            "module_id": "300",
            "offer_id": "4254",
            "grade": null,
            "code": "WA"
        },
        "1022146": {
            "studentNum": "R1509D1022146",
            "module_id": "300",
            "offer_id": "4254",
            "grade": null,
            "code": "WA"
        },
        "1041591": {
            "studentNum": "R1510D1041591",
            "module_id": "300",
            "offer_id": "4254",
            "grade": null,
            "code": "WA"
        },
        "1226958": {
            "studentNum": "R1601D1226958",
            "module_id": "300",
            "offer_id": "4254",
            "grade": null,
            "code": "WA"
        }
    }
}

This is the Jason file and below code using json.org

 final JSONObject obj = new JSONObject(JSON_DATA);
    final JSONArray participants = obj.getJSONArray("participants");
    final int n = participants.length();
    for (int i = 0; i < n; ++i) {
      final JSONObject person = participants.getJSONObject(i);
      System.out.println(person.getInt("studentNum"));

    }

Basically I need to retrieve this this information as an array.

It's JSON (JavaScript Object Notation), not Jason (the leader of the Argonauts). Also, if you're using MySQL, then why Tag Microsoft SQL Server? Please only tag the RDBMS you are *actually* using. — Thom A, Jun 10 '19 at 08:14
Hi @GeorgeGeorgiou, added an answer reflecting your model and how to get form String to the JSON object you want. — Brother, Jun 10 '19 at 08:44

score 0 · Answer 1 · answered Jun 10 '19 at 08:12

Use more robust OOPs. Use a library like GSON to convert JSON to objects and then further harness Java capabilities.

GSON Maven Dependency

public class Participant {
public String studentNum;
    public String module_id;
    public String offer_id;
    public String grade;
    public String code;
}

Main object:

public class ParticipantData
{

    public String capacity;

    public Map<String, Participant> participants;
}

Now using GSON to convert JSON to object and iterate:

 Gson gson = new Gson();

        Type dataType = (new TypeToken<ParticipantData>()
        {
        }).getType();

        ParticipantData data = gson.fromJson(jsonData, dataType);

        for(String e: data.participants.keySet()) {
            System.out.printf("For participant with id %s, details are %s\n", e, data.participants.get(e));
        }

Brother · Answer 2 · 2019-06-10T08:43:11.607

so I have this example using Jackson:

compile group: 'com.fasterxml.jackson.core', name: 'jackson-core', version: '2.9.9'
compile group: 'com.fasterxml.jackson.core', name: 'jackson-annotations', version: '2.9.9'
compile group: 'com.fasterxml.jackson.core', name: 'jackson-databind', version: '2.9.9'

Models:

public class ParticipantData {
    private String capacity;
    private Map<String, Student> participants;

    public String getCapacity() {
        return capacity;
    }

    public Map<String, Student> getParticipants() {
        return participants;
    }
}

public class Student {
    private String studentNum;
    private String module_id;
    private String offer_id;
    private String grade;
    private String code;

    public String getStudentNum() {
        return studentNum;
    }

    public String getModule_id() {
        return module_id;
    }

    public String getOffer_id() {
        return offer_id;
    }

    public String getGrade() {
        return grade;
    }

    public String getCode() {
        return code;
    }
    @Override
    public String toString() {
        return "Student{" +
                "studentNum='" + studentNum + '\'' +
                ", module_id='" + module_id + '\'' +
                ", offer_id='" + offer_id + '\'' +
                ", grade='" + grade + '\'' +
                ", code='" + code + '\'' +
                '}';
    }
}

Here it is the parser class, making use of the ObjectMapper:

import com.fasterxml.jackson.databind.ObjectMapper;
import model.ParticipantData;

import java.io.IOException;
import java.util.Optional;

public class MyJsonReader {

    private static Optional<ParticipantData> read(String jsonToParse) {
        try {
            return Optional.of(new ObjectMapper().readValue(jsonToParse, ParticipantData.class));
        } catch (IOException e) {
            e.printStackTrace();
            return Optional.empty();
       }
    }

    public static void main(String[] args) {
        String jsonToParse = getYourJsonToParseAsString();

        read(jsonToParse).ifPresent(participantData -> {
            System.out.println(participantData.getCapacity());
            participantData.getParticipants().forEach((key, value) -> {
                System.out.println(key);
                System.out.println(value);
            });
        });
    }
}

In this example, it is iterating through all the participants and printing it.

score -1 · Answer 3 · answered Jun 10 '19 at 08:35

-1

You could use Jackson. It is easy and handles all the logic for you returning a Map with key-values.

Map<String, Object> jsonMap = new ObjectMapper().readValue(stringResponseBody, new TypeReference<Map<String, Object>>() {});

answered Jun 10 '19 at 08:35

h4t0n

91
1
9

score -1 · Accepted Answer · answered Jun 10 '19 at 08:50

Your Code Changes using org.json

        JSONObject jsonObj = new JSONObject(JSON_DATA);

        // here participants is jsonObject, not jsonArray
        JSONObject participants = jsonObj.getJSONObject("participants");

        // this jsonObject participants has five key-value pairs.
        // so iterate through keys and fetch values (These values are also another
        // jsonObject)
        Iterator<String> key = participants.keys();
        while (key.hasNext()) {
            JSONObject person = participants.getJSONObject(key.next());
           // value of key studentNum is string type
            System.out.println(person.getString("studentNum"));
        }

Json structured file Parsing with Java

4 Answers4