Json deserialization using java enums

Question

I'm trying to deserialize the following JSON to a java object.

"distance":[
  {
    "weight": 60,
    "unit": "km"
  },
  {
    "weight": 100,
    "unit": "m"
  }
]

The java object should look like:

[

  {
    "km": 60
  },
  {
    "m": 100
  }
]

Answer 1

Here I have found two issues, One, it is not well formed JSON. It should be:

{
 "distance":[
   {
     "weight": 60,
     "unit": "km"
   },
   {
     "weight": 100,
     "unit": "m"
   }
 ]
}

OR

[
  {
    "weight": 60,
    "unit": "km"
  },
  {
    "weight": 100,
    "unit": "m"
  }
]

Another is, you did not mention your POJO clearly. You can achieve this response several ways. For simplicity I am assuming simple Map for it.

However, we need to make this string well formed at first.

String jsonStr = "\"distance\":[\n" +
                "  {\n" +
                "    \"weight\": 60,\n" +
                "    \"unit\": \"km\"\n" +
                "  },\n" +
                "  {\n" +
                "    \"weight\": 100,\n" +
                "    \"unit\": \"m\"\n" +
                "  }\n" +
                "]";
jsonStr = jsonStr.substring(jsonStr.indexOf(":") + 1);

Create a class:

@Data
public class Distance {
    private Double weight;
    private String unit;
}

Here I have used lombok for getter and setter.

Parse JSON string to list of Distance :

ObjectMapper objectMapper = new ObjectMapper();
List<Distance> distances = objectMapper.readValue(jsonStr, new TypeReference<List<Distance>>(){});

Now you can convert this list to your desired response. For converting this response to Map :

Map<String, Double> response = distances.stream().collect(Collectors.toMap(Distance::getUnit, Distance::getWeight));

OR, you can convert it to List<Pair<String, Double>> as well:

List<Pair<String, Double>> response = new ArrayList<>();
distances.forEach(distance -> {
    response.add(new Pair(distance.getUnit(), distance.getWeight()));
});

Happy coding...

Answer 2

The problem can be solved using JSONObject and HashMap

1. Use JSONObject to convert json string into java class object.
2. Loop items inside JSONObject to get distance and unit data.
3. Create HashMap object and put it inside ArrayList.

import org.json.JSONArray;
import org.json.JSONException;
import org.json.JSONObject;
import java.util.ArrayList;
import java.util.HashMap;

public class Main {
    public static void main(String args[]) throws JSONException {
        ArrayList<HashMap<String, Double>> result = new ArrayList<HashMap<String,Double>>();

        String jsonStr = "{\n" +
                "    \"distance\": [\n" +
                "        { \n" +
                "            \"weight\": 60, \n" +
                "            \"unit\": \"km\" \n" +
                "        }, \n" +
                "        { \n" +
                "            \"weight\": 100, \n" +
                "            \"unit\": \"m\" \n" +
                "        } \n" +
                "    ]\n" +
                "}";

        JSONObject json = new JSONObject(jsonStr);
        JSONArray distanceArray = json.getJSONArray("distance");

        for(int i = 0; i < distanceArray.length(); i++) {
            JSONObject distanceItems = distanceArray.getJSONObject(i);
            final String unit = distanceItems.getString("unit");
            final double weight = distanceItems.getDouble("weight");

            result.add(new HashMap<String, Double>() {{
                put(unit, weight);
            }});
        }

        System.out.println(result.toString()); // output [{km=60.0}, {m=100.0}]
    }
}