How to convert JSON response as shown below to POJO using Gson?

Question

The format of my JSON is :

{"abc": [{
  "field_1": "string_1",
  "value": 0.304
},
{
  "field_1": "string_2",
  "value": 0.193
}]}

"abc" is variable, "field_1" and "value" are field names. I want a class in Java which stores this JSON in some format for example:

 String t;                      // should store "abc"
 List<myClass> myClassObject;   // myClass should contain "field_1" and "value"

myClass.java

String field_1;              // should store "string_1" and "string_2"
Double value;                // should store 0.304 and 0.193

I want the class myClass.java because in future I may want to add more metadata in JSON response. This is complex object mapping, but I am not able to figure out what should my class be looking like in order to store the JSON response.

Answer 1

For root object do not create new POJO just use Map. Example could look like below:

import com.google.gson.Gson;
import com.google.gson.GsonBuilder;
import com.google.gson.annotations.SerializedName;
import com.google.gson.reflect.TypeToken;

import java.io.File;
import java.io.FileReader;
import java.lang.reflect.Type;
import java.util.List;
import java.util.Map;

public class GsonApp {

    public static void main(String[] args) throws Exception {
        File jsonFile = new File("./resource/test.json").getAbsoluteFile();

        Gson gson = new GsonBuilder().create();

        Type mapType = new TypeToken<Map<String, List<Item>>>() {
        }.getType();
        Map<String, List<Item>> map = gson.fromJson(new FileReader(jsonFile), mapType);
        Map.Entry<String, List<Item>> first = map.entrySet().stream().findFirst().get();
        Items items = new Items(first.getKey(), first.getValue());

        System.out.println(items);
    }
}

class Items {
    private final String key;
    private final List<Item> items;

    public Items(String key, List<Item> items) {
        this.key = key;
        this.items = items;
    }

    public String getKey() {
        return key;
    }

    public List<Item> getItems() {
        return items;
    }

    @Override
    public String toString() {
        return "Items{" +
                "key='" + key + '\'' +
                ", items=" + items +
                '}';
    }
}

class Item {

    @SerializedName("field_1")
    private String field;
    private Double value;

    public String getField() {
        return field;
    }

    public void setField(String field) {
        this.field = field;
    }

    public Double getValue() {
        return value;
    }

    public void setValue(Double value) {
        this.value = value;
    }

    @Override
    public String toString() {
        return "Item{" +
                "field='" + field + '\'' +
                ", value=" + value +
                '}';
    }
}

Above code prints:

Items{key='abc', items=[Item{field='string_1', value=0.304}, Item{field='string_2', value=0.193}]}