This is how you'd do it:

import java.util.Map;
import java.util.HashMap;
import java.util.Map.Entry;
import java.lang.reflect.Field;

public class Main {
    public int a = 3;
    public String b = "Hello";

    public static void main(String[] args) {
        Map<String, Object> map = parameters(new Main());
        for (Entry<String, Object> entry : map.entrySet()) {
            System.out.println(entry.getKey() + ": " + entry.getValue());
        }
    }

    public static Map<String, Object> parameters(Object obj) {
        Map<String, Object> map = new HashMap<>();
        for (Field field : obj.getClass().getDeclaredFields()) {
            field.setAccessible(true);
            try { map.put(field.getName(), field.get(obj)); } catch (Exception e) { }
        }
        return map;
    }
}

Basically, you use reflection to get all of the fields in the class. Then, you access all of those fields of the object. Keep in mind that this only works for fields accessible from the method that gets the fields.

A solution with Gson:

Gson gson = new Gson();
JsonElement jsonElement = gson.toJsonTree(map);
MyPojo pojo = gson.fromJson(jsonElement, MyPojo.class);

Library Josson can do the job.

https://github.com/octomix/josson

Deserialization

Josson josson = Josson.fromJsonString(
    "{" +
    "  \"code\": 1," +
    "  \"msg\": \"query success\"," +
    "  \"obj\": {" +
    "    \"data\": [" +
    "      {" +
    "        \"name\": \"\"," +
    "        \"value\": {" +
    "          \"num\": \"89\"" +
    "        }" +
    "      }," +
    "      {" +
    "        \"name\": \"1\"," +
    "        \"value\": {" +
    "          \"num\": \"19\"" +
    "        }" +
    "      }," +
    "      {" +
    "        \"name\": \"2\"," +
    "        \"value\": {" +
    "          \"num\": \"6\"" +
    "        }" +
    "      }" +
    "    ]" +
    "  }," +
    "  \"success\": true," +
    "  \"duration\": 523" +
    "}");

Transformation

JsonNode node = josson.getNode("obj.data.map(name::value.num)");
System.out.println(node);

List<Map<String, String>> result = Josson.readValueFor(node, List.class);
System.out.println(result);

Output

[{"":"89"},{"1":"19"},{"2":"6"}]

[{=89}, {1=19}, {2=6}]