Use custom object to receive json response for Rest API

I have set up a rest API service using Jersey which produces an json response. This is the server code:

@Path("/addNumservice")
public class AddNumService {
    @GET
    @Produces("application/json")
    public Response addNum() throws JSONException {
        JSONObject jsonObject = new JSONObject();
        int x = 5, y = 4;
        int z = x + y; 
        jsonObject.put("Sum Value", z); 

        String result = "@Produces(\"application/json\") Output: \n\nNumber adding Output: \n\n" + jsonObject;
        return Response.status(200).entity(result).build();
    }
}

When I run the server, I see the o/p as expected:

@Produces("application/json") Output: 
Number adding Output: 
{"Sum Value":9}

Now I want to set up a client class to receive the json response in a custom object which I have defined, AddNumResponseObject, but when I do this:

AddNumResponseObject object
      = webResource2.accept("application/json").get(AddNumResponseObject.class);

I get this error: A message body reader for Java class com.crunchify.client.AddNumResponseObject, and Java type class com.crunchify.client.AddNumResponseObject, and MIME media type application/json was not found

Can someone please help me?

Thanks!