Javax WS Response adds "type" in Response Entity

Question

I have an Endpoint written in Java WS. The DTO classes used are as follows:

public class MainResponseDTO extends PaginationResponseDTO {

    private List<MainDTO> mainDTOs;

    public List<MainDTO> getMainDTOs() {
        return mainDTOs;
    }

    public void setMainDTOs(List<MainDTO> mainDTOs) {
        this.mainDTOs = mainDTOs;
    }
}

And this class extends a PaginationResponseDTO

public class PaginationResponseDTO {

    private int page;
    private int limit;
    private long size;

    public int getPage() {
        return page;
    }
    public void setPage(int page) {
        this.page = page;
    }
    public int getLimit() {
        return limit;
    }
    public void setLimit(int limit) {
        this.limit = limit;
    }
    public long getSize() {
        return size;
    }
    public void setSize(long size) {
        this.size = size;
    }
}

This is how I am sending responses.

@POST
@Path("/demo")
@Produces(MediaType.APPLICATION_JSON)
@Consumes(MediaType.APPLICATION_JSON)
public Response getExpiredAndDueDocumentsWithPagination() {

    MainResponseDTO response = new MainResponseDTO();

    // Used this for diagnosis
    logger.debug(new Gson().toJson(response));

    return Response.ok().entity(response).build();
}

However, the response on accesing the above URL (via Postman or Curl) gives the following response:

{
    "type": "mainResponseDTO",
    "limit": 0,
    "page": 0,
    "size": 0
}

I have not used a "type" property in any one of the DTOs then how is it getting appended. I also logged the JSON String representation of the response DTO using GSON just before returning the response which is as follows.

{"page":0,"limit":0,"size":0}

How did the "type" property get inside the response? Where have I gone wrong in my approach?

If the details provided in the question are insufficient do let me know for additional ones that are required.