How to send JSON data as response in nanohttpd

I am using the nanohttpd server in my project. The GET Request should send a JSON file as response but nanohttpd only allows String. How can I do this?

This is the GET method:

class GetHandler{
JSONObject response;
JSONObject doGet(Queue queue){
    Map.Entry<String, Message> entry = (Map.Entry<String, Message>)queue.getQueue().entrySet().iterator().next();
    String key = entry.getKey();
    String value = entry.getValue().getText();
    int reCount = entry.getValue().increaseCount();
    queue.getQueue().remove(key);
    Date date = new Date();
    long time = date.getTime();
    queue.getInFlightQueue().put(key,new Message(value, reCount, time));
    System.out.println("ID : " + key + " Message Body : " + value);
    response.put("ID",key);
    response.put("Message Body", value);
    return response;
}
}

This is the Handling part:

if (uriComponents[1].equals("queue") && mainHashMap.containsKey(uriComponents[2])) {
    if (mainHashMap.get(uriComponents[2]).getQueue().isEmpty()) {
        response = "Error : trying to access an empty queue";
        return newFixedLengthResponse(NanoHTTPD.Response.Status.NOT_FOUND, "text/plain", response);
   } else {
        JSONObject message = implementGet.doGet(mainHashMap.get(uriComponents[2]));
        return newFixedLengthResponse(NanoHTTPD.Response.Status.OK, "text/plain", message.toJSONString());
        }
} else {
            response = "Error : Given queue name doesn't exits. Usage:- GET/queue/<queue_name>";
            return newFixedLengthResponse(NanoHTTPD.Response.Status.NOT_FOUND, "text/plain", response);
}