Parsing JSON string returns undefined

Question

This is my java method:

public String getEvents(String cat, int start, int step)  {
		
        //JSONArray list = new JSONArray();
        List<String> list = new ArrayList();
  
		if (cat.equalsIgnoreCase("D")){
			for (int i=0; i<listDog.size(); i++){
				JSONObject obj = new JSONObject();
				System.out.println("di=" + i + "=" + (String)listDog.get(i));
				obj.put("event_id",new String((String)listDog.get(i)));
				list.add(obj.toJSONString());
			}
		}
		else {
			for (int i=0; i<listHorse.size(); i++){
				JSONObject obj = new JSONObject();
				System.out.println("i=" + i + "=" + (String)listHorse.get(i));
				obj.put("event_id",new String((String)listHorse.get(i)));
				list.add(obj.toJSONString());
			}
		}
		
                
		return list.toString();
		
	}

and below is how I convert it to json:

Map map = new HashMap();
Gson gson = new GsonBuilder().disableHtmlEscaping().create()

....

String result = service.getEvents(cat, Integer.parseInt(start), Integer.parseInt(step));
String objs = gson.toJson(result);
map.put("result", objs);

.....

String output = gson.toJson(map);
System.out.println("output->" + output);
out.println(output);
out.flush();
out.close();

I'm getting the below JSON string from the server side:

{
  "result": "\"[{\\\"event_id\\\":\\\"2139114\\\"}]\"",
  "eventList": "[{\"eventId\":164151,\"meetingCode\":\"5G8QV\",\"meetingName\":\"Kranji\",\"eventTime\":\"13:20:00\",\"eventCode\":\"07:50:00\",\"category\":\"HR\"},{\"eventId\":163890,\"meetingCode\":\"5G8MW\",\"meetingName\":\"Henlow\",\"eventTime\":\"02:30:00\",\"eventCode\":\"21:00:00\",\"category\":\"DG\"}]"
}

and I convert it like this:

          success: function(data) {

              console.log("SERVLET DATA: " + data.replace("\n", ""));

              if (typeof(data) !== 'undefined' && data !== '' && data !== null)               {
                data = data.replace("\n", "");
                var jsonData = JSON.parse(data);
                //console.log(jsonData);
                for (var i in jsonData) {
                  var event = jsonData[i];
                  var event2 = JSON.stringify(jsonData[i]);
                  var event3 = JSON.parse(event);
                  console.log("init:" + event.event_id);
                }
              }

in all three logs, I'm getting "undefined". Please tell me how can I get the "event_id" value from the below json string?

"result": "\"[{\\\"event_id\\\":\\\"2139114\\\"}]\""

I checked validity of json string online and it says no errors found.

is there any issue here or how to access "event_id" field ?

Answer 1

I changed my java method as below:

public List < UKDashboardEventDTO > getEvents(String cat, int start, int step) {

  //JSONArray list = new JSONArray();
  List < UKDashboardEventDTO > list = new ArrayList();
  UKDashboardEventDTO dto;
  if (cat.equalsIgnoreCase("D")) {
    for (int i = start - 1; i < listDog.size(); i = i + step) {
      //JSONObject obj = new JSONObject();
      dto = new UKDashboardEventDTO();
      System.out.println("di=" + i + "=" + (String) listDog.get(i));
      dto.setEventId(Long.parseLong(listDog.get(i).toString()));
      list.add(dto);
    }
  } else if (cat.equalsIgnoreCase("H")) {
    for (int i = start - 1; i < listHorse.size(); i = i + step) {
      dto = new UKDashboardEventDTO();
      System.out.println("di=" + i + "=" + (String) listHorse.get(i));
      dto.setEventId(Long.parseLong(listHorse.get(i).toString()));
      list.add(dto);
    }
  } else if (cat.equalsIgnoreCase("A")) {
    for (int i = start - 1; i < listAll.size(); i = i + step) {
      dto = new UKDashboardEventDTO();
      System.out.println("di=" + i + "=" + (String) listAll.get(i));
      dto.setEventId(Long.parseLong(listAll.get(i).toString()));
      list.add(dto);
    }
  }

  return list;

}

and used the dto class as below:

public class UKDashboardEventDTO implements Serializable{

    private Long eventId;
  
  ............
  
 }

and pass the values like this:

List < UKDashboardEventDTO > result = service.getEvents(cat, Integer.parseInt(start), Integer.parseInt(step));
map.put("result", result);
//out.println(objs);   

.......


try {
  String objs = gson.toJson(map);
  System.out.println("objs->" + objs);
  out.println(objs);
  out.flush();
  out.close();
} catch (Exception ex) {

}

did not want to do any change to the client side logic. this works !

Answer 2

Firstly, you need to re-factor the getEvents method to return only plain objects (e.g. List), then, put the result in to the target map before passing it to the toJSON method

public class Event {
    String event_id;

    Event(String event_id) {
        this.event_id = event_id;
    }
}

public List<Event> getEvents(String cat, int start, int step)  {

    List<Event> list = new ArrayList<Event>();

    if (cat.equalsIgnoreCase("D")){
        for (int i=0; i<listDog.size(); i++){
            System.out.println("di=" + i + "=" + (String)listDog.get(i));

            list.add( new Event( (String)listDog.get(i) ));
        }
    }
    else {
        for (int i=0; i<listHorse.size(); i++){
            System.out.println("i=" + i + "=" + (String)listHorse.get(i));

            list.add( new Event( (String)listHorse.get(i) ));
        }
    }

    return list;

}

Then, convert your result like this:

Map map = new HashMap();
Gson gson = new GsonBuilder().disableHtmlEscaping().create()

....

List<Event> result = service.getEvents(cat, Integer.parseInt(start), Integer.parseInt(step));

map.put("result", result);

String objs = gson.toJson(map);