如何在两个不同的时间传递两个不同的JSON对象进行processFinish

Question

I have this MainActivity which does two HTTP calls and return the JSON object back to the MainActivity class. I have seperately implemented the AsyncTask class and used the AsyncResponse interface to get the JSON object to the MainActivity by using the processFinish function call.

At first I came up with one HTTP call which worked perfectly. Secondly I wanted to do another HTTP call in the same activity class. So I edit the code to cater the second HTTP call.

When I run the application, only the first HTTP call is working. When I call the second HTTP call it throws an exception saying reference to a null object

Then I checked by logging the onPostExecute method which calls the processFinish function. There I could see the JSON Object. So, that means the second JSON object doesn't get to the processFinish

How do I manage the second HTTP call? Please help me! I am new to Android.

Following is my AsyncTask class...

public class ServiceHandler extends AsyncTask<String, Void, JSONObject> {

    String startStationID;
    String endStationID;
    String searchDate;
    String startTime;
    String endTime;

    public ServiceHandler(String startStationID, String endStationID, String searchDate, String startTime, String endTime) {
        this.startStationID = startStationID;
        this.endStationID = endStationID;
        this.searchDate = searchDate;
        this.startTime = startTime;
        this.endTime = endTime;
    }

    public interface AsyncResponse {
        void processFinish(JSONObject output);
    }

    public AsyncResponse delegate=null;

    public ServiceHandler(AsyncResponse delegate) {
        this.delegate = delegate;
    }

    @Override
    protected JSONObject doInBackground(String... params) {

        String method = params[0];
        JSONObject JSON_Object = null;


        if (method.equals("getStations")) {

            JSON_Object = Constants.apiCall("http://api.lankagate.gov.lk:8280/railway/1.0/station/getAll?lang=en");

        } else if (method.equals("searchTrains")) {
            JSON_Object = Constants.apiCall("http://api.lankagate.gov.lk:8280/railway/1.0/train/searchTrain?" +
                    "startStationID="+this.startStationID+"&" +
                    "endStationID="+this.endStationID+"&" +
                    "searchDate="+this.searchDate+"&" +
                    "startTime="+this.startTime+"&" +
                    "endTime="+this.endTime+"&" +
                    "lang=en");
        }
        return JSON_Object;
    }

    @Override
    protected void onProgressUpdate(Void... values) {
        super.onProgressUpdate(values);
    }

    @Override
    protected void onPostExecute(JSONObject obj) {
        try{
Log.d("onPostExecute",obj.toString());
            delegate.processFinish(obj);

        }catch (Exception e){
            Log.e("onPostExecute",e.getMessage());
        }

    }

    @Override
    protected void onPreExecute() {
        super.onPreExecute();
    }
}

Following is my processFinish function...

    @Override
    public void processFinish(JSONObject output) {
        Log.d("processFinish",output.toString());

        if(!isSearchClicked) {
            //Get all the stations...
            if (output != null) {
                Toast.makeText(MainActivity.this, "Successfully Connected!", Toast.LENGTH_SHORT).show();
                try {
                    JSONObject obj = output.getJSONObject("RESULTS");
                    output = null;
                    JSONArray dataArray = obj.getJSONArray("stationList");

                    for (int i = 0; i < dataArray.length(); i++) {
                        JSONObject object1 = dataArray.getJSONObject(i);

                        String stationID = object1.getString("stationID");
                        String stationName = object1.getString("stationName");

                        stationNames.add(stationName);
                        stationIDs.add(stationID);
//                    stations.put(stationID,stationName);

                    }
                } catch (JSONException e) {
                    e.printStackTrace();
                }
            } else {
                Toast.makeText(MainActivity.this, " Connection Failed!", Toast.LENGTH_SHORT).show();
            }
        }else {
            //search click action...
            if (output != null) {
                Toast.makeText(MainActivity.this, "Successfully Searched!", Toast.LENGTH_SHORT).show();


                try {
                    JSONObject obj = output.getJSONObject("RESULTS");
                    JSONArray directTrains = obj.getJSONArray("directTrains");


//                    Log.d("array size",String.valueOf(directTrains.length()));
//                    for (int i = 0; i < directTrains.length(); i++) {
//                        JSONObject object1 = directTrains.getJSONObject(i);
//
//                        String stationID = object1.getString("stationID");
//                        String stationName = object1.getString("stationName");
//                        Log.d("JArr", stationID + " : " + stationName);
//
//                        stationNames.add(stationName);
//                        stationIDs.add(stationID);
////                    stations.put(stationID,stationName);
//                    }
//                    Log.d("stationNames", stationNames.toString());

                } catch (JSONException e) {
                    e.printStackTrace();
                }
            } else {
                Toast.makeText(MainActivity.this, " Connection Failed!", Toast.LENGTH_SHORT).show();
                Log.d("output",output.toString());
            }
        }
    }

Following is my first HTTP call...

ServiceHandler sh = new ServiceHandler(this);

String method = "getStations";
sh.execute(method);

Following is my second HTTP call...

String method = "searchTrains"
        ServiceHandler sh = new ServiceHandler(startStationID,endStationID,searchDate,startTime,endTime);
        sh.execute(method);

Answer 1

Although I don't understand exactly what your problem is. There are few things I suggest you to do.

Here I go.

1. Don't use AsyncTask to make your http calls , use an intent services instead.
2. Use a OkHTTP library for your networking source
3. On your intent service send local broadcast with LocalBroadcastManager to broadcast your results from the http call.
4. Register broadcastsReceivers within your activities or fragments that will listen for those broadcasts that comes from the intent service

Why not to use AsyncTask: Because of configuration change - if you rotate your device you will lose that network calls-

Read about intent services here