android - Attempt to invoke interface method 'void com.bustracker.AsyncTaskCallback.onAsyncTaskFinished(java.util.ArrayList)' on a null object reference -

i trying pass result doinbackground onpostexecute() method getting error:

java.lang.nullpointerexception: attempt invoke interface method 'void com.bustracker.asynctaskcallback.onasynctaskfinished(java.util.arraylist)' on null object reference

what cant understand checking in onpostexecute whether arraylist empty or not when output of routes the output of routes: [3,6,8] app crashes when arraylist empty, app not crash?!

how can fix that?

i appreciate help.

postdata class:

public class postdata {     string jsonstring;     private asynctaskcallback callback;       public postdata(asynctaskcallback callback) {         this.callback = callback;     }      public string getjsonstring() {         return jsonstring;      }      public void setjsonstring(string jsonstring) {         this.jsonstring = jsonstring;     }      public void post_data(string jsonstring, context context) {         this.jsonstring = jsonstring;          new myasynctask(context).execute(jsonstring);         // new myasynctask(context).cancel(true);      }      class myasynctask extends asynctask<string, integer, arraylist<integer>> {              arraylist<integer> routes = new arraylist<integer>();              @override             protected arraylist<integer> doinbackground(string... params) {                 // todo auto-generated method stub                 bufferedreader reader = null;                  try {                      url myurl = new url(                             "https://bustracker.rhcloud.com/webapi/test");                     httpurlconnection conn = (httpurlconnection) myurl                             .openconnection();                     conn.setrequestmethod("get");                     conn.setconnecttimeout(10000);                     conn.setreadtimeout(10000);                     conn.connect();                      stringbuilder sb = new stringbuilder();                     reader = new bufferedreader(new inputstreamreader(                             conn.getinputstream()));                     string line;                      while ((line = reader.readline()) != null) {                         sb.append(line + "\n");                      }                     system.out.println("stringbuilder: " +sb);                      gson gson = new gson();                     jsonstore data = gson.fromjson(sb.tostring(), jsonstore.class);                     routes = data.getroutes();                     system.out.println("the output of routes: " +routes);                    } catch (ioexception e) {                      e.printstacktrace();                     return null;                 } {                     if (reader != null) {                         try {                             reader.close();                             return null;                         } catch (exception e) {                             // todo auto-generated catch block                             e.printstacktrace();                         }                     }                 }                  return null;              }              protected void onpostexecute(arraylist<integer> result) {                  if (routes != null && !routes.isempty()) {                     callback.onasynctaskfinished(routes);                 } else {                     log.e("123", "avoiding null pointer in availableroutes class, routes null!!!");                 }             }          } } 

java.lang.nullpointerexception: attempt invoke interface method 'void com.bustracker.asynctaskcallback.onasynctaskfinished(java.util.arraylist)' on null object reference

looks interface instance null. initialize asynctaskcallback instance.