android - Passing data from onPostExecute() to adapter -

can't pass data onpostexecute() adapter autocomletetextview. logcat shows me:

an exception occurred during performfiltering()! java.lang.nullpointerexception: collection == null.

public class uzactivity extends activity {  private static final string debug_tag = "httpexample"; list<string> responselist;  @override protected void oncreate(bundle savedinstancestate) {     super.oncreate(savedinstancestate);     setcontentview(r.layout.activity_uz);      final string url = "http://booking.uz.gov.ua/purchase/station/%d0%9a%d0%b8%d0%b5/";     new fetchstationtask().execute(url);         autocompletetextview textview = (autocompletetextview)             findviewbyid(r.id.autocompletetextview1);      arrayadapter<string> adapter = new arrayadapter<string>(this,             android.r.layout.simple_dropdown_item_1line, responselist);      textview.setadapter(adapter);   }  private class fetchstationtask extends asynctask<string, void, string>{      @override     protected string doinbackground(string... urls) {         try {             return new uzfetcher().geturlstring(urls[0]);         } catch (ioexception e) {             return "unable retrieve web page. url may invalid.";         }     }      @override     protected void onpostexecute(string result){          try {             objectmapper objectmapper = new objectmapper();             objectmapper.configure(deserializationfeature.fail_on_unknown_properties, false);             stationresponse st = objectmapper.readvalue(result, stationresponse.class);             responselist = new arraylist<>();             (int = 0; i<st.mstations.size(); i++){                      responselist.add(st.mstations.get(i).gettitle());              }               log.i(debug_tag, responselist.get(0));         } catch (ioexception e) {             e.printstacktrace();         }         log.i(debug_tag, result);      } } 

java.lang.nullpointerexception: collection == null.

arraylist should initialized first.

just add,

responselist = new arraylist<string>();

after setcontentview();