java - simple adapter cant update database -

hey guys have made listview of data database using simple adapter dont know how update everytime database changes.any help?the notifydatasetchanged(); method doesnt work , cant find out do.

mainactivity

      @override public void oncreate(bundle savedinstancestate) {     super.oncreate(savedinstancestate);     setcontentview(r.layout.activity_general_discussion);     db = new sqlitehandler(this);      arraylist<hashmap<string, string>> emaillist = db.getmessage();     if (emaillist.size() != 0) {          listadapter adapter = new simpleadapter(mainactivity.this, emaillist,                 r.layout.raw_layout,                 new string[]{"user_posted", "post", "posted_at", "post_id"}, new int[]{                 r.id.text_user_name, r.id.text_user_post, r.id.text_user_date, r.id.text_user_number});         setlistadapter(adapter);      } 

helperclass

    public arraylist<hashmap<string, string>> getmessage(){     arraylist<hashmap<string, string>> message = new arraylist<hashmap<string, string>>();     string selectquery = "select  * " + table_posting;      sqlitedatabase db = this.getreadabledatabase();     cursor cursor = db.rawquery(selectquery, null);      if ( cursor.movetofirst()) {         {             hashmap<string, string> map = new hashmap<string, string>();             map.put("post_id", cursor.getstring(0));             map.put("user_posted", cursor.getstring(1));             map.put("post", cursor.getstring(2));             map.put("posted_at", cursor.getstring(3));             message.add(map);         } while (cursor.movetonext());     }      return message; } 

declare adapter & list global level

class example{ listadapter adapter; arraylist<hashmap<string, string>> emaillist;   @override public void oncreate(bundle savedinstancestate) {     super.oncreate(savedinstancestate);     setcontentview(r.layout.activity_general_discussion);     db = new sqlitehandler(this);      emaillist = db.getmessage();     if (emaillist.size() != 0) {          adapter = new simpleadapter(mainactivity.this, emaillist,                 r.layout.raw_layout,                 new string[]{"user_posted", "post", "posted_at", "post_id"}, new int[]{                 r.id.text_user_name, r.id.text_user_post, r.id.text_user_date, r.id.text_user_number});         setlistadapter(adapter);      } 

when want update

    adapter.notifydatasetchanged();