java - Android updating Activity when onBackPressed is called on another Activity -

as title suggests, aim update variables on 1 activity "mainactivity" when activity"addactivity" being pressed. @ moment, way can update variables on mainactivity when create new instance of it, don't want do. want update of variables happen on mainactivity instantiated. snippets of code below:

public class addactivity extends mainactivity {      ....     @override     public void onbackpressed() {         super.onbackpressed();         updatemain();     } }  public class mainactivity extends activity {     ....         // gets executed when activity gets created     @override     protected void onstart() {         super.onstart();         log.d("tag", "main onstart");         updatemain();     }      protected void updatemain() {             dbadapter dba = new dbadapter(this);         dba.open();         ....         double cost = dba.getcost();         string coststring = string.valueof(cost);         textview.settext(coststring);         dba.close();     }    } 

the addactivity extends mainactivity, when call updatemain() in addactivity, call addactivity's updatemain not mainactivity's, take no effect.

public class addactivity extends mainactivity {      ....     @override     public void onbackpressed() {         super.onbackpressed();         updatemain();// this.updatemain, while 'this' here means addactivity instance.     } } 

i think addactivity extends mainactivity not must(maybe...). can write data sharepreference, ant read mainactivty.