Android using fragments add to back stack wont work -

i building calculator app using fragment , want calculator fragment show when button pressed on menu item fragment. have been using addtobackstac(null) not working me.

here code below

public class mainactivity extends actionbaractivity {

fragmentmanager fragmentmanager;   protected void oncreate(bundle savedinstancestate) {     super.oncreate(savedinstancestate);     setcontentview(r.layout.activity_main);     if(savedinstancestate == null){     fragmentmanager = getfragmentmanager();     fragmenttransaction fragmenttransaction = fragmentmanager.begintransaction().addtobackstack(null);     calculatorfragment calc = new  calculatorfragment();     fragmenttransaction.add(r.id.rellejaut, calc);     fragmenttransaction.commit();     }  }  @override public boolean oncreateoptionsmenu(menu menu) {     // inflate menu; adds items action bar if present.     menuinflater inflater = getmenuinflater();     inflater.inflate(r.menu.main, menu);     return super.oncreateoptionsmenu(menu); }  @override public boolean onoptionsitemselected(menuitem item) {     // handle action bar item clicks here. action bar     // automatically handle clicks on home/up button, long     // specify parent activity in androidmanifest.xml.     int id = item.getitemid();     if (id == r.id.item) {          about= new about();         // insert fragment replacing existing fragment          fragmentmanager.begintransaction().addtobackstack(null).replace(r.id.rellejaut, about).commit();            return true;     }     return super.onoptionsitemselected(item); } 

}

you need first add() or replace() fragment in fragmenttransaction , call addtobackstack() work.
call way:

fragmentmanager.begintransaction().replace(r.id.rellejaut,about).addtobackstack(null).commit();