PassengerTravelFrg fragment = new PassengerTravelFrg();
                FragmentTransaction ft = getFragmentManager()
                        .beginTransaction();
            //    if (first == 1) {
                    ft.replace(R.id.travel, fragment).commit();
                //} else {
                //    ft.add(R.id.travel, fragment).commit();
            //        first = 1;
                //}

R.id.travel is id of linear layout which must not have any fragment inside to
add or replace fragments dynamically.replace can also be used for first time
adding.


PassengerTravelFrg fragment =

getSupportFragmentManager().findFragmentById(R.id.travel);

                FragmentTransaction ft = getFragmentManager()
                        .beginTransaction();
           ft.add(R.id.travel, fragment).commit();
            here R.id.travel is id of fragment declared in layout file to add fragment
only once initially.