android - Trouble returning a subclass of fragment as a fragment -

i have class named singlefragmentactivity want class workoutactivity extend. when override abstract method createfragment code in method, error, , when run code, ---

error:(12, 8) error: workoutactivity not abstract , not override abstract method createfragment() in singlefragmentactivity error:(16, 24) error: createfragment() in workoutactivity cannot override createfragment() in singlefragmentactivity return type android.app.fragment not compatible android.support.v4.app.fragment error:(15, 5) error: method not override or implement method supertype error:(20, 43) error: incompatible types: workoutfragment cannot converted fragment

i believe code correct, extend class , override method in same way in several other places in app. here code involved functions.

public class workoutfragment extends fragment {     public static final string extra_alreadycreated_id = "alreadycreated";      private exadapter adapter;     private listview listview;     private arraylist<set> sets;     private workout w;      public void oncreate(bundle savedinstancestate) {         super.oncreate(savedinstancestate);         uuid workoutid = (uuid) getarguments().getserializable(extra_alreadycreated_id);         w = workoutmaster.get(getactivity()).getworkout(workoutid);     }       public view oncreateview(layoutinflater inflater, viewgroup parent,bundle savedinstancestate)     {         view v = inflater.inflate(r.layout.fragment_workout, parent, false);         return v;      }     public static workoutfragment newinstance(uuid workoutid)     {         bundle args = new bundle();         args.putserializable(extra_alreadycreated_id, workoutid);         workoutfragment fragment = new workoutfragment();         fragment.setarguments(args);          return fragment;     } }  public class workoutactivity extends singlefragmentactivity {      @override     protected fragment createfragment()     {         uuid createdworkoutid = (uuid) getintent().getserializableextra(workoutfragment.extra_alreadycreated_id);          return workoutfragment.newinstance(createdworkoutid);     } }   public abstract class singlefragmentactivity extends fragmentactivity {     protected abstract fragment createfragment();      protected void oncreate(bundle savedinstancestate) {         super.oncreate(savedinstancestate);         setcontentview(r.layout.activity_fragment);          fragmentmanager fm = getsupportfragmentmanager(); //calling support part make compatible honeycomb devices...         //otherwise, call getfragmentmanager         fragment fragment = fm.findfragmentbyid(r.id.fragmentcontainer);          if(fragment == null)         {             fragment =  createfragment();             fm.begintransaction().add(r.id.fragmentcontainer, fragment).commit();         }     } } 

i've excluded imports, exist in project. before run project, error occurs @ return statement of createfragment() in workoutactivity. appreciate help.