android - getActionBar() returning null in FragmentActivity -

i trying build android tab layout swipeable views using this tutorial.

my fragmentactivity :

public class bookdetailsactivity extends fragmentactivity implements actionbar.tablistener {     private viewpager viewpager;     private tabspageradapter madapter;     private actionbar actionbar;     // tab titles     private string[] tabs = {"description", "find book"};     static arraylist<string> book;      @override     protected void oncreate(bundle savedinstancestate) {         super.oncreate(savedinstancestate);         getwindow().requestfeature(window.feature_action_bar);         setcontentview(r.layout.activity_books_details);          book = new arraylist<string>();         book = (arraylist<string>) getintent().getserializableextra("bookmap");          // initilization         viewpager = (viewpager) findviewbyid(r.id.pager);         actionbar = getactionbar();         madapter = new tabspageradapter(getsupportfragmentmanager(), book, getapplicationcontext());          viewpager.setadapter(madapter);         actionbar.setnavigationmode(actionbar.navigation_mode_tabs);          // adding tabs         (string tab_name : tabs) {             actionbar.addtab(actionbar.newtab().settext(tab_name)                     .settablistener(this));         }          /**          * on swiping viewpager make respective tab selected          * */         viewpager.setonpagechangelistener(new viewpager.onpagechangelistener() {              @override             public void onpageselected(int position) {                 // on changing page                 // make respected tab selected                 actionbar.setselectednavigationitem(position);             }              @override             public void onpagescrolled(int arg0, float arg1, int arg2) {             }              @override             public void onpagescrollstatechanged(int arg0) {             }         });     }      public static arraylist<string> getrequireddata() {         return book;     }      @override     public void ontabreselected(actionbar.tab tab, fragmenttransaction ft) {     }      @override     public void ontabselected(actionbar.tab tab, fragmenttransaction ft) {         // on tab selected         // show respected fragment view         viewpager.setcurrentitem(tab.getposition());     }      @override     public void ontabunselected(actionbar.tab tab, fragmenttransaction ft) {     }  } 

but getting error in:

actionbar.setnavigationmode(actionbar.navigation_mode_tabs);  

the error says:

attempt invoke virtual method 'void android.app.actionbar.setnavigationmode(int)' on null object reference 

why getting error?

android studio: actionbar.tablistener not work appcompatactivity?

actionbar.tablistener deprecated interface.i hope not use in future.

and, actionbartabs can replaced by:

• tablayout android design support library, or without viewpager; or

• a viewpager , favorite other tab solution it, whether pagertabstrip or any number of third-party tab implementations; or

• fragmenttabhost

like said before in answer: https://stackoverflow.com/a/35203962/4409113

but fixing issue, that's because actionbar not enabled right theme , extending fragmentactivity.

please check link: attempt invoke virtual method 'void android.app.actionbar.setdisplayhomeasupenabled(boolean)' on null object reference

use appcompatactivity instead add actionbar , if used it, need too:

actionbar actionbar = getsupportactionbar();