android - How to add a button in a Fragment? -

i have activity in want add button.

i've tried without luck. since i'm beginner in programming , have little experience, appeal help. sorry english. in advance.

    import android.app.actionbar;     import android.app.activity;     import android.app.fragment;     import android.app.fragmentmanager;     import android.content.intent;     import android.os.bundle;     import android.support.v4.widget.drawerlayout;     import android.view.layoutinflater;     import android.view.menu;     import android.view.menuitem;     import android.view.view;     import android.view.viewgroup;     import android.widget.button;       public class mainactivity extends activity         implements navigationdrawerfragment.navigationdrawercallbacks {         public charsequence mtitle;       protected void oncreatea(bundle savedinstancestate) {         super.oncreate(savedinstancestate);         setcontentview(r.layout.main);            mnavigationdrawerfragment = (navigationdrawerfragment)                                                                                                 getfragmentmanager().findfragmentbyid(r.id.navigation_drawer);           mtitle = gettitle();          // set drawer.         mnavigationdrawerfragment.setup(                 r.id.navigation_drawer,                 (drawerlayout) findviewbyid(r.id.drawer_layout));          }           @override         public void onnavigationdraweritemselected(int position) {         // update main content replacing fragments         fragmentmanager fragmentmanager = getfragmentmanager();         fragmentmanager.begintransaction()                 .replace(r.id.container,          placeholderfragment.newinstance(position + 1))                 .commit();     }      public void onsectionattached(int number) {         switch (number) {             case 1:                 mtitle = getstring(r.string.title_section1);                  break;             case 2:                 mtitle = getstring(r.string.title_section2);                 /*intent intent = new intent(this, cabinet.class);                 startactivity(intent);*/                  break;             case 3:                 mtitle = getstring(r.string.title_section3);                 break;             case 4:                 mtitle = getstring(r.string.title_section4);                 break;             case 5:                 mtitle = getstring(r.string.title_section5);                 break;         }     }      public void restoreactionbar() {         actionbar actionbar = getactionbar();         actionbar.setnavigationmode(actionbar.navigation_mode_standard);         actionbar.setdisplayshowtitleenabled(true);         actionbar.settitle(mtitle);     }       @override     public boolean oncreateoptionsmenu(menu menu) {         if (!mnavigationdrawerfragment.isdraweropen()) {             // show items in action bar relevant screen             // if drawer not showing. otherwise, let drawer             // decide show in action bar.             getmenuinflater().inflate(r.menu.main, menu);             restoreactionbar();             return true;         }         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();          //noinspection simplifiableifstatement         if (id == r.id.action_settings) {             return true;         }          return super.onoptionsitemselected(item);     }       /**      * placeholder fragment containing simple view.      */     public static class placeholderfragment extends fragment {         /**          * fragment argument representing section number          * fragment.          */          private static final string arg_section_number = "section_number";          /**          * returns new instance of fragment given section          * number.          */         public static placeholderfragment newinstance(int sectionnumber) {             placeholderfragment fragment = new placeholderfragment();             bundle args = new bundle();             args.putint(arg_section_number, sectionnumber);             fragment.setarguments(args);             return fragment;         }          public placeholderfragment() {         }          @override         public view oncreateview(layoutinflater inflater,                 container,                                  bundle savedinstancestate) {             view rootview = inflater.inflate(r.layout.fragment_main, container, false);             button button = (button) rootview.findviewbyid(r.id.button);             button.setonclicklistener(new view.onclicklistener() {                 @override                 public void onclick(view v) {                     switch (v.getid()) {                         case r.id.button:                              intent intent = new intent( placeholderfragment , categories.class);                             startactivity(intent);                             break;                      }                  }              });             return rootview;          }             @override         public void onattach(activity activity) {             super.onattach(activity);              ((mainactivity) activity).onsectionattached(                     getarguments().getint(arg_section_number));           }       }      }