java - set adapter null pointer -

i have been trying solve problem few hours now, failed figure out.i try set adapter expandable list view take error. thanks. "attempt invoke virtual method 'void android.widget.expandablelistview.setadapter(android.widget.expandablelistadapter)' on null object reference"

main_activity

 public class mainactivity extends appcompatactivity implements view.onclicklistener {      button b1;       private residemenu residemenu;     private context mcontext;     private residemenuitem itemanasayfa;     private residemenuitem itemrastgele;     private residemenuitem itemencok;     private residemenuitem itemtarifyaz;      private expandablelistview listview;     private expandablelistadapter listadapter;     private list<string> listdataheader;     private hashmap<string,list<string>> listhash;        @override     protected void oncreate(bundle savedinstancestate) {          listview=(expandablelistview)findviewbyid(r.id.exp2);         initdata();         listadapter=new com.yeni.expandablelistadapter(this,listdataheader,listhash);         listview.setadapter(listadapter);          super.oncreate(savedinstancestate);         setcontentview(r.layout.activity_main);         mcontext = this;         setupmenu();         if (savedinstancestate == null) {              changefragment(new f_ana_sayfa());          }      }      private void initdata() {         listdataheader=new arraylist<>();         listhash=new hashmap<>();          listdataheader.add("et");         listdataheader.add("tavuk");         listdataheader.add("sebze");         listdataheader.add("diger");          list<string> l_et=new arraylist<>();         l_et.add("kemikli");         l_et.add("kuşbaşı");          list<string> l_tavuk=new arraylist<>();         l_tavuk.add("bonfile");         l_tavuk.add("but");         l_tavuk.add("bütün tavuk");          list<string> l_sebze=new arraylist<>();         l_sebze.add("patates");         l_sebze.add("biber");         l_sebze.add("kabak");         l_sebze.add("patlıcan");          list<string> l_diger=new arraylist<>();         l_diger.add("tuz");         l_diger.add("yağ");         l_diger.add("kekik");          listhash.put(listdataheader.get(0),l_et);         listhash.put(listdataheader.get(1),l_tavuk);         listhash.put(listdataheader.get(2),l_sebze);         listhash.put(listdataheader.get(3),l_diger);      }      private void setupmenu() {         residemenu = new residemenu(this);         residemenu.setbackground(r.drawable.menu_background);         residemenu.attachtoactivity(this);           residemenu.setmenulistener(menulistener);         residemenu.setscalevalue(0.6f);         //create menu items;         itemanasayfa = new residemenuitem(this, r.drawable.icon_home, "ana sayfa");         itemrastgele = new residemenuitem(this, r.drawable.icon_profile, "random tarif bul");         itemencok = new residemenuitem(this, r.drawable.icon_calendar, "en cok begenilenler");         itemtarifyaz = new residemenuitem(this, r.drawable.icon_settings, "tarif yaz");           itemanasayfa.setonclicklistener(this);         itemrastgele.setonclicklistener(this);         itemencok.setonclicklistener(this);         itemtarifyaz.setonclicklistener(this);           residemenu.addmenuitem(itemanasayfa, residemenu.direction_left);         residemenu.addmenuitem(itemrastgele, residemenu.direction_left);         residemenu.addmenuitem(itemencok, residemenu.direction_right);         residemenu.addmenuitem(itemtarifyaz, residemenu.direction_right);          findviewbyid(r.id.title_bar_left_menu).setonclicklistener(new view.onclicklistener() {              @override              public void onclick(view view) {                  residemenu.openmenu(residemenu.direction_left);             }         });          findviewbyid(r.id.title_bar_right_menu).setonclicklistener(new view.onclicklistener() {              @override             public void onclick(view view) {                  residemenu.openmenu(residemenu.direction_right);             }         });     }      @override      public boolean dispatchtouchevent(motionevent ev) {         return residemenu.dispatchtouchevent(ev);     }      @override     public void onclick(view view) {          if (view == itemanasayfa) {             changefragment(new f_ana_sayfa());         } else if (view == itemrastgele) {             changefragment(new f_rastgele_tarif());         } else if (view == itemencok) {             changefragment(new f_encok_beg());         } else if (view == itemtarifyaz) {             changefragment(new f_tarif_yaz());         }         else             changefragment(new f_ana_sayfa());           residemenu.closemenu();     }       private residemenu.onmenulistener menulistener = new residemenu.onmenulistener() {         @override         public void openmenu() {             toast.maketext(mcontext, "menu opened!", toast.length_short).show();          }          @override         public void closemenu() {              toast.maketext(mcontext, "menu closed!", toast.length_short).show();         }       };         private void changefragment(fragment targetfragment) {         residemenu.clearignoredviewlist();         getsupportfragmentmanager()                 .begintransaction()                 .replace(r.id.main_fragment, targetfragment, "fragment")                 .settransitionstyle(fragmenttransaction.transit_fragment_fade)                 .commit();     }      public residemenu getresidemenu(){         return residemenu;} } 

adapter class

    public class expandablelistadapter extends baseexpandablelistadapter {      private context cont;     private list<string> listdataheader;     private hashmap<string,list<string>> listhashmap;      public expandablelistadapter(context cont, list<string> listdataheader, hashmap<string, list<string>> listhashmap) {         this.cont = cont;         this.listdataheader = listdataheader;         this.listhashmap = listhashmap;     }      @override     public int getgroupcount() {         return listdataheader.size();     }      @override     public int getchildrencount(int i) {         return listhashmap.get(listdataheader.get(i)).size();     }      @override     public object getgroup(int i) {         return listdataheader.get(i);     }      @override     public object getchild(int i, int i1) { // i=group position i=item position         return listhashmap.get(listdataheader.get(i)).get(i1);     }      @override     public long getgroupid(int i) {         return i;     }      @override     public long getchildid(int i, int i1) {         return i1;     }      @override     public boolean hasstableids() {         return false;     }      @override     public view getgroupview(int i, boolean b, view view, viewgroup viewgroup) {         string headertitle=(string)getgroup(i);          if (view==null){             layoutinflater inflater1=(layoutinflater)this.cont.getsystemservice(context.layout_inflater_service);             view=inflater1.inflate(r.layout.list_group,null);          }          textview lblheader=(textview)view.findviewbyid(r.id.header);         lblheader.settypeface(null, typeface.bold);         lblheader.settext(headertitle);         return view;     }      @override     public view getchildview(int i, int i1, boolean b, view view, viewgroup viewgroup) {         final string childtext=(string)getchild(i,i1);          if (view==null){             layoutinflater inflater1=(layoutinflater)this.cont.getsystemservice(context.layout_inflater_service);             view=inflater1.inflate(r.layout.list_item,null);          }          textview txtlistchild=(textview)view.findviewbyid(r.id.list_item);         txtlistchild.settext(childtext);         return view;     }      @override     public boolean ischildselectable(int i, int i1) {         return true;     } } 

activity_main xml

  
  
    <?xml version="1.0" encoding="utf-8"?>        <linearlayout xmlns:android="http://schemas.android.com/apk/res/android"          android:background="@android:color/white"          android:orientation="vertical"          android:layout_width="match_parent"          android:layout_height="match_parent">            <expandablelistview              android:layout_width="match_parent"              android:layout_height="match_parent"              android:id="@+id/exp2" />            <linearlayout              android:orientation="vertical"              android:layout_width="fill_parent"              android:layout_height="wrap_content"              android:id="@+id/layout_top">                <!--<imageview-->                  <!--android:layout_width="match_parent"-->                  <!--android:layout_height="3dp"-->                  <!--android:background="#2ea3fe"/>-->                <framelayout                  android:layout_width="fill_parent"                  android:layout_height="wrap_content">                    <button                      android:layout_width="28dp"                      android:layout_height="28dp"                      android:background="@drawable/titlebar_menu_selector"                      android:id="@+id/title_bar_left_menu"                      android:layout_gravity="left|center_vertical"                      android:layout_marginleft="10dp"/>                    <textview                      android:layout_width="wrap_content"                      android:layout_height="wrap_content"                      android:padding="7dp"                      android:text="yemek do"                      android:textsize="24sp"                      android:layout_gravity="center"                      android:textcolor="?attr/actionmodesplitbackground" />                    <button                      android:layout_width="28dp"                      android:layout_height="28dp"                      android:background="@drawable/titlebar_menu_selector"                      android:id="@+id/title_bar_right_menu"                      android:layout_gravity="right|center_vertical"                      android:layout_marginright="10dp"/>              </framelayout>                <view                  android:layout_width="match_parent"                  android:layout_height="5dp"                  android:background="#bcb8b8"/>            </linearlayout>            <framelayout              android:layout_width="match_parent"              android:layout_height="match_parent"              android:orientation="vertical"              android:id="@+id/main_fragment">                <relativelayout                  android:layout_width="match_parent"                  android:layout_height="match_parent">                    <button                      android:text="button"                      android:layout_width="wrap_content"                      android:layout_height="wrap_content"                      android:layout_alignparenttop="true"                      android:layout_alignparentleft="true"                      android:layout_alignparentstart="true"                      android:layout_marginleft="107dp"                      android:layout_marginstart="107dp"                      android:layout_margintop="188dp"                      android:id="@+id/button2" />                    <button                      android:text="button"                      android:layout_width="wrap_content"                      android:layout_height="wrap_content"                      android:layout_alignparenttop="true"                      android:layout_alignright="@+id/button2"                      android:layout_alignend="@+id/button2"                      android:layout_margintop="78dp"                      android:id="@+id/button3" />              </relativelayout>          </framelayout>        </linearlayout>
  
 
 
  
what doing:
  
 listview=(expandablelistview)findviewbyid(r.id.exp2); initdata(); listadapter=new com.yeni.expandablelistadapter(this,listdataheader,listhash); listview.setadapter(listadapter);  super.oncreate(savedinstancestate); setcontentview(r.layout.activity_main); 
  
you intialize listview object before setcontentview wrong. should this
  
super.oncreate(savedinstancestate); setcontentview(r.layout.activity_main); listview=(expandablelistview)findviewbyid(r.id.exp2); initdata(); listadapter=new            com.yeni.expandablelistadapter(this,listdataheader,listhash); listview.setadapter(listadapter);