android - viewpager = (ViewPager)findViewById(R.id.pager); returns NULL, how to use .addNext() and .showNext() -

i'm trying instantiate viewpager in app can't find way make viewpager object non-null. everytime call viewpager = (viewpager)findviewbyid(r.id.pager); oncreate() method in main activity, viewpager object returned null. why this?

i'm doing view manipulation in asynctask thread launches before , displays splash screen in addition loading files. in thread manipulate view using viewswitcher object , built in methods such .addview().
loadnotefiles.java

protected void onpreexecute(){     playfrets.viewswitch = new viewswitcher(mcontext);             playfrets.viewswitch.addview(viewswitcher.inflate(mcontext, r.layout.splash_screen, null));             instance = (activity)mcontext;  //cast context main activity activity access setcontentview method             instance.setcontentview(playfrets.viewswitch);         } ...      @override         protected void onpostexecute(void v){             playfrets.viewswitch.addview(viewswitcher.inflate(mcontext, r.layout.activity_play_frets, null));             playfrets.viewswitch.shownext();             ((playfrets)mcontext).configfretboard();         } 

should trying add each menu fragment view viewswitcher object using .addview()? causing viewpager null? i'm not clear on when , how need use viewswitcher object , methods .addview() , .shownext(), background explanation wonderful.below xml files , relevant code main activity.

playfrets.java

@override     protected void oncreate(bundle savedinstancestate) {         setrequestedorientation(activityinfo.screen_orientation_portrait);  //sets activity orientation portrait         //build gui         super.oncreate(savedinstancestate);         requestwindowfeature(window.feature_no_title);           //removes action bar display         getwindow().setflags(windowmanager.layoutparams.flag_fullscreen, windowmanager.layoutparams.flag_fullscreen);  //removes status bar display           //create asynctask load note files. splash screen displayed while task executing         new loadnotefiles(this).execute();          //set menu swipe capability         //implements viewpager class         viewpager = (viewpager)findviewbyid(r.id.pager); //viewpager null here         pageradapter pageadapter = new menupageradapter(getsupportfragmentmanager());         viewpager.setadapter(pageadapter);   //causes nullpointer exception here     } 

activity_play_frets.xml

<linearlayout     xmlns:android="http://schemas.android.com/apk/res/android"     android:layout_width="match_parent"     android:layout_height="match_parent"      android:orientation="horizontal"     android:baselinealigned="false"     android:keepscreenon="true">      <linearlayout             android:layout_width="0dp"         android:layout_height="match_parent"         android:layout_weight="1"         android:background="@drawable/gold_guitarsim_text"         android:orientation="vertical"          android:id="@+id/toolbargestureoverlay" >         <android.support.v4.view.viewpager             xmlns:android="http://schemas.android.com/apk/res/android"             android:id="@+id/pager"             android:layout_width="match_parent"             android:layout_height="match_parent">         </android.support.v4.view.viewpager>     </linearlayout>        <linearlayout         android:layout_width="0dp"         android:layout_height="wrap_content"         android:orientation="vertical"          android:layout_weight="4">       <com.example.guitarsim.multitouch         android:id="@+id/fretboard"           android:layout_width="wrap_content"         android:layout_height="match_parent"         android:background="@drawable/fret_board1" />     </linearlayout>     </linearlayout>  

sample menu fragment xml

<?xml version="1.0" encoding="utf-8"?> <linearlayout xmlns:android="http://schemas.android.com/apk/res/android"     android:layout_width="match_parent" android:layout_height="match_parent"> </linearlayout> 

call

setcontentview(r.layout.activity_play_frets)  

after getwindow() function