java - Why findViewById() doesn't work as getActivity().findViewById() here? -

    /**      * placeholder fragment containing simple view.      */     public static class placeholderfragment extends fragment {          public placeholderfragment() {         }          @override         public view oncreateview(layoutinflater inflater, viewgroup container,                 bundle savedinstancestate) {             view rootview = inflater.inflate(r.layout.fragment_detail,                     container, false);             // detail activity called via intent.  inspect intent forecast data.             intent intent=getactivity().getintent();             textview text=(textview)rootview.findviewbyid(r.id.text1);             string b=intent.getstringextra(intent.extra_text);             text.settext(b);              return rootview;         }     } 

in code when using

textview text=(textview)getactivity().findviewbyid(r.id.text1) 

then app crashes , logcat says that

caused by: java.lang.nullpointerexception: attempt invoke virtual method 'android.view.view android.view.window.findviewbyid(int)' on null object reference 

this xml file associated code

<relativelayout xmlns:android="http://schemas.android.com/apk/res/android"     xmlns:tools="http://schemas.android.com/tools"     android:layout_width="match_parent"     android:layout_height="match_parent"     android:paddingbottom="@dimen/activity_vertical_margin"     android:paddingleft="@dimen/activity_horizontal_margin"     android:paddingright="@dimen/activity_horizontal_margin"     android:paddingtop="@dimen/activity_vertical_margin"     tools:context="com.example.sunshine.app.detailactivity$placeholderfragment" >      <textview         android:id="@+id/text1"         android:layout_width="wrap_content"         android:layout_height="wrap_content"         android:text="@string/hello_world" />  </relativelayout> 

in code when using textview text=(textview)getactivity().findviewbyid(r.id.text1)

then app crashes , logcat says caused by: java.lang.nullpointerexception: attempt invoke virtual method 'android.view.view android.view.window.findviewbyid(int)' on null object reference

findviewbyid() on activity starts root view (what referred "decor view") , traverses tree of available widgets , containers, looking id match. hence, findviewbyid() called on activity find views part of overall activity view hierarchy.

however, inflate() call not attach inflated relativelayout activity view hierarchy. expected. views attached activity view hierarchy when fragment attached, sometime after return oncreateview(). result, when call findviewbyid() on activity from inside oncreateview(), cannot find textview, because textview being managed fragment , not yet part of activity view hierarchy.

findviewbyid() on view or viewgroup starts view or viewgroup , traverses tree of available widgets , containers, looking id match. relativelayout has access textview, when call findviewbyid() on relativelayout inflate, works.