android - Source not found. TextView -

i novice in java , android. want change text on activity via settext(). xml-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.p0081viewbyid.mainactivity$placeholderfragment" >      <textview         android:layout_width="wrap_content"         android:layout_height="wrap_content"         android:text="@string/hello_world"         android:id="@+id/mytext" />  </relativelayout> 

this part of java - code :

package com.example.p0081viewbyid;  import android.app.activity; import android.app.fragment; import android.os.bundle; import android.view.layoutinflater; import android.view.menu; import android.view.menuitem; import android.view.view; import android.view.viewgroup; import android.widget.textview;  public class mainactivity extends activity  {      @override     protected void oncreate(bundle savedinstancestate)      {         super.oncreate(savedinstancestate);         setcontentview(r.layout.activity_main);          textview mytextview = (textview)findviewbyid(r.id.mytext);         mytextview.settext("new text in textview");//application close on place          if (savedinstancestate == null)              {             getfragmentmanager().begintransaction()                     .add(r.id.container, new placeholderfragment()).commit();         }     } } 

this application close on settext method cause "source not found". error? tried fix changing such as:

mytextview.settext("new text in textview".tostring()); mytextview.settext(integer.tostring("new text in textview")) 

but same error.