android - addToBackStack() didn't work. Back to home not previous fragment -

i'm pretty new android. when try use addtobackstack(), encountered problem.

when press button in fragment 2, didn't go fragment 1 home.

why? there wrong in code? thanks!

maiinactivity.java

public class mainactivity extends actionbaractivity {      @override     protected void oncreate(bundle savedinstancestate) {         super.oncreate(savedinstancestate);         setcontentview(r.layout.activity_main);          fragmentmanager mfragmentmanager = getfragmentmanager();         fragmenttransaction mfragmenttransaction = mfragmentmanager.begintransaction();          mfragmenttransaction.add(r.id.id_frame, new firstfrag(), "firstfrag");         mfragmenttransaction.commit();       } } 

activity_main.xml

<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.myfragtest2.mainactivity" >      <framelayout          android:id="@+id/id_frame"         android:layout_width="fill_parent"         android:layout_height="fill_parent"          ></framelayout>  </relativelayout> 

firstfrag.java

public class firstfrag extends fragment {       private button button1;     @override     public view oncreateview(layoutinflater inflater, viewgroup container,             bundle savedinstancestate) {          view view = inflater.inflate(r.layout.first_frag, container, false);          button1 = (button)view.findviewbyid(r.id.btn1);         button1.setonclicklistener(new onclicklistener() {              @override             public void onclick(view v) {                 // todo auto-generated method stub                  fragmentmanager myfragmentmanager = getfragmentmanager();                 fragmenttransaction myftransaction = myfragmentmanager.begintransaction();                 myftransaction.replace(r.id.id_frame, new secondfrag());                 myftransaction.addtobackstack(null);                 myftransaction.commit();                }         });          return view;     }   } 

first_frag.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"     android:orientation="vertical" >      <edittext          android:id="@+id/et1"         android:layout_width="fill_parent"         android:layout_height="wrap_content"         android:text="this frag 1!!"         />      <button          android:id="@+id/btn1"         android:layout_width="wrap_content"         android:layout_height="wrap_content"         android:layout_gravity="center_vertical"         android:text="move frag 2"         />   </linearlayout> 

add snippet in main activity

@override public void onbackpressed() {     if (getfragmentmanager().getbackstackentrycount() == 0) {         super.onbackpressed();     } else {         getfragmentmanager().popbackstack();     } } 

and log count of fragments.