title: Why zero fragment in backstack
body: <p>My simple <strong>layout</strong> only has a fragment placeholder:</p>
  
  <pre><code>&lt;FrameLayout
     android:id="@+id/fragment_placeholder"
     android:layout_width="fill_parent"
     android:layout_height="fill_parent"
    /&gt;
  </code></pre>
  
  <p>I firstly add a <strong>1st fragment</strong> to this placeholder:</p>
  
  <pre><code>fragmentTransaction.add(R.id.fragment_placeholder, firstFragment, "first"); //I did not put to backstack
  </code></pre>
  
  <p>I have a <strong>2nd fragment</strong>, which replace the above fragment and <strong>put it to back stack</strong>:</p>
  
  <pre><code>FragmentManager fragMgr = getSupportFragmentManager();
  FragmentTransaction fragTrans = fragMgr.beginTransaction();
  
  //initialize an fragment instance
  Fragment secondFragment = initSecondFragment(); 
  
  //replace with the fragment 
  fragTrans.replace(R.id.fragment_placeholder, secondFragment, "second");
  
  //Add transaction to back stack
  fragTrans.addToBackStack(null);
  
  //commit the transaction
  fragTrans.setTransition(FragmentTransaction.TRANSIT_FRAGMENT_FADE);
  fragTrans.commit();  
  
  //The following log returns me 0 when counting the number of fragments in back stack, why?  
  Log.v("nr of fragment in back stack", fragMgr.getBackStackEntryCount()+"");
  </code></pre>
  
  <p>But I end up with <strong>0</strong> fragment in back stack, why??? </p>