android - findFragmentByTag() returns null after perform a FragmentTransaction using replace() method

Question

My Android app consists three fragments: A, B and C. They're loaded in the two containers defined in the MainActivity layout.

When the app is started, it shows the fragmentA loaded in the left_container and the fragmentC in the right_container.

If you press the button in the fragmentA, a FragmentTransaction changes FragmentC by FragmentB.

At the moment everything OK. But the trouble appears when I try to get a reference to the loaded fragmentB using findFragmentByTag(), because it returns null. I've used the method replace in the FragmentTransaction and I've finished it with commit(), but there isn't way to call FragmentB method. My code:

MainActivity.java:

 public class MainActivity extends Activity{
 static String fragmentTag = "FRAGMENTB_TAG";

 @Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);


    //Adds the left container's fragment
    getFragmentManager().beginTransaction().add(R.id.left_container, new FragmentA()).commit(); //Adds the fragment A to the left container

    //Adds the right container's fragment
    getFragmentManager().beginTransaction().add(R.id.right_container, new FragmentC()).commit(); //Adds the Fragment C to the right container
 }

 /**
 * Called when the button "Activate Fragment B" is pressed
 */
public void buttonListener(View v){

        FragmentTransaction ft = getFragmentManager().beginTransaction();
        ft.replace(R.id.right_container, new FragmentB(),fragmentTag); //Replaces the Fragment C previously in the right_container with a new Fragment B
        ft.commit(); //Finishes the transaction


        //!!HERE THE APP CRASHES (java.lang.NullPointerException = findFragmentByTag returns null
        ((FragmentB) getFragmentManager().findFragmentByTag(fragmentTag)).testView();


    }
}

FragmentB.java:

public class FragmentB extends Fragment {

public View onCreateView(LayoutInflater inflater,
        ViewGroup container,
        Bundle savedInstanceState) {
    return inflater.inflate(R.layout.fragment_b, container,false);

}


/**
 * Gets a reference to the text_fragment_b TextView and calls its method setText(), changing "It doesn't work" text by "It works!"
 */
public void testView(){
    TextView tv = (TextView)getView().findViewById(R.id.text_fragment_b);
    tv.setText("It works!");
}

}

activity_main.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="horizontal" >

<FrameLayout android:id="@+id/left_container" android:layout_width="0px" android:layout_weight="50" android:layout_height="match_parent"/>    
<FrameLayout android:id="@+id/right_container" android:layout_width="0px" android:layout_weight="50" android:layout_height="match_parent"/>

</LinearLayout>

fragment_b.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" 
android:layout_margin="5sp">

<TextView 
    android:id="@+id/text_fragment_b"
    android:text="It doesn't works!"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"/>

</LinearLayout>

Please help me! I'm a beginner in Android development!

Answer 1

I've fixed it! I called getSupportFragmentManager().executePendingTransactions() after doing the transaction and it worked! After calling that method I can get the fragment using both findFragmentById() and findFragmentByTag() methods.