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POfindFragmentByTag() returns null after perform a FragmentTransaction using replace() method
 

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    <p>My Android app consists three <strong>fragments: A, B and C</strong>. They're loaded in the two containers defined in the <code>MainActivity</code> layout. </p> <p>When the app is started, it shows the <strong>fragmentA loaded in the left_container</strong> and the <strong>fragmentC in the right_container</strong>. </p> <p>If you press the button in the <strong>fragmentA</strong>, a <code>FragmentTransaction</code> changes <strong>FragmentC</strong> by <strong>FragmentB</strong>. </p> <p>At the moment everything OK. <strong>But the trouble appears when I try to get a reference to the loaded fragmentB using</strong> <code>findFragmentByTag()</code>, because it returns <code>null</code>. I've used the method replace in the <code>FragmentTransaction</code> and I've finished it with <code>commit()</code>, but there isn't way to call <strong>FragmentB</strong> method. My code:</p> <p>MainActivity.java:</p> <pre><code> 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(); } } </code></pre> <p>FragmentB.java:</p> <pre><code>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!"); } } </code></pre> <p>activity_main.xml:</p> <pre><code>&lt;?xml version="1.0" encoding="utf-8"?&gt; &lt;LinearLayout xmlns:android="http://schemas.android.com/apk/res/android" android:layout_width="match_parent" android:layout_height="match_parent" android:orientation="horizontal" &gt; &lt;FrameLayout android:id="@+id/left_container" android:layout_width="0px" android:layout_weight="50" android:layout_height="match_parent"/&gt; &lt;FrameLayout android:id="@+id/right_container" android:layout_width="0px" android:layout_weight="50" android:layout_height="match_parent"/&gt; &lt;/LinearLayout&gt; </code></pre> <p>fragment_b.xml:</p> <pre><code>&lt;?xml version="1.0" encoding="utf-8"?&gt; &lt;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"&gt; &lt;TextView android:id="@+id/text_fragment_b" android:text="It doesn't works!" android:layout_width="match_parent" android:layout_height="wrap_content"/&gt; &lt;/LinearLayout&gt; </code></pre> <p>Please help me! I'm a beginner in Android development! </p>
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