Gestione dei frammenti su un'activity, con codice per l'espansione di un fragment.

Non ho lasciato nessun promemoria scritto di come caricare più fragments in un'activity scambiandone uno su comando dell'altro.
Ecco:

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:background="#AAFFFF"
    android:orientation="horizontal"
    android:id="@+id/container"
    android:layout_height="match_parent"
    android:layout_width="match_parent">

    <LinearLayout
        android:id="@+id/barra"
        android:orientation="horizontal"
        android:layout_width="0dp"
        android:layout_height="match_parent"
        android:layout_weight="2"/>

    <LinearLayout
        android:id="@+id/display"
        android:orientation="horizontal"
        android:layout_width="0dp"
        android:layout_height="match_parent"
        android:layout_weight="4"/>


</LinearLayout> 

public class MainActivity extends AppCompatActivity implements Frammento.OnFragmentInteractionListener{

    Frammento myFrag;
    FragmentManager fragmentManager;
    FragmentTransaction transaction;
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        myFrag=new Frammento();
        fragmentManager=getSupportFragmentManager();
        transaction=fragmentManager.beginTransaction();
        transaction.add(R.id.barra,myFrag,"barra");
        transaction.commit();

        Frammento2 frag2=new Frammento2();
        transaction=fragmentManager.beginTransaction();
        transaction.add(R.id.display,frag2,"display");
        transaction.commit();

    }

    public void onFragmentInteraction(){
        Frammento2 frammento2=(Frammento2)fragmentManager.findFragmentByTag("display");
        Button bottone=new Button(getApplicationContext());
        LinearLayout mainLayout=(LinearLayout)frammento2.getView().findViewById(R.id.frammento2);
        mainLayout.addView(bottone);
    }

}

---

---

Fragments:

<?xml version="1.0" encoding="utf-8"?>
<ScrollView xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_height="match_parent"
    android:layout_width="0dp"
    android:layout_weight="1"
    android:background="#FFFFFF"
    android:id="@+id/frammento">


    <Button
        android:text="Button"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:id="@+id/button2" />
</ScrollView> 

public class Frammento extends Fragment {

    OnFragmentInteractionListener mListener;
    Button button;
    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        ViewGroup rootView = (ViewGroup) inflater.inflate(
                R.layout.frammento, container, false);
        button=(Button)rootView.findViewById(R.id.button2);
        button.setOnClickListener(new View.OnClickListener(){

            @Override
            public void onClick(View v) {
                mListener.onFragmentInteraction();
            }
        });
        return rootView;
    }

    @Override
    public void onAttach(Context context){
        super.onAttach(context);
        if(context instanceof OnFragmentInteractionListener){
            mListener=(OnFragmentInteractionListener)context;
        }
    }

    public interface OnFragmentInteractionListener{
        public void onFragmentInteraction();
    }
}

---

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical"
    android:layout_width="0dp"
    android:layout_height="match_parent"
    android:layout_weight="4"
    android:id="@+id/frammento2"
    android:background="#FAA">

    <TextView
        android:text="SECONDO FRAMMENTO"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:id="@+id/textView" />
</LinearLayout> 

public class Frammento2 extends Fragment {

    OnFragmentInteractionListener mListener;
    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState){
        View v=inflater.inflate(R.layout.frammento2,container,false);
        return v;
    }

    @Override
    public void onAttach(Context context){
        super.onAttach(context);
        if(context instanceof OnFragmentInteractionListener){
            mListener=(OnFragmentInteractionListener)context;
        }
        System.out.println("FRAMMENTO 2 ONATTACH");

    }


}

---

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical"
    android:layout_width="0dp"
    android:layout_height="match_parent"
    android:background="#AFA"
    android:layout_weight="4">



    <TextView
        android:text="TERZO FRAMMENTO"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:id="@+id/textView2" />

    <Button
        android:text="Button"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:id="@+id/button4" />
</LinearLayout> 

public class Frammento3 extends Fragment {

    OnFragmentInteractionListener mListener;

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState){
        View v=inflater.inflate(R.layout.frammento3,container,false);
        return v;
    }

    @Override
    public void onAttach(Context context){
        super.onAttach(context);
        if(context instanceof OnFragmentInteractionListener){
            mListener=(OnFragmentInteractionListener)context;
        }
    }
}