• activity_main only has a placeholder (framelayout)
• it displays (load) one out of two fragments (FragmentOne, FragmentTwo)
• Fragment One has button to switch to Fragment Two
• Fragment Two has button to switch to Fragment One
• Button click, fragment switch are communicated via interface/listerner MyFragmentDataPassListener

<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <FrameLayout
    android:id="@+id/vv_fragContainer"
    android:layout_width="match_parent"
    android:layout_height="match_parent" />

</androidx.constraintlayout.widget.ConstraintLayout>

<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:background="@android:color/holo_orange_light"
    tools:context=".FragmentOne">

    <TextView
        android:id="@+id/vv_tvOne"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginTop="88dp"
        android:text="Hello First Fragment"
        android:textSize="20sp"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <Button
        android:id="@+id/vv_btnOne01"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginStart="4dp"
        android:layout_marginBottom="116dp"
        android:text="Upper"
        app:layout_constraintBottom_toTopOf="@+id/vv_btnOne02"
        app:layout_constraintStart_toStartOf="@+id/vv_btnOne02" />

    <Button
        android:id="@+id/vv_btnOne02"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginBottom="253dp"
        android:text="Lower"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent" />

</androidx.constraintlayout.widget.ConstraintLayout>	

android:background="@android:color/holo_blue_bright"
@+id/vv_tvTwo
@+id/vv_btnTwo01
@+id/vv_btnTwo02

public interface MyFragmentDataPassListener {
    public void cf_firedByFragment(String str, int source);
}

• create 2 fragements, load via FragmentManager
• implements MyFragmentDataPassListener
• overide method cf_firedByFragment
• cf_firedByFragment() does the fragment switch
• **Here, no .remove(frag01) why? -- not draw frag01 twice, but has problems -- see ver06

import androidx.appcompat.app.AppCompatActivity;
import androidx.fragment.app.Fragment;
import android.os.Bundle;
	
public class MainActivity extends AppCompatActivity implements MyFragmentDataPassListener {
    FragmentOne frag01;
    FragmentTwo frag02;

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

        frag01 = new FragmentOne();
        frag02 = new FragmentTwo();
        setContentFragment(1);
    }

    public static void loadFragment(AppCompatActivity activity, int containerId, Fragment fragment, String tag) {
        activity.getSupportFragmentManager().beginTransaction().
                replace(containerId, fragment,tag).commitAllowingStateLoss();
    }

    public void setContentFragment(int id) {
        switch (id) {
            case 1: loadFragment(this, R.id.vv_fragContainer,frag01,"fragment1");
                break;
            case 2: loadFragment(this, R.id.vv_fragContainer,frag02,"fragment2");
                break;
        }
    }

    @Override
    public void cf_firedByFragment(String str, int source) {
        if (source == 1) {
            setContentFragment(2);
        }
        else if (source == 2) {
            setContentFragment(1);
        }
    }

• MyFragmentDataPassListener instance cv_listener
• @Override onAttach(), assign cv_listener to MainActivity (which implements MyFragmentDataPassListener)
• Recall your JAVA knowledge of interface
• Button click calls cv_listener.cf_firedByFragment(String, int)
• cv_listener.cf_firedByFragment DOES the actually data passing from Fragment to Activity

import android.content.Context;
import android.os.Bundle;
import androidx.fragment.app.Fragment;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.Button;
import android.widget.TextView;

public class FragmentOne extends Fragment {
    TextView cv_tv;
    private MyFragmentDataPassListener cv_listener;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
    }

    @Override
    public void onAttach(Context context) {
        super.onAttach(context);
        if (context instanceof MyFragmentDataPassListener) {
            cv_listener = (MyFragmentDataPassListener) context;
        } else {
            throw new ClassCastException(context.toString() + " must MyFragmentDataPassListener");
        }
    }

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

        cv_tv = (TextView) v.findViewById(R.id.vv_tvOne);

        Button lv_btn01 = (Button) v.findViewById(R.id.vv_btnOne01);
        lv_btn01.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                cv_listener.cf_firedByFragment("UPPER", 1);
            }
        });
        Button lv_btn02 = (Button) v.findViewById(R.id.vv_btnOne02);
        lv_btn02.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                cv_listener.cf_firedByFragment("LOWER", 1);
            }
        });

        return v;
    }
}

almost identical
cv_listener.cf_firedByFragment("#FF0000", 2);
cv_listener.cf_firedByFragment("#00FF00", 2);

The important thing to keep in mind is that fragments should not directly communicate with each other
and should generally only communicate with their parent activity. 

• FragmentOne upper/lower button change cases in FragmentTwo
• FragmentTwo red/blue button change color in FragmentOne
• Fragment commuicate via MainActivity

• call cf_setTextColor() in FragmentOne
• call cf_setTextCase() in FragmentTwo
• trick: getSupportFragmentManager().executePendingTransactions();
• since did not .remove(frag01)

    public void cf_firedByFragment(String str, int source) {
        if (source == 1) {
            setContentFragment(2);

            // this will wait until frag02 onCreateView() done, otherwise null
            getSupportFragmentManager().executePendingTransactions();
            frag02.cf_setTextCase(str);
        }
        else if (source == 2) {
            setContentFragment(1);
            getSupportFragmentManager().executePendingTransactions();
            frag01.cf_setTextColor(str);
        }
    }

   public void cf_setTextColor(String str) {
        cv_tv.setTextColor(Color.parseColor(str));
    }

    public void cf_setTextCase(String str) {
        if (str.equals("UPPER")) {
           cv_tv.setText(cv_tv.getText().toString().toUpperCase());
        }
        else {
           cv_tv.setText(cv_tv.getText().toString().toLowerCase());
        }
    }

• After some fragement switches, press nav back button, what happens?
• It quits the app, NOT backing to any previous fragment
• Solution -- Fragment Backstack Lifecycle

• Activities -- DO NOT use
• Fragments -- the way

	<TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Hello World!"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.043"/>

    <androidx.viewpager2.widget.ViewPager2
        android:id="@+id/viewpager"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        tools:layout_editor_absoluteX="1dp"
        tools:layout_editor_absoluteY="49dp" />

<LinearLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <ImageView
        android:id="@+id/images"
        android:layout_width="match_parent"
        android:layout_height="match_parent" />

</LinearLayout>

    private ViewPager2 viewPager2;

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

        viewPager2 = findViewById(R.id.viewpager);
        viewPager2.setAdapter(new MyViewPager2Adapter(this));
    }

public class MyViewPager2Adapter extends RecyclerView.Adapter<MyViewPager2Adapter.MyViewHolder>  {
    private int[] images = {R.drawable.ic_launcher_background, R.drawable.ic_launcher_foreground};
    private Context ctx;

    MyViewPager2Adapter(Context ctx) {
        this.ctx = ctx;
    }

    // This method returns our layout
    @NonNull
    @Override
    public MyViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {
        View view = LayoutInflater.from(ctx).inflate(R.layout.images_holder, parent, false);
        return new MyViewHolder(view);
    }

    // This method binds the screen with the view
    @Override
    public void onBindViewHolder(@NonNull MyViewHolder holder, int position) {
        // This will set the images in imageview
        holder.images.setImageResource(images[position]);
    }

    // This Method returns the size of the Array
    @Override
    public int getItemCount() {
        return images.length;
    }

    // The ViewHolder class holds the view
    public static class MyViewHolder extends RecyclerView.ViewHolder {
        ImageView images;

        public MyViewHolder(@NonNull View itemView) {
            super(itemView);
            images = itemView.findViewById(R.id.images);
        }
    }
}

• Three pages with xml
• Only differ in background colors

<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:background="@android:color/holo_orange_dark">
</androidx.constraintlayout.widget.ConstraintLayout>	

    android:background="@android:color/holo_green_light">
    android:background="@android:color/holo_blue_light">	

    private int[] pages = {R.layout.page01, R.layout.page02, R.layout.page03};
...
        viewPager2 = findViewById(R.id.viewpager);
        viewPager2.setAdapter(new MyViewPager2Adapter(pages));
...

...
    private int[] layouts;

    MyViewPager2Adapter(int[] layouts) {
        this.layouts = layouts;
    }

    @NonNull
    @Override
    public MyViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {
        View view = LayoutInflater.from(parent.getContext())
                .inflate(viewType, parent, false);
        return new MyViewHolder(view);
    }

...

• Three pages one single xml
• Pass colors into constructor
• Button click callback

    <TextView
        android:id="@+id/textview"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginTop="112dp"
        android:text="TextView"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.498"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <Button
        android:id="@+id/button"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginBottom="184dp"
        android:text="Button"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.498"
        app:layout_constraintStart_toStartOf="parent" />

...
    private String[] pages = {"First Screen", "Second Screen", "Third Screen"};


    viewPager2 = findViewById(R.id.viewpager);
    viewPager2.setAdapter(new MyViewPager2Adapter(pages));
...

    private String[] layouts;
    private int[] colorArray = new int[]{android.R.color.holo_orange_light, android.R.color.holo_green_light,
            android.R.color.holo_blue_light};

    MyViewPager2Adapter(String[] layouts) {
        this.layouts = layouts;
    }

    // This method returns our layout
    @NonNull
    @Override
    public MyViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {
        View view = LayoutInflater.from(parent.getContext())
                .inflate(R.layout.single, parent, false);
        return new MyViewHolder(view);
    }

    // This method binds the screen with the view
    @Override
    public void onBindViewHolder(@NonNull MyViewHolder holder, int position) {
        //// HERE
        holder.textview.setText(layouts[position]);
        holder.constraintlayout.setBackgroundResource(colorArray[position]);
    }

    @Override
    public int getItemViewType(int position) {
        return colorArray[position];
    }

    // This Method returns the size of the Array
    @Override
    public int getItemCount() {
        return layouts.length;
    }

    // The ViewHolder class holds the view
    public static class MyViewHolder extends RecyclerView.ViewHolder {
        TextView textview;
        Button button;
        ConstraintLayout constraintlayout;

        public MyViewHolder(@NonNull View itemView) {
            super(itemView);
            textview = itemView.findViewById(R.id.textview);
            button = itemView.findViewById(R.id.button);
            constraintlayout = itemView.findViewById(R.id.constraintlayout);

            button.setOnClickListener(new View.OnClickListener() {
                @Override
                public void onClick(View v) {
                    textview.setTextSize(textview.getTextSize() + 0.1f);
                }
            });
        }
    }