How to pass text from one fragment to another? Both hosted by the MainActivity

Question

I have been trying to use Fragments in Android Studio and have been successful in the implementation of it. But, when I am trying to pass any data (Text/String here) I'm getting a Null Pointer Exception.

The Fragment doesn't appear to get received by the second Fragment. Here is the code:

MainActivity.java

public class MainActivity extends AppCompatActivity {

    TabLayout tb;
    ViewPager2 vp;
    viewPagerAdapter vpa;


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

        tb=findViewById(R.id.tab_lyt);
        vp= findViewById(R.id.pager);
        vpa= new viewPagerAdapter(this);

        vp.setAdapter(vpa);

        tb.addOnTabSelectedListener(new TabLayout.OnTabSelectedListener() {
            @Override
            public void onTabSelected(TabLayout.Tab tab) {
                vp.setCurrentItem(tab.getPosition());
            }

            @Override
            public void onTabUnselected(TabLayout.Tab tab) {

            }

            @Override
            public void onTabReselected(TabLayout.Tab tab) {

            }
        });

        vp.registerOnPageChangeCallback(new ViewPager2.OnPageChangeCallback() {
            @Override
            public void onPageSelected(int position) {
                super.onPageSelected(position);

                tb.getTabAt(position).select();
            }
        });
    }
}

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout 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">

    <androidx.viewpager2.widget.ViewPager2
        android:id="@+id/pager"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:layout_below="@id/tab_lyt"
        android:layout_marginTop="0dp" />

    <com.google.android.material.tabs.TabLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:id="@+id/tab_lyt">

        <com.google.android.material.tabs.TabItem
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:text="Menu 1"/>

        <com.google.android.material.tabs.TabItem
            android:id="@+id/frag2"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:text="Menu 2"/>



    </com.google.android.material.tabs.TabLayout>




</RelativeLayout>

viewPagerAdapter.java

public class viewPagerAdapter extends FragmentStateAdapter {
    public viewPagerAdapter(@NonNull FragmentActivity fragmentActivity) {
        super(fragmentActivity);
    }

    @NonNull
    @Override
    public Fragment createFragment(int position) {
        switch (position)
        {
            case 0: return new frag1();

            case 1: return new frag2();

            default: return new frag1();
        }
    }

    @Override
    public int getItemCount() {
        return 2;
    }
}

frag1.java

public class frag1 extends Fragment {
    private EditText editText;
    private Button button;

    public frag1(){

    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        View view = inflater.inflate(R.layout.fragment_frag1, container, false);

        editText = view.findViewById(R.id.frag1ET);
        button = view.findViewById(R.id.frag1btn);


        button.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                String text = editText.getText().toString();
                Bundle bundle = new Bundle();
                bundle.putString("tex", text);
                Toast.makeText(getContext(), ""+bundle.toString(), Toast.LENGTH_LONG).show();
                frag2 f2 = new frag2();
                f2.setArguments(bundle);
                editText.setText("");
            }
        });

        return view;
    }
}

frag1.xml

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    tools:context=".frgmnts.frag1">


    <EditText
        android:id="@+id/frag1ET"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginTop="116dp"
        android:ems="10"
        android:hint="Enter text"
        android:inputType="textPersonName"
        app:layout_constraintHorizontal_bias="0.497"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <Button
        android:id="@+id/frag1btn"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginTop="28dp"
        android:text="Button"
        app:layout_constraintEnd_toEndOf="@+id/frag1ET"
        app:layout_constraintStart_toStartOf="@+id/frag1ET"
        app:layout_constraintTop_toBottomOf="@+id/frag1ET" />

    <TextView
        android:id="@+id/frag1tv"
        android:layout_width="260dp"
        android:layout_height="56dp"
        android:layout_marginTop="56dp"
        android:textAlignment="textStart"
        android:hint="Text from Frag 2"
        android:textSize="20sp"
        app:layout_constraintEnd_toEndOf="@+id/frag1ET"
        app:layout_constraintHorizontal_bias="0.506"
        app:layout_constraintStart_toStartOf="@+id/frag1ET"
        app:layout_constraintTop_toBottomOf="@+id/frag1btn" />
</androidx.constraintlayout.widget.ConstraintLayout>

frag2.java

public class frag2 extends Fragment {
    private TextView tv2;

    public frag2(){

    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        View view = inflater.inflate(R.layout.fragment_frag2, container, false);

        tv2 = view.findViewById(R.id.frag2tv);


        return view;
    }

    @Override
    public void onResume() {
        super.onResume();
        Bundle bundle = getArguments();
        if (bundle != null) {
            String text = bundle.getString("tex");
            tv2.setText(text);
        }
        else
            tv2.setText("Text not received");
    }
}

frag2.xml

<?xml version="1.0" encoding="utf-8"?>
<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=".frgmnts.frag2">

    <EditText
        android:id="@+id/frag2ET"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginTop="116dp"
        android:ems="10"
        android:hint="Enter text"
        android:inputType="textPersonName"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.497"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <Button
        android:id="@+id/frag2btn"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginTop="40dp"
        android:text="Button"
        app:layout_constraintEnd_toEndOf="@+id/frag2ET"
        app:layout_constraintStart_toStartOf="@+id/frag2ET"
        app:layout_constraintTop_toBottomOf="@+id/frag2ET" />

    <TextView
        android:id="@+id/frag2tv"
        android:layout_width="260dp"
        android:layout_height="56dp"
        android:layout_marginTop="44dp"
        android:hint="Text from Frag 1"
        android:textAlignment="textStart"
        android:textSize="20sp"
        app:layout_constraintEnd_toEndOf="@+id/frag2ET"
        app:layout_constraintStart_toStartOf="@+id/frag2ET"
        app:layout_constraintTop_toBottomOf="@+id/frag2btn" />

</androidx.constraintlayout.widget.ConstraintLayout>

Any help and inputs in this regard will be helpful. Thank you

Answer 1

You're creating a new fragment when sending data, which you souldn't. The viewPager has no reference of the new fragment with data. That's why you're getting the NullPointerException.

There are several ways to pass data from one activity to another. The most convinient way is to use a common ViewModel class. Here is an example:

Create a Common view model class.

class CommonViewModel: ViewModel() {

// your variables to share

}

Send data from a fragment

class FragmentOne: Fragment() {
    private lateinit var viewModel: CommonViewModel
    override fun onViewCreated() {
        veiwModel = ViewModelProvider(requireActivity())[CommonViewModel::class.java]
        
        veiwModel.data_1 = "some_data"
    }
}

Get the data from the other fragment.

class FragmentTwo: Fragment() {
    private lateinit var viewModel: CommonViewModel
    override fun onViewCreated() {
        veiwModel = ViewModelProvider(requireActivity())[CommonViewModel::class.java]
        
        val dataFromOtherFragment = veiwModel.data_1
    }
}

Remember the requireActivity() is crucial since we want the same viewModel in both fragments.

Answer 2

I'm not sure it is the best idea, but it can solve your requirement.

You can create a listener to listen to your button and pass the text to the other fragment.

public interface OnFragmentSubmitListener {
    void onSubmitted(String text);
}

And then place the listener on the first fragment.

public class Frag1 extends Fragment {
    private EditText editText;
    private Button button;
    private OnFragmentSubmitListener listener;

    public Frag1(){

    }

    public void setOnFragmentSubmit(OnFragmentSubmitListener listener) {
        this.listener = listener;
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        View view = inflater.inflate(R.layout.fragment_frag1, container, false);

        editText = view.findViewById(R.id.frag1ET);
        button = view.findViewById(R.id.frag1btn);


        button.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                String text = editText.getText().toString();
                listener.onSubmitted(text);
                editText.setText("");
            }
        });

        return view;
    }

}

Then finally handle the listener on Adapter.

public class ViewPagerAdapter extends FragmentStateAdapter {
    private Frag1 frag1 = new Frag1();
    private Frag2 frag2 = new Frag2();

    public ViewPagerAdapter(@NonNull FragmentActivity fragmentActivity) {
        super(fragmentActivity);
        frag1.setOnFragmentSubmit((text -> {
            Bundle bundle = new Bundle();
            bundle.putString("tex", text);
            frag2.setArguments(bundle);
        }));
    }

    @NonNull
    @Override
    public Fragment createFragment(int position) {
        switch (position)
        {
            case 0: return frag1;

            case 1: return frag2;

            default: return frag1;
        }
    }

    @Override
    public int getItemCount() {
        return 2;
    }
}