Accessing Parent Activity Button in Fragment Crashes App

Question

Hey guys I am a beginner in Android Develpoment. I am currently learning Navigation Components, but stuck in this app. I am setting listener to the button present in the activity_main.xml inside the FirstFragment.java and SecondFragment.java but my app crashes. I don't know what I am doing wrong.

Main Activity.java

package com.ahstore.inventory;

import android.os.Bundle;

import com.google.android.material.snackbar.Snackbar;

import androidx.appcompat.app.AppCompatActivity;

import android.view.View;

import androidx.navigation.NavController;
import androidx.navigation.Navigation;
import androidx.navigation.fragment.NavHostFragment;
import androidx.navigation.ui.AppBarConfiguration;
import androidx.navigation.ui.NavigationUI;

import com.ahstore.inventory.databinding.ActivityMainBinding;

import android.view.Menu;
import android.view.MenuItem;
import android.widget.Button;

public class MainActivity extends AppCompatActivity {

    private AppBarConfiguration appBarConfiguration;
    private ActivityMainBinding binding;

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

        binding = ActivityMainBinding.inflate(getLayoutInflater());
        setContentView(binding.getRoot());

        setSupportActionBar(binding.toolbar);

        NavController navController = Navigation.findNavController(this, R.id.nav_host_fragment_content_main);
        appBarConfiguration = new AppBarConfiguration.Builder(navController.getGraph()).build();
        NavigationUI.setupActionBarWithNavController(this, navController, appBarConfiguration);




        binding.fab.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Snackbar.make(view, "Replace with your own action", Snackbar.LENGTH_LONG)
                        .setAction("Action", null).show();
            }
        });
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.menu_main, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();

        //noinspection SimplifiableIfStatement
        if (id == R.id.action_settings) {
            return true;
        }

        return super.onOptionsItemSelected(item);
    }

    @Override
    public boolean onSupportNavigateUp() {
        NavController navController = Navigation.findNavController(this, R.id.nav_host_fragment_content_main);
        return NavigationUI.navigateUp(navController, appBarConfiguration)
                || super.onSupportNavigateUp();
    }
}

activity_main.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:id="@+id/coordinatorLayout"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">


    <com.google.android.material.appbar.AppBarLayout
        android:id="@+id/appBarLayout"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:theme="@style/Theme.Inventory.AppBarOverlay"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent">

        <androidx.appcompat.widget.Toolbar
            android:id="@+id/toolbar"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:background="?attr/colorPrimary"
            app:popupTheme="@style/Theme.Inventory.PopupOverlay" />

    </com.google.android.material.appbar.AppBarLayout>


    <HorizontalScrollView
        android:id="@+id/horizontalScrollView"
        android:layout_width="409dp"
        android:layout_height="50dp"
        android:layout_marginStart="10dp"
        android:layout_marginEnd="10dp"
        android:scrollIndicators="none"
        android:scrollbarAlwaysDrawHorizontalTrack="false"
        android:scrollbarAlwaysDrawVerticalTrack="false"
        android:scrollbars="none"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/appBarLayout">

        <LinearLayout
            android:layout_width="wrap_content"
            android:layout_height="match_parent"
            android:orientation="horizontal">

            <Button
                android:id="@+id/button1"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:layout_marginRight="5dp"
                android:layout_weight="1"
                android:text="Button" />

            <Button
                android:id="@+id/button2"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:layout_marginRight="5dp"
                android:layout_weight="1"
                android:text="Button" />

        </LinearLayout>
    </HorizontalScrollView>

    <fragment
        android:id="@+id/nav_host_fragment_content_main"
        android:name="androidx.navigation.fragment.NavHostFragment"
        android:layout_width="0dp"
        android:layout_height="0dp"
        app:defaultNavHost="true"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.5"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/horizontalScrollView"
        app:navGraph="@navigation/nav_graph" />

    <com.google.android.material.floatingactionbutton.FloatingActionButton
        android:id="@+id/fab"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginEnd="16dp"
        android:layout_marginBottom="16dp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:srcCompat="@android:drawable/ic_input_add"
        tools:ignore="SpeakableTextPresentCheck,SpeakableTextPresentCheck" />



</androidx.constraintlayout.widget.ConstraintLayout>

FirstFragment.java

package com.ahstore.inventory;

import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.Button;

import androidx.annotation.NonNull;
import androidx.fragment.app.Fragment;
import androidx.navigation.fragment.NavHostFragment;

import com.ahstore.inventory.databinding.FragmentFirstBinding;

public class FirstFragment extends Fragment {

    private FragmentFirstBinding binding;

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


        return view;

    }

    public void onViewCreated(@NonNull View view, Bundle savedInstanceState) {
        super.onViewCreated(view, savedInstanceState);

        Button button1=view.findViewById(R.id.button1);
        button1.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                NavHostFragment.findNavController(FirstFragment.this)
                        .navigate(R.id.action_FirstFragment_to_SecondFragment);
            }
        });



    }

    @Override
    public void onDestroyView() {
        super.onDestroyView();
        binding = null;
    }

}

fragment_first.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=".FirstFragment">

    <ScrollView
        android:layout_width="0dp"
        android:layout_height="0dp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent">

        <LinearLayout
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:orientation="vertical">

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

        </LinearLayout>
    </ScrollView>
</androidx.constraintlayout.widget.ConstraintLayout>

SecondFragment.java

package com.ahstore.inventory;

import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.Button;

import androidx.annotation.NonNull;
import androidx.fragment.app.Fragment;
import androidx.navigation.fragment.NavHostFragment;

import com.ahstore.inventory.databinding.FragmentSecondBinding;

public class SecondFragment extends Fragment {

    private FragmentSecondBinding binding;

    @Override
    public View onCreateView(
            LayoutInflater inflater, ViewGroup container,
            Bundle savedInstanceState
    ) {

        View view=inflater.inflate(R.layout.fragment_second,container,false);

        return view;

    }

    public void onViewCreated(@NonNull View view, Bundle savedInstanceState) {
        super.onViewCreated(view, savedInstanceState);
        Button button=view.findViewById(R.id.button2);
        button.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                NavHostFragment.findNavController(SecondFragment.this)
                        .navigate(R.id.action_SecondFragment_to_FirstFragment);
            }
        });
    }

    @Override
    public void onDestroyView() {
        super.onDestroyView();
        binding = null;
    }

}

fragment_second.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:id="@+id/cl"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".SecondFragment">

    <TextView
        android:id="@+id/textview_second"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="am second"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

</androidx.constraintlayout.widget.ConstraintLayout>

Error

2022-03-04 16:54:11.473 2392-2392/com.ahstore.inventory E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.ahstore.inventory, PID: 2392
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.ahstore.inventory/com.ahstore.inventory.MainActivity}: android.view.InflateException: Binary XML file line #79 in com.ahstore.inventory:layout/activity_main: Binary XML file line #79 in com.ahstore.inventory:layout/activity_main: Error inflating class fragment
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3800)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3976)
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:85)
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135)
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2315)
        at android.os.Handler.dispatchMessage(Handler.java:106)
        at android.os.Looper.loop(Looper.java:246)
        at android.app.ActivityThread.main(ActivityThread.java:8550)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:602)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1130)
     Caused by: android.view.InflateException: Binary XML file line #79 in com.ahstore.inventory:layout/activity_main: Binary XML file line #79 in com.ahstore.inventory:layout/activity_main: Error inflating class fragment
     Caused by: android.view.InflateException: Binary XML file line #79 in com.ahstore.inventory:layout/activity_main: Error inflating class fragment
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
        at com.ahstore.inventory.FirstFragment.onViewCreated(FirstFragment.java:35)
        at androidx.fragment.app.Fragment.performViewCreated(Fragment.java:2987)
        at androidx.fragment.app.FragmentStateManager.createView(FragmentStateManager.java:546)
        at androidx.fragment.app.FragmentStateManager.moveToExpectedState(FragmentStateManager.java:282)
        at androidx.fragment.app.FragmentStore.moveToExpectedState(FragmentStore.java:112)
        at androidx.fragment.app.FragmentManager.moveToState(FragmentManager.java:1647)
        at androidx.fragment.app.FragmentManager.dispatchStateChange(FragmentManager.java:3128)
        at androidx.fragment.app.FragmentManager.dispatchViewCreated(FragmentManager.java:3065)
        at androidx.fragment.app.Fragment.performViewCreated(Fragment.java:2988)
        at androidx.fragment.app.FragmentStateManager.ensureInflatedView(FragmentStateManager.java:392)
        at androidx.fragment.app.FragmentStateManager.moveToExpectedState(FragmentStateManager.java:281)
        at androidx.fragment.app.FragmentLayoutInflaterFactory.onCreateView(FragmentLayoutInflaterFactory.java:140)
        at androidx.fragment.app.FragmentController.onCreateView(FragmentController.java:135)
        at androidx.fragment.app.FragmentActivity.dispatchFragmentsOnCreateView(FragmentActivity.java:319)
        at androidx.fragment.app.FragmentActivity.onCreateView(FragmentActivity.java:298)
        at android.view.LayoutInflater.tryCreateView(LayoutInflater.java:1067)
        at android.view.LayoutInflater.createViewFromTag(LayoutInflater.java:995)
        at android.view.LayoutInflater.createViewFromTag(LayoutInflater.java:959)
        at android.view.LayoutInflater.rInflate(LayoutInflater.java:1121)
        at android.view.LayoutInflater.rInflateChildren(LayoutInflater.java:1082)
        at android.view.LayoutInflater.inflate(LayoutInflater.java:680)
        at android.view.LayoutInflater.inflate(LayoutInflater.java:532)
        at com.ahstore.inventory.databinding.ActivityMainBinding.inflate(ActivityMainBinding.java:75)
        at com.ahstore.inventory.databinding.ActivityMainBinding.inflate(ActivityMainBinding.java:69)
        at com.ahstore.inventory.MainActivity.onCreate(MainActivity.java:32)
        at android.app.Activity.performCreate(Activity.java:8198)
2022-03-04 16:54:11.474 2392-2392/com.ahstore.inventory E/AndroidRuntime:     at android.app.Activity.performCreate(Activity.java:8182)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1309)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3773)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3976)
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:85)
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135)
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2315)
        at android.os.Handler.dispatchMessage(Handler.java:106)
        at android.os.Looper.loop(Looper.java:246)
        at android.app.ActivityThread.main(ActivityThread.java:8550)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:602)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1130)
 

Answer 1

Here are a few points that will help you understand your issue better

1. Your activity and fragments have their own respective layout files (xmls)
2. View are bound within their layout files. Simply put, if an element with id, say A, is present in activity_main.xml, then it can only be accessed when the layout being inflated is activity_main.xml. If the contentView of your Activity is activity_main.xml, it will be able to access all Views inside the layout file.
3. In fragments, the layout is inflated in the onCreateView method, which returns the resulting View for the Fragment. The findViewById(id) method is called against that View object. The method will look for the View with the given id inside that View only. If it is able to find the View it will return it, else it will return null. Same goes for finding a View inside an Activity (you do not need to specify the View for an Activity as it will automatically reference the layout that is specified in setContentView).

Now that we have the above points cleared, we can see the issue that you are facing. Firstly, the buttons with ids button1 and button2 are in your activity_main.xml layout file. However you are trying to access these from your Fragment1 and Fragment2 classes. But your Fragment1 and Fragment2 classes have their own layout files which do not have the two buttons. So, when you try to retrieve the buttons (using findViewById), it returns null because the buttons are not present in the layout. You then try to set a click listener. Since, the button object is null, calling any method on it will result in a Null Pointer Exception being throw.

We now understand what you are doing wrong. Now, how do we fix this. First you need to understand what it is you want to achieve. In this case I would suggest that you move both buttons to the respective Fragment layouts i.e. the View with id button1 will go to Fragment1 and the View with id button2 will go to Fragment2. This should help resolve the NPE that you are facing.

Still there are some cases where a Fragment might want to access a View that is present in its parent Activity. One of the ways to do this would be to

1. Create a public method in your Activity class which returns a View with the given id.

   public View getViewById(int id) {
       return findViewById(id);
   }
   

2. Use this method in your Fragment by doing so

   ((MainActivity) getActivity()).getViewById(R.id.button1)
   

Other approaches involve using Interfaces which I am sure you will learn about as you continue learning more.

Just to wrap things up, you have two ways to go about fixing your issue. Either move the buttons to the fragment layout files or create a public method to retrieve those views.