NullPointerException accessing views in onCreate()

Question

This is a canonical question for a problem frequently posted on StackOverflow.

I'm following a tutorial. I've created a new activity using a wizard. I get NullPointerException when attempting to call a method on Views obtained with findViewById() in my activity onCreate().

Activity onCreate():

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

    View something = findViewById(R.id.something);
    something.setOnClickListener(new View.OnClickListener() { ... }); // NPE HERE

    if (savedInstanceState == null) {
        getSupportFragmentManager().beginTransaction()
                .add(R.id.container, new PlaceholderFragment()).commit();
    }
}

Layout XML (fragment_main.xml):

<RelativeLayout 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"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    tools:context="packagename.MainActivity$PlaceholderFragment" >

    <View
        android:layout_width="100dp"
        android:layout_height="100dp"
        android:id="@+id/something" />

</RelativeLayout>

Answer 1

The tutorial is probably outdated, attempting to create an activity-based UI instead of the fragment-based UI preferred by wizard-generated code.

The view is in the fragment layout (fragment_main.xml) and not in the activity layout (activity_main.xml). onCreate() is too early in the lifecycle to find it in the activity view hierarchy, and a null is returned. Invoking a method on null causes the NPE.

The preferred solution is to move the code to the fragment onCreateView(), calling findViewById() on the inflated fragment layout rootView:

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

  View something = rootView.findViewById(R.id.something); // not activity findViewById()
  something.setOnClickListener(new View.OnClickListener() { ... });

  return rootView;
}

As a side note, the fragment layout will eventually be a part of the activity view hierarchy and discoverable with activity findViewById() but only after the fragment transaction has been run. Pending fragment transactions get executed in super.onStart() after onCreate().

Answer 2

Try OnStart() method and just use

View view = getView().findViewById(R.id.something);

or Declare any View using getView().findViewById method in onStart()

Declare click listener on view by anyView.setOnClickListener(this);

Answer 3

Try to shift your accessing views to the onViewCreated method of fragment because sometimes when you try to access the views in onCreate method they are not rendered at the time resulting null pointer exception.

 @Override
public void onViewCreated(View view, @Nullable Bundle savedInstanceState) {
    super.onViewCreated(view, savedInstanceState);
     View something = findViewById(R.id.something);
     something.setOnClickListener(new View.OnClickListener() { ... }); // NPE HERE

     if (savedInstanceState == null) {
           getSupportFragmentManager().beginTransaction()
            .add(R.id.container, new PlaceholderFragment()).commit();
    }
 }

Answer 4

Agreed, this is a typical error because people often don't really understand how Fragments work when they begin working on Android development. To alleviate confusion, I created a simple example code that I originally posted on Application is stopped in android emulator , but I posted it here as well.

An example is the following:

public class ContainerActivity extends FragmentActivity implements ExampleFragment.Callback
{
    @Override
    public void onCreate(Bundle saveInstanceState)
    {
        super.onCreate(saveInstanceState);
        this.setContentView(R.layout.activity_container);
        if (saveInstanceState == null)
        {               
             getSupportFragmentManager().beginTransaction()
                .add(R.id.activity_container_container, new ExampleFragment())
                .addToBackStack(null)
             .commit();
        }
        getSupportFragmentManager().addOnBackStackChangedListener(new OnBackStackChangedListener()
        {
            public void onBackStackChanged()
            {
                int backCount = getSupportFragmentManager().getBackStackEntryCount();
                if (backCount == 0)
                {
                    finish();
                }
            }
        });
    }

    @Override
    public void exampleFragmentCallback()
    {
        Toast.makeText(this, "Hello!", Toast.LENGTH_LONG).show();
    }
}

activity_container.xml:

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical" >

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

</RelativeLayout>

ExampleFragment:

public class ExampleFragment extends Fragment implements View.OnClickListener
{
    public static interface Callback
    {
        void exampleFragmentCallback();
    }

    private Button btnOne;
    private Button btnTwo;
    private Button btnThree;

    private Callback callback;

    @Override
    public void onAttach(Activity activity)
    {
        super.onAttach(activity);
        try
        {
            this.callback = (Callback) activity;
        }
        catch (ClassCastException e)
        {
            Log.e(this.getClass().getSimpleName(), "Activity must implement Callback interface.", e);
            throw e;
        }
    }

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

        btnOne = (Button) rootView.findViewById(R.id.example_button_one);
        btnTwo = (Button) rootView.findViewById(R.id.example_button_two);
        btnThree = (Button) rootView.findViewById(R.id.example_button_three);

        btnOne.setOnClickListener(this);
        btnTwo.setOnClickListener(this);
        btnThree.setOnClickListener(this);
        return rootView;
    }

    @Override
    public void onClick(View v)
    {
        if (btnOne == v)
        {
            Toast.makeText(getActivity(), "One.", Toast.LENGTH_LONG).show();
        }
        else if (btnTwo == v)
        {
            Toast.makeText(getActivity(), "Two.", Toast.LENGTH_LONG).show();
        }
        else if (btnThree == v)
        {
            callback.exampleFragmentCallback();
        }
    }
}

fragment_example.xml:

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
        android:layout_width="match_parent"
        android:layout_height="match_parent" >

        <Button
            android:id="@+id/example_button_one"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:layout_alignParentTop="true"
            android:layout_centerHorizontal="true"
            android:layout_marginTop="30dp"
            android:text="@string/hello" 
            android:layout_marginLeft="20dp"
            android:layout_marginRight="20dp"/>

        <Button
            android:id="@+id/example_button_two"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_alignLeft="@+id/example_button_one"
            android:layout_alignRight="@+id/example_button_one"
            android:layout_below="@+id/example_button_one"
            android:layout_marginTop="30dp"
            android:text="@string/hello" />

        <Button
            android:id="@+id/example_button_three"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_alignLeft="@+id/example_button_two"
            android:layout_alignRight="@+id/example_button_two"
            android:layout_below="@+id/example_button_two"
            android:layout_marginTop="30dp"
            android:text="@string/hello" />

</RelativeLayout>

And that should be a valid example, it shows how you can use an Activity to display a Fragment, and handle events in that Fragment. And also how to communicate with the containing Activity.

Answer 5

The view "something" is in fragment and not in activity, so instead of accessing it in activity you must access it in the fragment class like

In PlaceholderFragment.class

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

View something = root .findViewById(R.id.something);
something.setOnClickListener(new View.OnClickListener() { ... });

return root;
}

Answer 6

You are trying to access UI elements in the onCreate() but , it is too early to access them , since in fragment views can be created in onCreateView() method. And onActivityCreated() method is reliable to handle any actions on them, since activity is fully loaded in this state.

Answer 7

Add the following in your activity_main.xml

<fragment
    android:id="@+id/myFragment"
    android:name="packagename.MainActivity$PlaceholderFragment"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content" >
</fragment>

Answer 8

Since you have declared your View in the fragment_main.xml,move that piece of code where you get the NPE in the onCreateView() method of the fragment. This should solve the issue.

Answer 9

in the posted code above in the question there is a problem : you are using R.layout.activity_main in oncreate method, but the xml files name is "fragment_main.xml" , means you are trying to get the view of fragment_main.xml file which is not being shown so it gives null pointer exception. change the code like :

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.fragment_main);// your xml layout ,where the views are

    View something = findViewById(R.id.something);
    something.setOnClickListener(new View.OnClickListener() { ... }); // NPE HERE

    if (savedInstanceState == null) {
        getSupportFragmentManager().beginTransaction()
                .add(R.id.container, new PlaceholderFragment()).commit();
    }
}

Answer 10

You have to remember important thing is : NullPointerException occurs when you have declared your variable and trying to retreive its value before assigning value to it.

Answer 11

Use onViewCreated() Method whenever using or calling views from fragments.

override fun onViewCreated(view: View?, savedInstanceState: Bundle?) {
    super.onViewCreated(view, savedInstanceState)
      View v = view.findViewById(R.id.whatever)
}

Answer 12

I've got the same NullPointerException initializing a listener after calling findViewById() onCreate() and onCreateView() methods.

But when I've used the onActivityCreated(Bundle savedInstanceState) {...} it works. So, I could access the GroupView and set my listener.

I hope it be helpful.

Answer 13

Most popular library for finding views which is used by almost every developer.

ButterKnife

As I can their are enough answers explaining finding views with proper methodology. But if you are android developer and code frequently on daily basis then you can use butter-knife which saves a lot time in finding views and you don't have write code for it, With in 2-3 steps you can find views in milliseconds.

Add dependency in app level gradle:

implementation 'com.jakewharton:butterknife:8.8.1'
annotationProcessor 'com.jakewharton:butterknife-compiler:8.8.1'

Add plugin for butter knife:

File -> Settings -> plugins-> 

Then search for Android ButterKnife Zelezny and install plugin and restart your studio and you are done with it.

Now just go to Oncreate method of your activity and right click on your layout_name and tap on generate button and select butterknife injection option and your views references will be automatically created like mention below:

    @BindView(R.id.rv_featured_artist)
    ViewPager rvFeaturedArtist;
    @BindView(R.id.indicator)
    PageIndicator indicator;
    @BindView(R.id.rv_artist)
    RecyclerView rvArtist;
    @BindView(R.id.nsv)
    NestedScrollingView nsv;
    @BindView(R.id.btn_filter)
    Button btnFilter;