NullPointerException accessing views in onCreate()

Question

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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

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 2

The tutorial is probably outdated, attempting to create an activity-based UI instead of the fragment-based UI preferred by wizard-generated code.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 3

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 4

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 5

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;

Answer 6

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);