NullPointerException, setting adapter on null object

Question

I'm trying to get a Staggered Grid layout using a RecyclerView in one of my ViewPagers fragments. When I start the app, it immediately crashes and throws this error:

Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.support.v7.widget.RecyclerView.setAdapter(android.support.v7.widget.RecyclerView$Adapter)' on a null object reference at xxx.HomeActivity.onCreate(HomeActivity.java:39) at android.app.Activity.performCreate(Activity.java:5990) at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1106) at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2278)

My first thought is that this might be caused by the order in which the views are loaded but i'm not sure. I have a ViewPager with 2 fragments. I have the RecyclerView declared in the first fragments xml, like so:

res/layout/fragment_home.xml:

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

    <android.support.v7.widget.RecyclerView
        android:id="@+id/home_grid"
        android:layout_width="match_parent"
        android:layout_height="match_parent" />

</RelativeLayout>

Heres the relative parts of my Activity:

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_home);
    ButterKnife.bind(this);

    setupToolbar();
    setupTabs();

    HomeGridAdapter adapter = new HomeGridAdapter(Contacts.createFakeContacts(20));

    RecyclerView recyclerView = (RecyclerView) findViewById(R.id.home_grid);

    recyclerView.setAdapter(adapter);
    recyclerView.setLayoutManager(new StaggeredGridLayoutManager(2, StaggeredGridLayoutManager.VERTICAL));


}

Here is the first fragments Java file:

public class HomeFragment extends Fragment {

public static final String ARG_PAGE = "ARG_PAGE";

private int mPage;

public static HomeFragment newInstance(int page) {
    Bundle args = new Bundle();
    args.putInt(ARG_PAGE, page);
    HomeFragment fragment = new HomeFragment();
    fragment.setArguments(args);
    return fragment;
}

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    mPage = getArguments().getInt(ARG_PAGE);

}

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {
    // Inflate the layout for this fragment
    return inflater.inflate(R.layout.fragment_home, container, false);
}


}

I dont think i need to post the RecyclerViews adapter code as I dont think that is part of the issue, but if its needed I can.

Any input on this issue is appreciated!

EDIT: As requested, heres my activitys xml file:

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
            xmlns:tools="http://schemas.android.com/tools"
            xmlns:app="http://schemas.android.com/apk/res-auto"
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:orientation="vertical"
            tools:context=".HomeActivity">

<android.support.v7.widget.Toolbar
    android:id="@+id/toolbar"
    android:minHeight="?attr/actionBarSize"
    android:layout_width="match_parent"
    android:layout_height="75dp"
    style="@style/ToolbarStyle"
    >

    <android.support.design.widget.TabLayout
        android:id="@+id/tab_layout"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        app:tabMode="scrollable"/>

</android.support.v7.widget.Toolbar>

<android.support.v4.view.ViewPager
    android:id="@+id/viewpager"
    android:layout_width="match_parent"
    android:layout_height="0dp"
    android:layout_weight="1"/>

</LinearLayout>

Answer 1

You are loading activity_home in activity

setContentView(R.layout.activity_home);

and your home_grid is located in first_fragment

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

<android.support.v7.widget.RecyclerView
    android:id="@+id/home_grid"
    android:layout_width="match_parent"
    android:layout_height="match_parent" />

</RelativeLayout>

Answer 2

Within onCreate() of your Activity, first find your ViewPager:

    ViewPager pager = (ViewPager) findViewById(R.id.viewpager);
    MyFragmentPagerAdaptor adaptor = new
        MyFragmentPagerAdapter(getSupportFragmentManager());
    pager.setAdapter(adaptor);

Within the Activity Class, Create a new FragmentPagerAdapter nested class called MyFragmentPagerAdapter, which returns your Fragment on the getItem() method.

Currently, your ViewPager doesn't have any content inside and you are not loading the fragment into the ViewPager. (Purely from the code I see. I may be wrong)

Now that your ViewPager has the fragment content inside, you need a way to access the child View within the fragment (within the Viewpager).

This will not be directly possible, so you need to create a instance of the fragment you are looking for using your adaptor's getItem() method:

Fragment f = (Fragment) adaptor.getItem(pager.getCurrentItem());

Although getCurrentItem() can be replaced with an index pointing directly to your Fragment, which should have already been defined within the getItem().

Finally, You can find your child view within the fragment like so:

RecyclerView theViewINeed = (RecyclerView)f.getView().findViewById(R.id.home_grid);

---

edit: seems like the Fragment's OnCreateView() is not called during the parent Activity's OnCreate().

Try moving the below code to OnStart. (Put the Fragment reference you obtain from onCreate() as a field, and access it during onStart())

RecyclerView theViewINeed = (RecyclerView)f.getView().findViewById(R.id.home_grid);

Answer 3

You have specified activity_home.xml layout for Activity

while your RecyclerView tag is in fragment_home.xml

try to initialize RecyclerView in fragment Or add RecyclerView tag in activity_home.xml