How to resolve a NullPointerException while using RecyclerView? - android

I am using two RecyclerViews in my app, one in the MainActivity and one in another activity. The one in the MainActivity works fine but the other has a NullPointerException and I don't know where the problem here is.
Here is a code snippet of the second RecyclerView, where the logcat says that there is a Null object reference. I used the same code as the first RecyclerView where everything works.
The error message says that there is a problem in this line: recyclerView.setLayoutManager(layoutManager);
and in this line
initRecyclerViewFriendOverView();
Here is where I implemented my second RecyclerView:
private void initRecyclerViewFriendOverView() {
LinearLayoutManager layoutManager = new LinearLayoutManager(this, RecyclerView.VERTICAL, false);
RecyclerView recyclerView = findViewById(R.id.recyclerViewFriendOverView);
recyclerView.setLayoutManager(layoutManager);
RecyclerViewAdapterFriendOverView adpater = new RecyclerViewAdapterFriendOverView(mNames, mImageUrls, this);
recyclerView.setAdapter(adpater);
}
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_play_overview);
getSupportActionBar().setDisplayHomeAsUpEnabled(true);
init();
initRecyclerViewFriendOverView();
getImages();
}
Here ist the XML where the second RecyclerView is used:
<?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"
android:orientation="vertical" android:layout_width="match_parent"
android:layout_height="match_parent">
<androidx.recyclerview.widget.RecyclerView
android:id="#+id/recyclerViewFreundesUebersicht"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginStart="8dp"
android:layout_marginTop="8dp"
android:orientation="vertical"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="parent" />
</androidx.constraintlayout.widget.ConstraintLayout>
Edit:
The problem was that I called the RecyclerView before initializing it in the onCreate method.


EDIT:
You are including another xml file in your activity using include statement, findViewById() won't be able to find views from included layouts. Give an id to your include statement, find that included view first and then call findViewById () in that view:
In Activity layout file:
<include id="includedRecyclerLayout"
....
</include>
And in your code:
ConstraintLayout includedLayout = findViewById(R.id.inckudedRecyclerLayout);
RecyclerView recyclerview = includedLayout.findViewById(R.id.RECYCLER_VIEW_ID);
If it shows error at recyclerView.setLayoutManager(layoutManager); it means your recyclerView is null. Then you look at where you initialized it:
RecyclerView recyclerView = findViewById(R.id.recyclerViewFreundesUebersicht);
It can be null because of two reasons:
You didn't add the RecyclerView to XML
Your R.id.recyclerViewFreundesUebersicht is referring to a view from another xml
Check your XML and make sure your recyclerview was added to XML and the id you are using here is same as the one used in activity.


Initialize LinearLayoutManager like this
LinearLayoutManager linearLayoutManager = new LinearLayoutManager(context);
linearLayoutManager.setOrientation(LinearLayoutManager.VERTICAL);
recyclerView.setLayoutManager(linearLayoutManager);
recyclerView.setItemAnimator(new DefaultItemAnimator());

Related

how is the recyclerView maintains its scroll position without any extra coding

Created a simple activity which has a recyclerView and displays the data list. When turned on the setting's 'don't keep activity alive' and set 'Background process limit' to 'No background process'. Expect the acitivity will be killed by os in background.
When minimize the app and reopen it, saw the activity's onCreate() is called on a new instance and it does all again to create a new set of recyclerView and adapter with new datalist.
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
Toolbar toolbar = findViewById(R.id.toolbar);
mRecyclerView = findViewById(R.id.recyclerView);
setSupportActionBar(toolbar);
RecyclerView.LayoutManager layoutManager = new LinearLayoutManager(this);
mRecyclerView.setLayoutManager(layoutManager);
}
But the ui still shows the item in last scrolled position (correctly restored).
Anyone knows how the previous position is transfered to the new recyclerView instance, etc.?
the layout:
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical">
<android.support.v7.widget.Toolbar
android:id="#+id/toolbar"
android:layout_width="match_parent"
android:layout_height="?attr/actionBarSize"
android:background="?attr/colorPrimary"
app:titleTextColor="#android:color/white"
/>
<ScrollView
android:layout_width="match_parent"
android:layout_height="match_parent"
android:fillViewport="true">
<android.support.v7.widget.RecyclerView
android:id="#+id/recyclerView"
android:layout_width="match_parent"
android:layout_height="wrap_content" />
</ScrollView>
</LinearLayout>
the activity:
public class MainActivity extends AppCompatActivity {
private RecyclerView mRecyclerView;
private ItemAdapter mAdapter;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
Toolbar toolbar = findViewById(R.id.toolbar);
mRecyclerView = findViewById(R.id. recyclerView);
setSupportActionBar(toolbar);
RecyclerView.LayoutManager layoutManager = new LinearLayoutManager(this);
mRecyclerView.setLayoutManager(layoutManager);
}
#Override
protected void onResume() {
super.onResume();
new FetchItemASyncTask(this).execute();
// in the asyncTask it will do setup adapter when data ready
// mAdapter = new ItemAdapter(dataList)
// mRecyclerView.setAdapter(mAdapter);
}
}

This is part of the default saving of state that Android performs for many views. The only thing you need to do to enable this is to give the view an android:id attribute (if you take that id away, you'll notice that it no longer saves the scroll position). The same is true for e.g. the text entered into an EditText.

RecyclerView Layout file throws the exception: Failed to find view with ID ...:id/recyclerView in item layout

I'm currently trying to implement my first ever RecyclerView in Xamarin.Android.
In my main layout file, i have a DrawerLayout which contains a FrameLayout and a ListView. What i am trying to achieve is that each time an item from the ListView is clicked, a Fragment containing a RecyclerView will be loaded using the FragmentManager.
The issue i face is that each time i return from OnCreateView in the Fragment, i get the above exception.
What confuses me most is that when i step through the code in OnCreateView, FindViewById does not throw an exception and seems to populate recyclerView correctly.
Any help will be greatly appreciated. Thank you.
DiscoverNew.axml
<?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">
<android.support.v7.widget.RecyclerView
android:id="#+id/recyclerView"
android:layout_width="match_parent"
android:layout_height="wrap_content" />
</RelativeLayout>
DiscoverNewFragment.cs
public class DiscoverNewFragment : Fragment
{
RecyclerView recyclerView;
IngredientsAdapter ingredientsAdapter;
List<Ingredients> listOfIngredients;
public override View OnCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState)
{
var view = inflater.Inflate(Resource.Layout.DiscoverNew, container, false);
recyclerView = view.FindViewById(Resource.Id.recyclerView) as RecyclerView;
var layoutMan = new GridLayoutManager(recyclerView.Context, 2, GridLayoutManager.Vertical, false);
recyclerView.SetLayoutManager(layoutMan);
listOfIngredients = IngredientsData.LoadIngredients(this.Activity);
ingredientsAdapter = new IngredientsAdapter(listOfIngredients);
recyclerView.SetAdapter(ingredientsAdapter);
return view;
}
}
MainActivity.cs
[Activity(Label = "MyApp", Theme = "#style/AppTheme", MainLauncher = true)]
public class MainActivity : AppCompatActivity
{
. . .
protected override void OnCreate(Bundle savedInstanceState)
{
base.OnCreate(savedInstanceState);
SetContentView(Resource.Layout.Main);
. . .
OnMenuItemClick(0); // Load Fragment 0 at startup
}
. . .
void OnMenuItemClick(int position)
{
base.FragmentManager.BeginTransaction().Replace(Resource.Id.frameLayout, new DiscoverNewFragment()).Commit();
this.Title = "Discover";
drawerLayout.CloseDrawer(drawerListView);
}
}
NB: i have omitted some code from MainActivity.cs which is related to the setting up of the drawer layout

I have just resolved this issue by reverting the following nuget package from version 27.0.2.1 to version 26.1.0.1:- xamarin.android.support.design
I know it's more of a work around than a solution but after trying different things, this was the only option that seemed to work.

How to set Horizontal Recyclerview in one of the fragment in ViewPager

I m having a ViewPager with three fragments , one fragment has something that scrolls horizontally but when m trying to scroll it slides the page

if you need a horizontal RecyclerView you just need to add a the RecyclerView to your fragment and use LinearLayoutManager.HORIZONTAL
LinearLayoutManager layoutManager
= new LinearLayoutManager(this, LinearLayoutManager.HORIZONTAL, false);
RecyclerView myList = (RecyclerView) findViewById(R.id.my_recycler_view);
myList.setLayoutManager(layoutManager);
OR
just add this RecyclerView in your fragment
<android.support.v7.widget.RecyclerView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:orientation="horizontal"
app:layoutManager="android.support.v7.widget.LinearLayoutManager" />

"LayoutManager is already attached to a RecyclerView" error

I am trying to have multiple RecyclerViews in a layout but I get the following error: "LayoutManager is already attached to a RecyclerView"
The Java code is:
#Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
// Inflate the layout for this fragment
View view = inflater.inflate(R.layout.fragment_squad, container, false);
Activity parentActivity = getActivity();
final ObservableScrollView scrollView = (ObservableScrollView) view.findViewById(R.id.squad_scrollview);
final RecyclerView gkRecyclerView = (RecyclerView) view.findViewById(R.id.gk_recycler);
final RecyclerView coachRecyclerView = (RecyclerView) view.findViewById(R.id.coach_recycler);
coachRecyclerView.setAdapter(new SquadRecyclerAdapter(parentActivity, getSquadDummyData(0)));
coachRecyclerView.setLayoutManager(new MyLinearLayoutManager(parentActivity, LinearLayoutManager.VERTICAL, false));
coachRecyclerView.setHasFixedSize(false);
gkRecyclerView.setAdapter(new SquadRecyclerAdapter(parentActivity, getSquadDummyData(1)));
gkRecyclerView.setLayoutManager(new MyLinearLayoutManager(parentActivity, LinearLayoutManager.VERTICAL, false));
gkRecyclerView.setHasFixedSize(false);
scrollView.setTouchInterceptionViewGroup((ViewGroup) parentActivity.findViewById(R.id.container));
if (parentActivity instanceof ObservableScrollViewCallbacks) {
scrollView.setScrollViewCallbacks((ObservableScrollViewCallbacks) parentActivity);
}
return view;
}
The XML layout code is:
<com.github.ksoichiro.android.observablescrollview.ObservableScrollView xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:id="#+id/squad_scrollview"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:layout_margin="#dimen/margin_medium"
>
<LinearLayout
android:id="#+id/squad_container"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:background="#drawable/seasons_scrollview"
android:divider="#drawable/nav_bar_divider"
android:elevation="#dimen/card_elevation"
android:orientation="vertical"
android:showDividers="middle">
<LinearLayout
android:id="#+id/coach_group"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:orientation="vertical"
android:paddingBottom="#dimen/margin_small"
android:paddingLeft="#dimen/margin_standard"
android:paddingRight="#dimen/margin_standard"
android:paddingTop="#dimen/margin_small">
<TextView
android:id="#+id/squad_coach_header"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="Coach"
android:textSize="#dimen/text_size_standard" />
<android.support.v7.widget.RecyclerView
android:id="#+id/coach_recycler"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:paddingTop="#dimen/margin_small"
android:paddingBottom="#dimen/margin_small"
android:scrollbars="none">
</android.support.v7.widget.RecyclerView>
</LinearLayout>
<LinearLayout
android:id="#+id/gk_group"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:orientation="vertical"
android:paddingBottom="#dimen/margin_small"
android:paddingLeft="#dimen/margin_standard"
android:paddingRight="#dimen/margin_standard"
android:paddingTop="#dimen/margin_small">
<TextView
android:id="#+id/squad_gk_header"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="Goalkeepers"
android:textSize="#dimen/text_size_standard" />
<android.support.v7.widget.RecyclerView
android:id="#+id/gk_recycler"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:paddingTop="#dimen/margin_small"
android:paddingBottom="#dimen/margin_small"
android:scrollbars="none">
</android.support.v7.widget.RecyclerView>
</LinearLayout>
<LinearLayout
android:id="#+id/def_group"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:orientation="vertical"
android:paddingBottom="#dimen/margin_small"
android:paddingLeft="#dimen/margin_standard"
android:paddingRight="#dimen/margin_standard"
android:paddingTop="#dimen/margin_small">
<TextView
android:id="#+id/squad_def_header"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="Defense"
android:textSize="#dimen/text_size_standard" />
</LinearLayout>
<LinearLayout
android:id="#+id/mid_group"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:orientation="vertical"
android:paddingBottom="#dimen/margin_small"
android:paddingLeft="#dimen/margin_standard"
android:paddingRight="#dimen/margin_standard"
android:paddingTop="#dimen/margin_small">
<TextView
android:id="#+id/squad_mid_header"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="Midfielders"
android:textSize="#dimen/text_size_standard" />
</LinearLayout>
<LinearLayout
android:id="#+id/for_group"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:orientation="vertical"
android:paddingBottom="#dimen/margin_small"
android:paddingLeft="#dimen/margin_standard"
android:paddingRight="#dimen/margin_standard"
android:paddingTop="#dimen/margin_small">
<TextView
android:id="#+id/squad_for_header"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="Forwards"
android:textSize="#dimen/text_size_standard" />
</LinearLayout>
</LinearLayout>
</com.github.ksoichiro.android.observablescrollview.ObservableScrollView>
The MyLinearLayoutManager is a custom LinearLayoutManager I found online in order to solve the wrap-content issue of the SDK LinearLayoutManager.
Is there any way I can have multiple RecyclerViews in a single layout? It seems that I cannot attach more than one LayoutManagers per layout.
Any assistance would be very welcome :)

I had this problem too. My Activity uses Tabs, with three fragments, when I go to third tab, and back to first (or second), this error is thrown.
After searching a lot, I found out that may be the garbage collector, because I was using a strong reference.
Since the constructor LinearLayoutManager uses the activity as the parameter (not the fragment), a Tabs Activity stays active during tabs changes.
Removing the local field in mLinearLayoutManager from the class, and using a weak reference, I could get rid of this problem:
before:
public class MyFragment1 extends Fragment
private LinearLayoutManager linearLayoutManager;
#Override
public void onCreate(Bundle savedInstanceState) {
linearLayoutManager = new LinearLayoutManager(getActivity());
(...)
}
#Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
(...)
mRecyclerView.setLayoutManager(linearLayoutManager);
}
}
I changed to:
public class MyFragment1 extends Fragment {
// private LinearLayoutManager linearLayoutManager;
#Override
public void onCreate(Bundle savedInstanceState) {
// linearLayoutManager = new LinearLayoutManager(getActivity());
(...)
}
#Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
(...)
mRecyclerView.setLayoutManager(
new LinearLayoutManager(getActivity()));
}
}

Just need to create a new instance:
RecyclerView recyclerView = new RecyclerView(getContext());
recyclerView.setLayoutManager(new LinearLayoutManager(getContext()))

I faced this error when providing LayoutManager via Dagger. The solution is to replace layout manager injection with layout manager javax.inject.Provider injection.
#Inject
lateinit var layoutManager: Provider<RecyclerView.LayoutManager>
...
recyclerView.setLayoutManager(layoutManager.get())

I have the same problem as well. I work around it by setting null to LinearLayoutManager instance.
public class MyFragment extends Fragment {
protected LinearLayoutManager mLinearLayoutManager;
...
#Override
public void onDestroy() {
super.onDestroy();
if(mLinearLayoutManager != null) // Workaround: android.support.v7.widget.LinearLayoutManager is already attached to a RecyclerView
mLinearLayoutManager = null;
}

I opened FragmentB from FragmentA. Then returned back to FragmentA and got this exception. I found an error. I created in onCreate:
linearLayoutManager = LinearLayoutManager(requireContext())
and used in it onCreateView:
view.recycler_view.layoutManager = linearLayoutManager
Because linearLayoutManager hadn't recreated when we returned from FragmentB, RecyclerView used an old linearLayoutManager. So, I moved linearLayoutManager = LinearLayoutManager(requireContext()) to onCreateView.

In my case, I had declared a LinearLayoutManager globally and I was trying to attach the same instance of LinearLayoutManager to multiple RecyclerView's so I was getting this error.
The Solution is to attach different LayoutManager to each RecyclerView because one LayoutManager can only be attached to one Recyclerview.

I had this problem with Dagger2 and it was solved by removing the Scope Annotation.
I had annotated the code with some scope and when i removed the scope the problem was fixed.
I hope this would be helpful to you.
#MyScope #Provides static LinearLayoutManager provideLinearLayoutManager
I deleted the #MyScope.

This error I faced with Daggar2
and simply remove by using Provider before the LinearLayoutManager like;
#Inject
Provider <LinearLayoutManager> linearLayoutManager;
mViewDataBinding.rvResult.setLayoutManager(linearLayoutManager.get());

Possible options to fix this crash:
Create an instance of LayoutManager in onViewCreated of Fragment:
override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
super.onViewCreated(view, savedInstanceState)
recyclerView.layoutManager = LinearLayoutManager(context)
}
If it is necessary to have an instance of LayoutManager as a variable in Fragment, then detach the instance from RecyclerView in onDestroyView:
class MyFragment : Fragment() {
private val myLayoutManager by lazy { LinearLayoutManager(context) }
override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
super.onViewCreated(view, savedInstanceState)
recyclerView.layoutManager = myLayoutManager
}
override fun onDestroyView() {
recyclerView.layoutManager = null
super.onDestroyView()
}
}
The second option will work, because the condition for throwing an exception will be false. Here is the code from the method setLayoutManager of RecyclerView:
...
if (layout != null) {
if (layout.mRecyclerView != null) { // will be false
throw new IllegalArgumentException("LayoutManager " + layout
+ " is already attached to a RecyclerView:"
+ layout.mRecyclerView.exceptionLabel());
}
mLayout.setRecyclerView(this);
if (mIsAttached) {
mLayout.dispatchAttachedToWindow(this);
}
}

Setting scrollbars property in RecyclerView crashes my app

I recently started using RecyclerView replacing older ListView. But whenever I use android:scrollbars="vertical" in android.support.v7.widget.RecyclerView element, my app crashes with the following error:
Attempt to invoke virtual method 'boolean android.support.v7.widget.RecyclerView$LayoutManager.canScrollVertically()' on a null object reference
This is how I'm using RecyclerView:
View rootView;
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
rootView = inflater.inflate(R.layout.my_fragment, container, false);
return rootView;
}
protected void onPostExecute(Boolean result) {
RecyclerView rv = (RecyclerView) rootView.findViewById(R.id.recyclerview);
LinearLayoutManager llm = new LinearLayoutManager(context);
rv.setLayoutManager(llm);
RecycleViewAdapter adapter = new RecycleViewAdapter(myRecyclerViewAdapter);
rv.setHasFixedSize(true);
rv.setAdapter(adapter);
}
And this is my_fragment.xml file:
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical">
<android.support.v7.widget.RecyclerView
android:id="#+id/recyclerview"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:scrollbars="vertical">
</android.support.v7.widget.RecyclerView>
</LinearLayout>
Everything works perfectly fine if I don't use android:scrollbars="vertical". Thanks for your help.

When using the RecyclerView you don't really need to specify the scrollbars display; it is the LayoutManager job where you specify if it is Vertical or Horizontal orientation and of course the scrollbar display will automatically change whatever orientation you choose.
//for vertical scrollbar and orientation
LinearLayoutManager(context, LinearLayoutManager.VERTICAL, false);
//for horizontal scrollbar and orientation
LinearLayoutManager(context, LinearLayoutManager.HORIZONTAL, false);

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