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Viewpager fragment reference to new copy after activity restore

When my activity containing viewpager is killed by system in background and then restores its state, fragments are correctly created and viewpager adapter can also point to them correctly.
But when I get a fragment reference and try to access its fields, they are all null (checked by using breakpoint).
I checked this by placing breakpoints in fragment onCreateView() and in my activity button's clickListener.
((WelcomeFragment)homeActivityFragmentPageAdapter.getItem(POSITION_HOME)).setdata(myData);
Now this method will through null pointer exception since setdata(data) is internally accessing arraylist field of fragment.
This creates a problem for me since, my activity has to continuously feed network data to the fragment by calling its public method (as suggested by documentation).
How to insure that after state restored; correct instance is pointed in my activity.

Try to use instantiateItem adapter method instead getItem.
((WelcomeFragment)homeActivityFragmentPageAdapter.instantiateItem(mViewPager, POSITION_HOME)).setdata(myData);
Method getItem is overrided method, and common use is creation of child fragments.
EDIT:
In case of the question's scenario, you also need to store the state of FragmentStatePagerAdapter manually:
#Override
public void onSaveInstanceState(Bundle savedInstanceState) {
savedInstanceState
.putParcelable("pages",homeActivityFragmentPageAdapter.saveState());
super.onSaveInstanceState(savedInstanceState);
}
Then you can retrieve the state in oncreate:
#Override
protected void onCreate(final Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
if (savedInstanceState != null) {
homeActivityFragmentPageAdapter.restoreState(savedInstanceState.getParcelable("pages"),this.getClassLoader());
welcomeFragment = (WelcomeFragment) homeActivityFragmentPageAdapter.instantiateItem(mViewPager, POSITION_HOME);
}
else { //simply create a new instance here}
homeActivityFragmentPageAdapter.addFragmentToAdapter(welcomeFragment);
homeActivityFragmentPageAdapter.notifyDataSetChanged();
}

Prevent Fragment recovery in Android

We are using Fragments and we don't need them to be automatically recovered when the Activity is recreated.
But Android every time when Activity::onCreate(Bundle savedInstanceState) -> super.onCreate(savedInstanceState) is called, restores Fragments even if we use setRetainInstance(false) for those Fragments.
Moreover, in those Fragments Fragment.performCreateView() is called directly without going through Fragment::onAttach() and so on. Plus, some of the fields are null inside restored Fragment...
Does anybody know how to prevent Android from restoring fragments?
P.S. We know that in case of recreating Activity for config changes it could be done by adding to manifest android:configChanges="orientation|screenSize|screenLayout. But what about recreating activity in case of automatic memory cleaning?

We finished by adding to activity:
#Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(null);
}
It suppresses any saved data on create/recreate cycle of an Activity and avoids fragments auto re-creation.

#goRGon 's answer was very useful for me, but such use cause serious problems when there is some more information you needs to forward to your activity after recreate.
Here is improved version that only removes "fragments", but keep every other parameters.
ID that is removed from bundle is part of android.support.v4.app.FragmentActivity class as FRAGMENTS_TAG field. It may of course change over time, but it's not expected.
#Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(createBundleNoFragmentRestore(savedInstanceState));
}
/**
* Improve bundle to prevent restoring of fragments.
* #param bundle bundle container
* #return improved bundle with removed "fragments parcelable"
*/
private static Bundle createBundleNoFragmentRestore(Bundle bundle) {
if (bundle != null) {
bundle.remove("android:support:fragments");
}
return bundle;
}

I was having a problem with TransactionTooLargeException. So thankfully after using tolargetool I founded that the fragments (android:support:fragments) were been in memory, and the transaction became too large. So finally I did this, and it worked great.
#Override
public void onSaveInstanceState(final Bundle outState) {
super.onSaveInstanceState(outState);
outState.putSerializable("android:support:fragments", null);
}
Edit: I added it to the Activity. In my case I have one single Activity app and Multiple Fragments.

Those who got NPE with ViewPager when use this method described in the accepted answer, please override
ViewPager.onRestoreInstanceState(Parcelable state)
method and call
super.onRestoreInstanceState(null);
instead.

I removed the fragments in Activity's onCreate.

For an app with a ViewPager, I remove the fragments in onCreate(), before their creation.
Based on this thread: Remove all fragments from container, we have:
FragmentManager fm = getSupportFragmentManager();
for (Fragment fragment: fm.getFragments()) {
fm.beginTransaction().remove(fragment).commitNow();
}

Use this one for androidx
override fun onCreate(savedInstanceState: Bundle?) {
preventFragmentRecreation()
super.onCreate(savedInstanceState)
}
private fun preventFragmentRecreation() {
supportFragmentManager.addFragmentOnAttachListener { _, _ ->
savedStateRegistry.unregisterSavedStateProvider("android:support:fragments")
}
}

This worked for me
#Override
public void onSaveInstanceState(final Bundle outState) {
super.onSaveInstanceState(outState);
outState.remove("androidx.lifecycle.BundlableSavedStateRegistry.key");
}

View hierarchy in not restored automatically. So, in Fragment.onCreateView() or Activity.onCreate(), you have to restore all views (from xml or programmatically). Each ViewGroup that contains a fragment, must have the same ID as when you created it the first time. Once the view hierarchy is created, Android restores all fragments and put theirs views in the right ViewGroup thanks to the ID. Let say that Android remembers the ID of the ViewGroup on which a fragment was. This happens somewhere between onCreateView() and onStart().

Calling super.onCreate() with null parameter?

In onCreate(Bundle bdl){}, we must call its super constructor by super.onCreate(bdl).
For newly created activities, we got a null Bundle in onCreate(Bundle bdl){}. So when we call super.onCreate(bdl), it is the same as calling super.onCreate(null).
For reconstructed activities (like after rotate), we got a non-null Bundle. But I notice even if we call super.onCreate(null), instead of super.onCreate(bdl), it seems to be just the same. The layout restoration works are done in super.onRestoreInstanceState(bdl).
So, is it really true that calling super.onCreate(null) is the same as calling super.onCreate(bdl) in all the cases?
Thanks.

According to the Android Source code, the Activity.onCreate() method forwards the saveInstanceState bundle to the activity's fragments. To be more specific, it fetches a parcelable with the "android:fragments" key and forwards this parcelable to the fragments using the FragmentManager.restoreAllStates() method, which itself restore the state on all fragments.
The Activity.onRestoreInstanceState() method forwards the bundle to the activity's window. Again it fetches the "android:viewHierarchyState" bundle from the saved instance and forwards it the the window using the Window.restoreHierarchyState() method.
So to answer your question, if your activity doesn't use Fragments, then indeed calling super.onCreate(null) won't change anything. But as best practice, I'll advise you to always forward the exact savedInstance bundle (unless you know what you're doing).
Edit : here are the sample source codes I talked about, taken from AOSP v17 :
Activity.java
protected void onCreate(Bundle savedInstanceState) {
// [... some content ellipsed for readability purposes]
if (savedInstanceState != null) {
Parcelable p = savedInstanceState.getParcelable(FRAGMENTS_TAG);
mFragments.restoreAllState(p, mLastNonConfigurationInstances != null
? mLastNonConfigurationInstances.fragments : null);
}
mFragments.dispatchCreate();
getApplication().dispatchActivityCreated(this, savedInstanceState);
mCalled = true;
}
// [...]
protected void onRestoreInstanceState(Bundle savedInstanceState) {
if (mWindow != null) {
Bundle windowState = savedInstanceState.getBundle(WINDOW_HIERARCHY_TAG);
if (windowState != null) {
mWindow.restoreHierarchyState(windowState);
}
}
}

The easiest way for you to find this out is by using Log() utility.
Although, bare in mind that you can put stuff into the bundle with
Bundle bdl = new Bundle(1);
bdl.putString("file_absolute_path", f.getAbsolutePath());
cf.setArguments(bdl);
And retrieve them with getArguments().
So in short - it depends on whether you're using bundle arguments in your app. If no, then it's probably the same.

Fragment saveInstanceState is coming as null after orientation change

I have an activity with action bar tab. Each tab contain a fragment. Now when I rotate my device, bundle in my corresponding fragment is coming as null. This is taken care when I using device post android 3.2, but it is happening when device is Andoird3.0. I am having a headache after working on this issue. I crossed check various link on SO, but no help. Although I have given enough details, still will provide some code snippet as at various cases user ask for code snippet.
In my fragment class I am storing this value
#Override
public void onSaveInstanceState(Bundle outState)
{
super.onSaveInstanceState(outState);
outState.putBoolean("textboxVisible", true);
}
this is storing one boolean variable which it retrived as below.
/**
* Function called after activity is created. Use this
* method to restore the previous state of the fragment
*/
#Override
public void onActivityCreated(Bundle savedInstanceState)
{
super.onActivityCreated(savedInstanceState);
if (savedInstanceState != null)
{
//restore the state of the text box
boolean textboxVisible = savedInstanceState.getBoolean("textboxVisible");
if (textboxVisible)
{
//do some stuff
}
}
}
but after rotation savedInstanceState is coming as null.
I don't what is going wrong. I have read in some document that below 3.2 the onCreateView() of
fragment is not called with bundle value. But to deal with this. Any help will be appreciated.

if you use setRetainInstance(true) the savedInstance bundle is always gonna be null after orientation changed. SO you cannot really save something with it, but what you can do if you need to save something, is to put it in a data member of the fragment, because setRetainInstance(true) preserves the fragment and doesn't destroy it, so after the device was rotated you gonna have the same values.

Try to get the savedInstanceState in onCreate of the Fragment.
Like
#Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setRetainInstance(true);
if (savedInstanceState != null) {
// IT MUST NOT BE NULL HERE
}
}
Please try... i hope it will work

two different Bundle objects?

i have question about two different Bundle object in below methods :
onSaveInstanceState(Bundle outState);
onCreate (Bundle savedInstanceState);
how android system know that bundle object in onCreate method is object that programmer used for save his/her activity states and onCreate method use that Bundle object to get activity state that is killed by system?
is the Bundle object one of the Activity class Members and super.saveInstanceState(outState);
save the Bundle in the Bundle object of Activity and when an activity call onCreate(Bundle ) method this member send to onCreate method?how can i use Bundle in onCreate( ) method?
please help me...

The values you save in the onSaveInstanceState method's bundle will be sent back to you in onCreate. As an example of how this works.
You get a phone call.
Your Activity is stopped and onSaveInstanceState is called. You put a value into this bundle.
Android finishes your activity and destroys that instance because the OS needs memory.
The user returns to your application.
The bundle is recreated from some type of persistent storage that Android maintains on your behalf. Now your onCreate can grab the value you placed in the bundle during onSaveInstanceState
EXAMPLE
#Override
protected void onCreate(Bundle savedInstanceState)
{
super.onCreate(savedInstanceState);
this.setContentView(R.layout.lldr_activity);
mFilterCheckbox = (CheckBox) findViewById(R.id.checkbox_id);
if(savedInstanceState != null) {
mFilterCheckbox.setChecked(savedInstanceState.getBoolean("FILTER_STATE", false));
}
}
#Override
protected void onSaveInstanceState(Bundle outState)
{
super.onSaveInstanceState(outState);
outState.putParcelable("FILTER_STATE", mFilterCheckbox.isChecked());
}