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App crashes when running code from in fragment

I am making my first app in android studio. So far I have built a bottom navigation bar using fragments. I am now trying to add a calendar feature to one of the fragments, I have copied a tutorial using activities to run the code and think there is a problem with using "View".
This is the code to open for the nav bar to open the fragment.
#Override
public boolean onNavigationItemSelected(MenuItem item) {
switch (item.getItemId()){
case R.id.home:
getSupportFragmentManager().beginTransaction().replace(R.id.container,homeFragment).commit();
return true;
case R.id.calendar:
getSupportFragmentManager().beginTransaction().replace(R.id.container,calendarFragment).commit();
return true;
case R.id.wellbeing:
getSupportFragmentManager().beginTransaction().replace(R.id.container,wellbeingFragment).commit();
return true;
}
return false;
This is java in my calendar fragment, which when opened from the navigation bar crashes the app
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
// Inflate the layout for this fragment
initWidgets();
selectedDate = LocalDate.now();
setMonthView();
return inflater.inflate(R.layout.fragment_calendar, container, false);
}
private void initWidgets()
{
calendarRecyclerView = requireView().findViewById(R.id.calendarRecyclerView);
monthYearText = requireView().findViewById(R.id.monthYearTV);
}
private void setMonthView()
{
monthYearText.setText(monthYearFromDate(selectedDate));
ArrayList<String> daysInMonth = daysInMonthArray(selectedDate);
CalendarAdapter calendarAdapter = new CalendarAdapter(daysInMonth, this);
RecyclerView.LayoutManager layoutManager = new GridLayoutManager(getContext(),7);
calendarRecyclerView.setLayoutManager(layoutManager);
calendarRecyclerView.setAdapter(calendarAdapter);
}
This is the error message I receive in my logcat
2022-05-12 12:45:05.305 10950-10950/com.example.myapplication E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.myapplication, PID: 10950
java.lang.IllegalStateException: Fragment CalendarFragment{c4de180} (94b1c782-ca39-403f-b45b-5d0d7a042f15 id=0x7f080087) did not return a View from onCreateView() or this was called before onCreateView().
at androidx.fragment.app.Fragment.requireView(Fragment.java:1964)
at com.example.myapplication.CalendarFragment.initWidgets(CalendarFragment.java:41)
at com.example.myapplication.CalendarFragment.onCreateView(CalendarFragment.java:30)
at androidx.fragment.app.Fragment.performCreateView(Fragment.java:2963)
at androidx.fragment.app.FragmentStateManager.createView(FragmentStateManager.java:518)
at androidx.fragment.app.FragmentStateManager.moveToExpectedState(FragmentStateManager.java:282)
at androidx.fragment.app.FragmentManager.executeOpsTogether(FragmentManager.java:2189)
at androidx.fragment.app.FragmentManager.removeRedundantOperationsAndExecute(FragmentManager.java:2100)
at androidx.fragment.app.FragmentManager.execPendingActions(FragmentManager.java:2002)
at androidx.fragment.app.FragmentManager$5.run(FragmentManager.java:524)
at android.os.Handler.handleCallback(Handler.java:938)
at android.os.Handler.dispatchMessage(Handler.java:99)
at android.os.Looper.loopOnce(Looper.java:201)
at android.os.Looper.loop(Looper.java:288)
at android.app.ActivityThread.main(ActivityThread.java:7842)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:548)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1003)
I believe there is no error in the calendar code and is something to do with onCreateView in the fragment code.

You cannot call requireView() before returning a non-null value from onCreateView().
Move these lines
initWidgets();
selectedDate = LocalDate.now();
setMonthView();
(where initWidgets() calls requireView()) from onCreateView() to e.g. onViewCreated().

Yes it crashes because findViewById wasn't called from the fragment_calendar layout before returning inflater therefore anything called after the return statement will not be executed and will return a NullPointerException. Unless you override onViewCreated method and put down the codes
So to avoid that, you need to override onViewCreated method or you must return view. Remember, if you want to return view then all codes or public methods must be reached before the return view statement. Else, it'll crash due to exception
Check this codes, it'll fix it correctly
public class TestFragment extends Fragment {
RecyclerView calendarRecyclerView;
TextView monthYearText;
#Nullable
#org.jetbrains.annotations.Nullable
#Override
public View onCreateView(#NonNull LayoutInflater inflater, #Nullable #org.jetbrains.annotations.Nullable ViewGroup container, #Nullable #org.jetbrains.annotations.Nullable Bundle savedInstanceState) {
selectedDate = LocalDate.now();
return inflater.inflate(R.layout.fragment_calendar, container, false);
}
#Override
public void onViewCreated(#NonNull View view, #Nullable #org.jetbrains.annotations.Nullable Bundle savedInstanceState) {
super.onViewCreated(view, savedInstanceState);
initWidgets(view);
setMonthView();
}
private void initWidgets(View view) {
calendarRecyclerView = view.findViewById(R.id.calendarRecyclerView);
monthYearText = view.findViewById(R.id.monthYearTV);
}
private void setMonthView() {
monthYearText.setText(monthYearFromDate(selectedDate));
ArrayList<String> daysInMonth = daysInMonthArray(selectedDate);
CalendarAdapter calendarAdapter = new CalendarAdapter(daysInMonth, getActivity());
RecyclerView.LayoutManager layoutManager = new GridLayoutManager(getActivity(), 7);
calendarRecyclerView.setLayoutManager(layoutManager);
calendarAdapter.setAdapter(calendarAdapter);
}
}

Using a ArrayAdapter won't work with fragment

I am trying to get an ArrayList from an activity and then display it inside a fragment in a custom adapter. The problem it's that I cannot setAdapter to do so. The error it says the context it's wrong, when I created the Adapter.
Creare_istoricAdapter class:
public class Creare_istoricAdapter extends ArrayAdapter<Creare_istoric> {
private static final String TAG = "Creareistoric_adapter";
private Context mContext;
int mResource;
public Creare_istoricAdapter(#NonNull Context context, int resource, #NonNull ArrayList<Creare_istoric> objects) {
super(context, resource, objects);
mContext = context;
mResource = resource;
}
#NonNull
#Override
public View getView(int position, #Nullable View convertView, #NonNull ViewGroup parent) {
String id_alerta = getItem(position).getId_alerta();
String tip_problema = getItem(position).getTip_problema();
String data_ora= getItem(position).getData_ora();
String stare_problema = getItem(position).getStare_problema();
String descriere = getItem(position).getDescriere();
Creare_istoric istoric = new Creare_istoric(id_alerta, tip_problema, data_ora, stare_problema, descriere);
LayoutInflater inflater = LayoutInflater.from(mContext);
convertView = inflater.inflate(mResource, parent, false);
TextView text_tip_alerta_istoric = convertView.findViewById(R.id.text_tip_alerta_istoric);
TextView text_ora_alerta_istoric = convertView.findViewById(R.id.text_ora_alerta_istoric);
TextView text_stare_alerta_istoric = convertView.findViewById(R.id.text_stare_alerta_istoric);
text_tip_alerta_istoric.setText(tip_problema);
text_ora_alerta_istoric.setText(data_ora);
text_stare_alerta_istoric.setText(stare_problema);
return convertView;
}
}
The fragment class:
public class Fragment_alerte_trimise extends Fragment {
public Fragment_alerte_trimise(){}
ListView list_view_alerte_trimise;
ArrayList<Creare_istoric> alerteTrimise= new ArrayList<>();
#Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState){
View alerte_trimise = inflater.inflate(R.layout.fragment_alerte_trimise,container, false);
return alerte_trimise;
Bundle bundle_trimise;
if (bundle_trimise != null) {
alerteTrimise = bundle_trimise.getParcelableArrayList("alerte_trimise");
}
list_view_alerte_trimise =getView().findViewById(R.id.list_view_alerte_trimise);
Creare_istoricAdapter adapter = new Creare_istoricAdapter(this, R.layout.istoric_alerte_adapter, alerteTrimise);
list_view_alerte_trimise.setAdapter(adapter);
}
}
The problems it's in new Creare_istoricAdapter(this, R.layout.istoric_alerte_adapter, alerteTrimise) , about the context, and is also greyed out, saying it's redundant.
EDIT:
I have changed the code following the answers below, but now when I open the activity that uses the fragment, it does not show the list at all. It's only an blank screen.
The fragment code:
public class Fragment_alerte_trimise extends Fragment {
public Fragment_alerte_trimise(){}
private ListView list_view_alerte_trimise;
ArrayList<Creare_istoric> alerteTrimise= new ArrayList<>();
#Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState){
View alerte_trimise = inflater.inflate(R.layout.fragment_alerte_trimise,container, false);
return alerte_trimise;
}
#Override
public void onViewCreated(#NonNull View view, #Nullable Bundle savedInstanceState) {
super.onViewCreated(view, savedInstanceState);
Bundle bundle_trimise = getArguments();
if (bundle_trimise != null) {
alerteTrimise = bundle_trimise.getParcelableArrayList("alerte_trimise");
}
list_view_alerte_trimise =getView().findViewById(R.id.list_view_alerte_trimise);
list_view_alerte_trimise.setAdapter(new Creare_istoricAdapter(getContext(), R.layout.istoric_alerte_adapter, alerteTrimise));
}
}
The way I created the Parceable:
Bundle bundle_trimise = new Bundle();
bundle_trimise.putParcelableArrayList("alerte_trimise", alerteTrimise);
Fragment_alerte_trimise fg = new Fragment_alerte_trimise();
fg.setArguments(bundle_trimise);
I have the ListView created in the layout that's inflated by the onCreateView, but when I use getView().findViewById it says that it may produce 'NullPointerException', and so does the getContext(), which says it might be null

Please try to use getContext() instead of this.

You have to use activity context instead of fragment context this. Instead of using getContext() or getActivity() you should initialize the context on onAttach to avoid NullPointerException. Check below:
private Context activityContext;
#Override
public void onAttach(#NonNull Context context) {
super.onAttach(context);
activityContext = context;
}
#Override
public void onDetach() {
super.onDetach();
activityContext = null;
}
Beside this, you have to return the view inside onCreateView at the very end. Check below:
#Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState){
....
// use alerte_trimise instead of getView()
list_view_alerte_trimise = alerte_trimise.findViewById(R.id.list_view_alerte_trimise);
//use activityContext instead of this
Creare_istoricAdapter adapter = new Creare_istoricAdapter(activityContext, R.layout.istoric_alerte_adapter, alerteTrimise);
....
return alerte_trimise;
}
In your code, you try to use Bundle after call return. That's why rest of your code is unreachable.

You are using Creare_istoricAdapter in your Fragment_alerte_trimise so it needs context. therefor we can give activity context or application context(beyond the activity level)
Moreover...
getContext() - Returns the context view only current running activity.
getActivity()- Return the Activity this fragment is currently associated with.
getActivity() can be used in a Fragment for getting the parent Activity of the Fragment.
Add this code changes to your code.
getActivity().getApplicationContext()

Use getContext() instead of this. this refers to current object (Fragment), which is not subclass of Context.
Create adapter in onViewCreated(), because getContext() returns null in onCreateView().

App not responding error due to Fragment code

So my code for schedule fragment are listed below. These codes are fixed from my later error, but the problem is, it gives a Not responding error once I click "My Schedule" in my navigation drawer. Remember that my Schedule Fragment is connected to Navigation Drawer it self.
#TargetApi(Build.VERSION_CODES.N)
public class ScheduleFragment extends Fragment {
CompactCalendarView compactCalendar;
private SimpleDateFormat dateFormatMonth = new SimpleDateFormat("MMM - yyyy", Locale.getDefault());
public ScheduleFragment() {
// Required empty public constructor
}
#Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState){
// Inflate the layout for this fragment
View v = inflater.inflate(R.layout.fragment_schedule, container, false);
final ActionBar actionBar = ((AppCompatActivity) getActivity()).getSupportActionBar();
actionBar.setDisplayHomeAsUpEnabled(false);
actionBar.setTitle(null);
compactCalendar = (CompactCalendarView) v.findViewById(R.id.compactcalendar_view);
compactCalendar.setUseThreeLetterAbbreviation(true);
Event ev1 = new Event(Color.RED, 1477040400000L, "Teachers' Professional Day");
compactCalendar.addEvent(ev1);
compactCalendar.setListener(new CompactCalendarView.CompactCalendarViewListener() {
#Override
public void onDayClick(Date dateClicked) {
Context c = getContext().getApplicationContext();
if (dateClicked.toString().compareTo("Fri Oct 21 00:00:00 AST 2016") == 0) {
Toast.makeText(c, "Teachers' Professional Day", Toast.LENGTH_SHORT).show();
} else {
Toast.makeText(c, "No Events Planned for that day", Toast.LENGTH_SHORT).show();
}
}
#Override
public void onMonthScroll(Date firstDayOfNewMonth) {
actionBar.setTitle(dateFormatMonth.format(firstDayOfNewMonth));
}
});
return v;
}
}
What seems to be the problem? Do I need to implement the "extends ActionBarActivity" or "AppCompatActivity"? Either of these to will result in error of #Override method.

Android Fragment Back press

I have a couple of fragments. But all the fragments lead to PeriodFragment when they press a button. My question is how do I implement so that when the user presses the BACK button on his mobile phone, the PeriodFragment will switch back to the fragment it was entered from.
This is the java code from PeriodFragment.java:
public class PeriodFragment extends Fragment {
Button btnPretrazi;
public PeriodFragment() {
// Required empty public constructor
}
#Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
// Inflate the layout for this fragment
View view = inflater.inflate(R.layout.fragment_period, container, false);
//buttonPretraziPeriod
btnPretrazi = (Button) view.findViewById(R.id.buttonPretraziPeriod);
btnPretrazi.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View v) {
Toast.makeText(getContext(),"test", Toast.LENGTH_SHORT).show();
}
});
return view;
}
}
This is my TvrdjavaFragment.java (one of the fragments that have the button to switch to PeriodFragment.java) :
package com.example.ivanp.msnis;
public class TvrdjavaFragment extends Fragment {
Button btnIdinaperiod;
public TvrdjavaFragment() {
// Required empty public constructor
}
#Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
View view = inflater.inflate(R.layout.fragment_tvrdjava, container, false);
// Inflate the layout for this fragment
//show date
TextView datumprikaz = (TextView) view.findViewById(R.id.datumprikaz);
Date danas = new Date();
SimpleDateFormat sdf = new SimpleDateFormat("dd.MM.yyyy");
String novDatum = sdf.format(danas);
datumprikaz.setText(novDatum);
//End of show date
btnIdinaperiod = (Button) view.findViewById(R.id.buttonIdinaperiod);
btnIdinaperiod.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View v) {
PeriodFragment periodFragment = new PeriodFragment();
FragmentTransaction periodFragmentTransaction = getActivity().getSupportFragmentManager().beginTransaction();
periodFragmentTransaction.replace(R.id.frame, periodFragment);
periodFragmentTransaction.commit();
}
});
return view;
}
}
I'm new to android studio, so please tell the details.

According to android docs When using fragments in your app, individual FragmentTransaction objects may represent context changes that should be added to the back stack. For example, if you are implementing a master/detail flow on a handset by swapping out fragments, you should ensure that pressing the Back button on a detail screen returns the user to the master screen. To do so, call addToBackStack() before you commit the transaction:
// Works with either the framework FragmentManager or the
// support package FragmentManager (getSupportFragmentManager).
getSupportFragmentManager().beginTransaction()
.add(detailFragment, "detail")
// Add this transaction to the back stack
.addToBackStack()
.commit();

Adding to backstack will solve the problem:
periodFragmentTransaction.replace(R.id.frame, periodFragment);
periodFragmentTransaction.addToBackStack(null);
periodFragmentTransaction.commit();

Android Studio - Fragment button action

I tried to make an application that shows a fragment at start then you can change that fragment to an other one with a button in the first fragment. But when I put the button action in the fragments java, it won't start and I get the nullpointerexception error. Could you tell me why?
public class Fragment_main extends Fragment {
FragmentManager fm = getFragmentManager();
#Nullable
#Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
Button button_inditas = (Button) getView().findViewById(R.id.button_inditas);
button_inditas.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View v) {
fm.beginTransaction().replace(R.id.content_frame, new Fragment_1()).commit();
}
});
return inflater.inflate(R.layout.fragment_main,container,false);
}
}

But when I put the button action in the fragments java, it won't start and I get the nullpointerexception error. Could you tell me why?
Well I see some errors on your code...
First of all you can't call getView() if you haven't inflated one, it means that you MUST inflate a view as i'll put on my answer and then you can avoid the getView() and use that view itself.
And instead of returning the inflater you have to return your View
This is how your Fragment should look like:
public class Fragment_main extends Fragment {
public Fragment_main() {
}
#Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
View rootView = inflater.inflate(R.layout.fragment_main,container,false);
Button button_inditas = (Button)rootView.findViewById(R.id.button_inditas);
button_inditas.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View v) {
FragmentManager fragmentManager = getFragmentManager ();
FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction ();
fragmentTransaction.add (R.id.content_frame, new Fragment_1());
fragmentTransaction.commit ();
});
return rootView;
}
}

You have to inflate the view first and use the view returned by that instead of getView ().
View view = inflater.inflate (...);
and then
... = (Button) view.findView(...);
This happens because the view that is returned by getView () hasn't been created yet, so getView() returns null.