Why CalendarView in android show error when declare or initialize?

good day, I meet some error when I try to code this.

If I initial when declare, it show like this .

public class FragmentCalendar extends Fragment {
    Calendar calendar;
    CalendarView calendarView = (CalendarView) findViewById(R.id.calendarView);
    Date date;
}

the error shown as below:

error: cannot find symbol CalendarView calendarView = (CalendarView)findViewById(R.id.calendarView); ^ symbol: method findViewById(int) location: class FragmentCalendar

If I initial after declare, it show like this.

public class FragmentCalendar extends Fragment {
    Calendar calendar;
    CalendarView calendarView;
    Date date;

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState){
        calendar = calendar.getInstance(); 
        date = calendar.getTime();
        System.out.println("current date : "+date);

        calendarView.findViewById(R.id.calendarView);
        View view = inflater
                .inflate(R.layout.fragment_calendar, container, false); 
        return view;
    }

then show the error as below:

Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.View android.widget.CalendarView.findViewById(int)' on a null object reference at com.example.doodoobii.FragmentCalendar.onCreateView(FragmentCalendar.java:50)

Is already "import android.widget.CalendarView;" and "import java.util.Calendar;".

I can't get what is the cause, and no idea how to fix this. Please teach me , thanks for all senior here.

This is the xml file of it.

<CalendarView
            android:id="@+id/calendarView"
            android:layout_height="100dp"
            android:layout_width="wrap_content"
            android:theme="@style/CalendarViewCustom"
            android:dateTextAppearance="@style/CalendarDateCustomText"
            android:weekDayTextAppearance="@style/CalendarWeekCustomText"
            app:layout_constraintBottom_toBottomOf="parent"
            app:layout_constraintLeft_toLeftOf="parent"
            app:layout_constraintTop_toTopOf="parent"/>

I wish it can work nicely. I wish to know why it will error, what I did wrong to it? Thank you.

calendarView is initialized by finding the CalendarView within the inflated view using view.findViewById(). This should resolve the NullPointerException that you were encountering.

public class FragmentCalendar extends Fragment {
    Calendar calendar;
    CalendarView calendarView;
    Date date;

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState){
        View view = inflater.inflate(R.layout.fragment_calendar, container, 
                           false);
        calendarView = view.findViewById(R.id.calendarView); // Initialize calendarView here
        calendar = Calendar.getInstance(); 
        date = calendar.getTime();
        System.out.println("current date : "+date);
        return view;
    }
}