我正在嘗試在我的一個片段中創建日歷，但我不知道如何將日歷類調用到onCreate（）;在片段中起作用兩.java。我已經嘗試過這個，但我不斷得到錯誤：致命錯誤 嘗試在空對象引用上調用虛擬方法“安卓.view.窗口$回調安卓.視圖.窗口.getCallback（）”.片段兩.java：@Overridepublic void onCreate(Bundle savedInstanceState) {    super.onCreate(savedInstanceState);    Calendar calendar = new Calendar();    calendar.setContentView(R.layout.calendar);}日歷.java：public class Calendar extends AppCompatActivity {    CalendarView calendarView;    TextView dateDisplay;    @Override    protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(R.layout.calendar);        calendarView = (CalendarView) findViewById(R.id.calendarView);        dateDisplay = (TextView) findViewById(R.id.date_display);        dateDisplay.setText("Date: ");        calendarView.setOnDateChangeListener(new CalendarView.OnDateChangeListener() {            @Override            public void onSelectedDayChange(CalendarView calendarView, int i, int i1, int i2) {                dateDisplay.setText("Date: " + i2 + " / " + i1 + " / " + i);                Toast.makeText(getApplicationContext(), "Selected Date:\n" + "Day = " + i2 + "\n" + "Month = " + i1 + "\n" + "Year = " + i, Toast.LENGTH_LONG).show();            }        });    }}如何在我的片段兩個函數中應用日歷類？onCreate()