Fragment Force Close 中的 DatePickerDialog

Question

我需要你的帮助。我是 Android App 的新手，我想用 DatePickerDialog 创建一个日期选择器。我可以让对话框显示并选择日期，但是当我单击“确定”时，应用程序已关闭。我不知道我哪里出错了，我试图寻找所有的解决方案，但我仍然无法解决这个问题。

public class NotificationsFragment extends Fragment {

private NotificationsViewModel notificationsViewModel;
private DatePickerDialog datePickerDialog;
private Button lihatBtn;
private TextView tanggalDisplay, hariDisplay, bulanDisplay;
private CardView cardView;
private SimpleDateFormat simpleDateFormat;
private String hari, tanggal, bulan;
private DatePickerDialog.OnDateSetListener dateSetListener;

public View onCreateView(@NonNull LayoutInflater inflater,
                         ViewGroup container, Bundle savedInstanceState) {
    notificationsViewModel =
            new ViewModelProvider(this).get(NotificationsViewModel.class);
    View root = inflater.inflate(R.layout.fragment_notifications, container, false);
    simpleDateFormat = new SimpleDateFormat("dd-MM-yyyy", Locale.US);
    TextView hariDisplay = root.findViewById(R.id.text_hari_riwayat);
    TextView tanggalDisplay = root.findViewById(R.id.text_tanggal_riwayat);
    TextView bulanDisplay = root.findViewById(R.id.text_riwayat);
    CardView cardView = root.findViewById(R.id.card_date);
    Button lihatBtn = root.findViewById(R.id.lihat);

    cardView.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            showDateDialog();
        }
    });

    return root;
}

private void showDateDialog(){

    /**
     * Calendar untuk mendapatkan tanggal sekarang
     */
    Calendar calendar = Calendar.getInstance();
    int year = calendar.get(Calendar.YEAR);
    int month = calendar.get(Calendar.MONTH);
    int day = calendar.get(Calendar.DAY_OF_MONTH);
    datePickerDialog = new DatePickerDialog(requireContext(), dateSetListener, year, month, day);

    datePickerDialog.getWindow();
    datePickerDialog.show();


    /**
     * Initiate DatePicker dialog
     */
    dateSetListener = new DatePickerDialog.OnDateSetListener(){
        @Override
        public void onDateSet(DatePicker datePicker, int year, int month, int day) {
            month = month + 1;
            bulan = month + " " + year;
            tanggal = String.valueOf(day);
            bulanDisplay.setText(bulan);
            tanggalDisplay.setText(tanggal);
        }
    };
  }
}

这是XML

    <androidx.cardview.widget.CardView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:id="@+id/card_date"
        android:layout_marginTop="4dp">

        <RelativeLayout
            android:layout_width="wrap_content"
            android:layout_height="wrap_content">

            <TextView
                android:id="@+id/text_tanggal_riwayat"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:layout_marginStart="8dp"
                android:layout_marginBottom="10dp"
                android:layout_marginTop="8dp"
                android:layout_marginEnd="8dp"
                android:textAlignment="textStart"
                android:textSize="30sp"
                android:textColor="@color/primary"
                android:textStyle="bold"
                android:text="27"
                />
            <TextView
                android:id="@+id/text_hari_riwayat"
                android:layout_toEndOf="@+id/text_tanggal_riwayat"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:layout_marginStart="8dp"
                android:layout_marginTop="8dp"
                android:layout_marginEnd="8dp"
                android:textAlignment="textStart"
                android:textStyle="bold"
                android:textColor="@color/black"
                android:textSize="15sp"
                android:text="Sabtu"
                />
            <TextView
                android:id="@+id/text_bulan_riwayat"
                android:layout_toEndOf="@+id/text_tanggal_riwayat"
                android:layout_below="@+id/text_hari_riwayat"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:layout_marginStart="8dp"
                android:layout_marginEnd="8dp"
                android:textAlignment="textStart"
                android:textStyle="bold"
                android:textColor="@color/black"
                android:textSize="15sp"
                android:text="November 2020"
                />
        </RelativeLayout>
    </androidx.cardview.widget.CardView>

这是错误日志

2020-11-29 14:22:57.243 25192-25192/com.github.otoponik E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.github.otoponik, PID: 25192
java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.TextView.setText(java.lang.CharSequence)' on a null object reference
    at com.github.otoponik.ui.notifications.NotificationsFragment.onDateSet(NotificationsFragment.java:112)
    at android.app.DatePickerDialog.onClick(DatePickerDialog.java:173)
    at com.android.internal.app.AlertController$ButtonHandler.handleMessage(AlertController.java:185)
    at android.os.Handler.dispatchMessage(Handler.java:107)
    at android.os.Looper.loop(Looper.java:228)
    at android.app.ActivityThread.main(ActivityThread.java:7782)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:492)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:981)

Answer 1

您必须在使用前实例化 dateSetListener :

 private void showDateDialog(){

 dateSetListener = new DatePickerDialog.OnDateSetListener(){
        @Override
        public void onDateSet(DatePicker datePicker, int year, int month, int day) {
            month = month + 1;
            bulan = month + " " + year;
            tanggal = String.valueOf(day);
            bulanDisplay.setText(bulan);
            tanggalDisplay.setText(tanggal);
        }
    };
    /**
     * Calendar untuk mendapatkan tanggal sekarang
     */
    Calendar calendar = Calendar.getInstance();
    int year = calendar.get(Calendar.YEAR);
    int month = calendar.get(Calendar.MONTH);
    int day = calendar.get(Calendar.DAY_OF_MONTH);
    datePickerDialog = new DatePickerDialog(requireContext(), dateSetListener, year, month, day);

    datePickerDialog.getWindow();
    datePickerDialog.show();


    /**
     * Initiate DatePicker dialog
     */
   
  }

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