java.lang.NullPointerException：尝试在空对象引用上调用虚方法 'boolean java.util.Calendar.before(java.lang.Object)'

Question

用户可以按下三个按钮：一个会调出时钟 select 时间，另一个会调出日历 select 日期。第三个按钮是最后按下的，意思是发出通知。

当我按下最后一个按钮时，出现错误：

java.lang.NullPointerException: Attempt to invoke virtual method 'boolean java.util.Calendar.before(java.lang.Object)' on a null object reference
        at com.example.meeldetuletuserakendus.MeeldetuletusActivity.startAlarm(MeeldetuletusActivity.java:105)

尝试执行 startAlarm(c) 方法时出现问题。我可以肯定地知道，当代码只能选择一个时间时，一切都运行良好，但是当我编写代码的日期选择部分时，出现了一些问题。所以问题可能与日期有关。

MeeldetuletusActivity.java:

public class MeeldetuletusActivity extends AppCompatActivity implements TimePickerDialog.OnTimeSetListener, DatePickerDialog.OnDateSetListener {
    String timeText;
    TextView textTime;
    Button nupp_vali, nupp_katkesta, nupp_kuupaev, nupp_alarm;
    Calendar c;



    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_meeldetuletus);

        nupp_vali = findViewById(R.id.button2);
        nupp_katkesta = findViewById(R.id.nupp_katkesta);
        nupp_kuupaev = findViewById(R.id.button);
        nupp_alarm = findViewById(R.id.button3);

        nupp_alarm.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {

                startAlarm(c);
            }
        });

        nupp_vali.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                DialogFragment timePicker = new TimePickerFragment();
                timePicker.show(getSupportFragmentManager(), "time picker");

            }
        });

        nupp_kuupaev.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                DialogFragment kuupaevaValija = new KuupaevaFragment();
                kuupaevaValija.show(getSupportFragmentManager(), "date picker");
            }
        });

        nupp_katkesta.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                katkestaAlarm();
                Toast.makeText(MeeldetuletusActivity.this, "Meeldetuletus unustatud", Toast.LENGTH_SHORT).show();

            }
        });




    }


    @Override
    public void onTimeSet(TimePicker view, int hourOfDay, int minute) {
        Calendar c = Calendar.getInstance();
        c.set(Calendar.HOUR_OF_DAY, hourOfDay);
        c.set(Calendar.MINUTE, minute);
        c.set(Calendar.SECOND, 0);
    }


    public void onDateSet (DatePicker view, int aasta, int kuu, int paev) {
        Calendar c = Calendar.getInstance();
        c.set(Calendar.YEAR, aasta);
        c.set(Calendar.MONTH, kuu);
        c.set(Calendar.DAY_OF_MONTH, paev);
    }



    private void startAlarm(Calendar c) {
        AlarmManager alarmManager = (AlarmManager) getSystemService(Context.ALARM_SERVICE);
        Intent intent = new Intent(this, NotificationPublisher.class);
        PendingIntent pendingIntent = PendingIntent.getBroadcast(this, 1, intent, 0);
        if (c.before(Calendar.getInstance())) {
            c.add(Calendar.DATE, 1);
        }
        Objects.requireNonNull(alarmManager).setExact(AlarmManager.RTC_WAKEUP,
                c.getTimeInMillis(), pendingIntent);
    }

    private void katkestaAlarm() {
        AlarmManager alarmManager = (AlarmManager) getSystemService(Context.ALARM_SERVICE);
        Intent myIntent = new Intent(getApplicationContext(), NotificationPublisher.class);
        PendingIntent pendingIntent = PendingIntent.getBroadcast(
                getApplicationContext(), 1, myIntent,
                PendingIntent.FLAG_UPDATE_CURRENT);

        alarmManager.cancel(pendingIntent);
    }
}

第 105 行是：

if (c.before(Calendar.getInstance())) {

KuupaevaFragment.java:

public class KuupaevaFragment extends DialogFragment {
    @NonNull
    @Override
    public Dialog onCreateDialog(Bundle savedInstanceState) {
        Calendar c = Calendar.getInstance();
        int aasta = c.get(Calendar.YEAR);
        int kuu = c.get(Calendar.MONTH);
        int paev = c.get(Calendar.DAY_OF_MONTH);
        return new DatePickerDialog(getActivity(), (DatePickerDialog.OnDateSetListener) getActivity(),
                aasta, kuu, paev);


    }


}

TimePickerFragment.java：（这之前工作得很好，我只是把它放在这里所以你也可以看到 KuupaevaFragment.java 的意思）

public class TimePickerFragment extends DialogFragment {
    @NonNull
    @Override
    public Dialog onCreateDialog(Bundle savedInstanceState) {
        Calendar c = Calendar.getInstance();
        int hour = c.get(Calendar.HOUR_OF_DAY);
        int minute = c.get(Calendar.MINUTE);
        return new TimePickerDialog(getActivity(), (TimePickerDialog.OnTimeSetListener) getActivity(),
                hour, minute, DateFormat.is24HourFormat(getActivity()));


    }


}

NotificationPublisher.java:

public class NotificationPublisher extends BroadcastReceiver {
    @Override
    public void onReceive(Context context, Intent intent) {
        NotificationHelper notificationHelper = new NotificationHelper(context);
        NotificationCompat.Builder nb = notificationHelper.getChannelNotification();
        notificationHelper.getManager().notify(1, nb.build());
        Notification notification = nb.build();
        notification.defaults |= Notification.DEFAULT_VIBRATE;
        notification.defaults |= Notification.DEFAULT_SOUND;
    }
}

NotificationHelper.Java:

class NotificationHelper extends ContextWrapper {
    public static final String channelID = "channelID";
    public static final String channelName = "Channel Name";
    private NotificationManager notificationManager;
    public NotificationHelper(Context base) {
        super(base);
        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
            createChannel();
        }
    }
    @TargetApi(Build.VERSION_CODES.O)
    private void createChannel() {
        NotificationChannel channel = new NotificationChannel(channelID, channelName,
                NotificationManager.IMPORTANCE_HIGH);
        getManager().createNotificationChannel(channel);
    }
    public NotificationManager getManager() {
        if (notificationManager == null) {
            notificationManager = (NotificationManager)
                    getSystemService(Context.NOTIFICATION_SERVICE);
        }
        return notificationManager;
    }
    public NotificationCompat.Builder getChannelNotification() {
        return new NotificationCompat.Builder(getApplicationContext(), channelID)
                .setContentTitle("Meeldetuletus")
                .setContentText("Teie valitud aeg on käes")
                .setSmallIcon(R.drawable.ic_baseline_notifications_active_24);
    }
}

Answer 1

尝试在onTimeSet和onDateSet中引用Activity的字段c：

@Override
public void onTimeSet(TimePicker view, int hourOfDay, int minute) {
    c = Calendar.getInstance();
    c.set(Calendar.HOUR_OF_DAY, hourOfDay);
    c.set(Calendar.MINUTE, minute);
    c.set(Calendar.SECOND, 0);
}


public void onDateSet(DatePicker view, int aasta, int kuu, int paev) {
    c = Calendar.getInstance();
    c.set(Calendar.YEAR, aasta);
    c.set(Calendar.MONTH, kuu);
    c.set(Calendar.DAY_OF_MONTH, paev);
}

所以不要像上面的代码那样写 Calendar c = Calendar.getInstance(); c = Calendar.getInstance();。

或者你可以尝试在Activity中初始化c属性:

Calendar c = Calendar.getInstance();

并使用它来更改日历的属性，例如在 onTimeSet 方法中：

@Override
public void onTimeSet(TimePicker view, int hourOfDay, int minute) {
    c.set(Calendar.HOUR_OF_DAY, hourOfDay);
    c.set(Calendar.MINUTE, minute);
    c.set(Calendar.SECOND, 0);
}

java.lang.NullPointerException：尝试在空对象引用上调用虚方法 'boolean java.util.Calendar.before(java.lang.Object)'

java.lang.NullPointerException: Attempt to invoke virtual method 'boolean java.util.Calendar.before(java.lang.Object)' on a null object reference

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