警报未被取消

Question

我在一个活动中设置闹钟，并希望在另一个作为服务的类中取消它。以下是我的代码：

Intent intent = new Intent(SetAlarm.this, AlertFriend.class);
                PendingIntent pi = PendingIntent.getActivity(SetAlarm.this, unique_id, inent,
                        PendingIntent.FLAG_UPDATE_CURRENT);

                Calendar c1 = Calendar.getInstance();

                 c1.set(Calendar.HOUR_OF_DAY, hour);
                 c1.set(Calendar.MINUTE, min);
                 c1.set(Calendar.SECOND, 0);
                 c1.set(Calendar.MILLISECOND, 0);

                 AlarmManager alarmManager = (AlarmManager) getSystemService(Context.ALARM_SERVICE);
                alarmManager.set(AlarmManager.RTC_WAKEUP, c1.getTimeInMillis(), pi);

以下是我要删除警报的地方（其中a是pi）：

Intent intent = new Intent(context, AlertFriend.class);
AlarmManager alarm = (AlarmManager)context.getSystemService(Context.ALARM_SERVICE);
alarm.cancel(PendingIntent.getActivity(context, a, intent,PendingIntent.FLAG_UPDATE_CURRENT));