AlarmManager

AlarmActivity.java

public class AlarmActivity extends AppCompatActivity{

    @Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        AlarmManager alarmManager = (AlarmManager) getSystemService(Context.ALARM_SERVICE);

        Calendar calendar = Calendar.getInstance();

        // 現在的 18:30:00
        calendar.set(Calendar.HOUR, 18);
        calendar.set(Calendar.MINUTE, 30);
        calendar.set(Calendar.SECOND, 0);

        long alarmTimes;
        
        // 避免設定到過去的時間，如果是過去的時間 day + 1
        if (calendar.getTimeInMillis() > new Date().getTime()) {
            alarmTimes = calendar.getTimeInMillis();
        } else {
            calendar.add(Calendar.DAY_OF_MONTH, 1);
            alarmTimes = calendar.getTimeInMillis();
        }

        // 現在三秒後
        calendar.add(Calendar.SECOND, 3);
        
        PendingIntent pendingIntent =
                PendingIntent.getBroadcast(this, 0, new Intent(this, AlarmReceiver.class), PendingIntent.FLAG_ONE_SHOT);

        alarmManager.set(AlarmManager.RTC_WAKEUP, calendar.getTimeInMillis(), pendingIntent);
        
//        RTC 真實時間 WAKEUP 要不要喚醒螢幕
//        AlarmManager.RTC
//        AlarmManager.RTC_WAKEUP

//        ELAPSED_REALTIME 開機時間 WAKEUP 要不要喚醒螢幕
//        AlarmManager.ELAPSED_REALTIME
//        AlarmManager.ELAPSED_REALTIME_WAKEUP
    }
}

AlarmReceiver.java

public class AlarmReceiver extends BroadcastReceiver {

    @Override
    public void onReceive(Context context, Intent intent) {

        NotificationManager notificationManager
                = (NotificationManager) context.getSystemService(Context.NOTIFICATION_SERVICE);

        PendingIntent pendingIntent = 
                PendingIntent.getActivity(context, 0, new Intent(context, AlarmActivity.class), PendingIntent.FLAG_ONE_SHOT);

        Notification notification = new NotificationCompat.Builder(context)
                .setSmallIcon(R.mipmap.ic_launcher)
                .setContentTitle("起床別睡了")
                .setContentText("我要把你公諸於世")
                .setContentIntent(pendingIntent)
                .build();

        notificationManager.notify(0, notification);
    }
}

manifest.xml

<receiver android:name=".AlarmReceiver">
    <intent-filter>
        <action android:name="android.intent.action.BOOT_COMPLETED"/>
    </intent-filter>
</receiver>