How to set an alarm Android (AlarmManager)

AlarmBroadcaster.java

public class AlarmBroadcaster extends BroadcastReceiver {

    @Override
    public void onReceive(Context context, Intent intent) {
        //Remember in the SetAlarm file we made an intent to this, this is way this work, otherwise you would have to put an action
        //and here listen to the action, like in a normal receiver
        createNotification(context);
    }

    public void createNotification(Context context) {
        NotificationTrigger.starNotification(context);
    }

}

AndroidManifest.xml

//whatever goes in your manifest

//You need the wake lock so the phone can be activated
<uses-permission android:name="android.permission.WAKE_LOCK" />

//You need the permision to set the alarm
<uses-permission android:name="your.package.alarm.permission.SET_ALARM" />
//And you need the permision to listen the boot
<uses-permission android:name="android.permission.RECEIVE_BOOT_COMPLETED" />

<!--This goes inside the application tags-->
<!--This is the receiver you called when the alarm was executed-->
<receiver android:name="AlarmBroadcaster" />
<!--This is the service that will run when the alarm time is reached-->
<service
  android:name="NotificationTrigger"
  android:exported="false" />
<!--You see how the filter here is boot completed, that is why that receiver works, cause you declared the filter here-->
<receiver
    android:name="DeviceBootReceiver"
    android:enabled="true">
    <intent-filter>
        <action android:name="android.intent.action.BOOT_COMPLETED" />
    </intent-filter>
</receiver>

DeviceBootReceiver.java

public class DeviceBootReceiver extends BroadcastReceiver {
    @Override
    public void onReceive(Context context, Intent intent) {
        if (intent != null) {
            if ("android.intent.action.BOOT_COMPLETED".equals(intent.getAction())) {
                new SetAlarm.alarmSetter(context);
            }
        }
    }
}

NotificationTrigger.java

public class NotificationTrigger extends IntentService {

    private static final String ALARM = "your.package.ALARM";

    public NotificationTrigger() {
        super("NotificationTrigger");
    }

    public static void starNotification(Context context) {
        Intent intent = new Intent(context, NotificationTrigger.class);
        intent.setAction(ALARM);
        context.startService(intent);
    }


    @Override
    protected void onHandleIntent(Intent intent) {
        if (intent != null) {
            final String action = intent.getAction();
            if (ALARM.equals(action)) {
                handleAlarm();
            }
        }
    }

    private void handleAlarm() {
      
      PendingIntent notificationIntent = PendingIntent.getActivity(this, 0, new Intent(this, CoolClassToOpenOnAlarm.class), 0);
      
      NotificationCompat.Builder notificationBuilder = new NotificationCompat.Builder(context)
                .setSmallIcon(R.drawable.cool_icon)
                .setContentTitle("title")
                .setTicker(context.getString(R.string.app_name))
                .setContentText("wake up user!");

        notificationBuilder.setContentIntent(notificationIntent);

        notificationBuilder.setDefaults(NotificationCompat.DEFAULT_VIBRATE);

        notificationBuilder.setAutoCancel(true);

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

        notificationManager.notify(0, notificationBuilder.build());
    }


}

SetAlarm.java

public class SetAlarm {

    public void alarmSetter(Context context) {

        //This will set the alarm time to be at the 14:30
        Calendar calendar = Calendar.getInstance();
        calendar.setTimeInMillis(System.currentTimeMillis());
        calendar.set(Calendar.HOUR_OF_DAY, 14);
        calendar.set(Calendar.MINUTE, 30);

        //This intent, will execute the AlarmBroadcaster when the alarm is triggered
        Intent alertIntent = new Intent(context, AlarmBroadcaster.class);

        AlarmManager alarmManager = (AlarmManager) context.getSystemService(Context.ALARM_SERVICE);

        PendingIntent pendingIntent = PendingIntent.getBroadcast(context, 1, alertIntent, PendingIntent.FLAG_UPDATE_CURRENT);

        alarmManager.setRepeating(AlarmManager.RTC_WAKEUP, calendar.getTimeInMillis(), AlarmManager.INTERVAL_DAY, pendingIntent);

    }

}