How to send notification without being online?

Let me add some workaround you can find more tutorial outside..

First create receiver class extends BroadcastReceiver.

public class ReminderReceiver extends BroadcastReceiver {


@Override
public void onReceive(Context context, Intent intent) {
    int Request_Code = intent.getExtras().getInt("TIME",0);
    showNotification(context, MainActivity.class,
            "New Notification Alert..!", "scheduled for " + Request_Code + " seconds",Request_Code);
}

public void showNotification(Context context, Class<?> cls, String title, String content,int RequestCode)
{
    Uri alarmSound = RingtoneManager.getDefaultUri(RingtoneManager.TYPE_NOTIFICATION);

    Intent notificationIntent = new Intent(context, cls);
    notificationIntent.setFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);

    TaskStackBuilder stackBuilder = TaskStackBuilder.create(context);
    stackBuilder.addParentStack(cls);
    stackBuilder.addNextIntent(notificationIntent);

    PendingIntent pendingIntent = stackBuilder.getPendingIntent(
            RequestCode,PendingIntent.FLAG_ONE_SHOT);

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

   if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
        CharSequence name = context.getString(R.string.channel_name);
        String description = context.getString(R.string.channel_description);
        int importance = NotificationManager.IMPORTANCE_DEFAULT;
        NotificationChannel channel = new NotificationChannel("my_channel_01", name, importance);
        channel.setDescription(description);
        // Register the channel with the system; you can't change the importance
        // or other notification behaviors after this
        notificationManager.createNotificationChannel(channel);
    }

    NotificationCompat.Builder builder = new NotificationCompat.Builder(context,"my_channel_01");
    Notification notification = builder.setContentTitle(title)
            .setContentText(content).setAutoCancel(true)
       .setSound(alarmSound).setSmallIcon(R.drawable.ic_launcher_background)
            .setContentIntent(pendingIntent).build();


    notificationManager.notify(RequestCode,notification);
}

}

Declare receiver in manifest class below activity tag..

 <receiver android:enabled="true" android:name=".ReminderReceiver"/>

Then set reminder to alarm manager.

 public void setReminder(Context context,Class<?> cls,int sec)
{
    Intent intent = new Intent(context, cls);
        intent.putExtra("TIME",sec);
    PendingIntent pendingIntent = PendingIntent.getBroadcast(context, sec, intent,
            PendingIntent.FLAG_ONE_SHOT);/* Find more about flags: https://developer.android.com/reference/android/app/PendingIntent */

    AlarmManager am = (AlarmManager) context.getSystemService(Context.ALARM_SERVICE);
    am.set( AlarmManager.RTC_WAKEUP, System.currentTimeMillis() + (sec * 1000), pendingIntent );//Add time in milliseconds. if you want to minute or hour mutiply by 60.. For ex: You want to trigger 5 Min then here you need to change 5 * 60 * 1000


}

Finally set your reminder

setReminder(_Context,ReminderReceiver.class,time);

Updated

For support android version 8.0 and above you have to create notification channel. Find more here Manage Channels

Add this in above code:

 if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
        CharSequence name = context.getString(R.string.channel_name);
        String description = context.getString(R.string.channel_description);
        int importance = NotificationManager.IMPORTANCE_DEFAULT;
        NotificationChannel channel = new NotificationChannel("my_channel_01", name, importance);
        channel.setDescription(description);
        // Register the channel with the system; you can't change the importance
        // or other notification behaviors after this
        notificationManager.createNotificationChannel(channel);
    }

Note use drawable for small icons not use mipmap or adaptive icons.Android Oreo Notification Crashes System UI

To Cancel the scheduled notification

 public void cancelReminder(Context context,Class<?> cls)
{
    Intent intent1 = new Intent(context, cls);
    intent1.putExtra("TIME",time);
    PendingIntent pendingIntent = PendingIntent.getBroadcast(context,
            time, intent1, PendingIntent.FLAG_ONE_SHOT);
    AlarmManager am = (AlarmManager) context.getSystemService(Context.ALARM_SERVICE);

    if(pendingIntent != null) {
        am.cancel(pendingIntent);
    }
}

And use above method to delete

cancelReminder(_Context,ReminderReceiver.class);

Note: _Context should be same as used in setreminder() method

I suggest you read Notifications Overview. This well help you to understand how notification works.

To now build the notification, here is the Here is the official documentation for notification.

Read and understand. When you encounter any problem then, you can come back here for the solution.