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 Intent notification_intent = new Intent(context, Dashboard.class);
 notification_intent.putExtra("username", dataSnapshot.getKey()); // sending extra data with the notification, this data can be used when the user clicks on the notification

 PendingIntent pendingIntent = PendingIntent.getActivity(context, 0, notification_intent, 0);


 if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {

     channel = new NotificationChannel("CHANNEL-01", "request", NotificationManager.IMPORTANCE_HIGH);
     notificationManager.createNotificationChannel(channel);

     notificationBuilder = new NotificationCompat.Builder(context, channel.getId())
             .setSmallIcon(R.drawable.account_icon)
             .setContentTitle("Notification.")
             .setContentText(notification.getUser() + " " + notification.getContent())
             .setAutoCancel(true)
             .setContentIntent(pendingIntent);
 } else
 {
     notificationBuilder = new NotificationCompat.Builder(context, "CHANNEL-01")
             .setSmallIcon(R.drawable.account_icon)
             .setContentTitle("Notification.")
              .setContentText(notification.getUser() + " " + notification.getContent())
             .setAutoCancel(true)
             .setContentIntent(pendingIntent);
 }

 notificationManager.notify(notificationId, notificationBuilder.build());