NotificationSample.java

 private void channelMaker() {
        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
            NotificationChannel serviceChannel = new NotificationChannel(
                    ChannelID,
                    "Dowload Channel",
                    NotificationManager.IMPORTANCE_DEFAULT
            );
            NotificationManager manager = getSystemService(NotificationManager.class);
            manager.createNotificationChannel(serviceChannel);
        }
    }

NotificationSample2.java

new NotificationCompat.Builder(this, ChannelID)
                .setContentTitle("Downloader")
                .setContentText(contentText)
                .setSmallIcon(R.drawable.ic_baseline_cloud_download_24)
                .build();