To Make a notification just call it from a method or anywhere

NotificationMaker.java

//for greater than android O
if(Build.VERSION.SDK_INT >= Build.VERSION_CODES.O){
            NotificationChannel channel = new NotificationChannel(CHANNEL_ID,CHANNEL_NAME, NotificationManager.IMPORTANCE_DEFAULT);
            channel.setDescription(CHANNEL_DESC);
            NotificationManager manager = getSystemService(NotificationManager.class);
            manager.createNotificationChannel(channel);
        }
// to make notification details
NotificationCompat.Builder builder = new NotificationCompat.Builder(MainActivity.this,CHANNEL_ID)
        .setSmallIcon(R.drawable.ic_notifications)
        .setContentTitle("Title")
        .setContentText("Description")
        .setPriority(NotificationCompat.PRIORITY_DEFAULT);

NotificationManagerCompat managerCompat = NotificationManagerCompat.from(MainActivity.this);
                        
//to notify the notification
managerCompat.notify(1,builder.build());
}