public class Main extends AppCompatActivity {

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.notification);


        Button btn = findViewById(R.id.button);
        btn.setOnClickListener(v -> {
            NotificationManager manager = getSystemService(NotificationManager.class);

            if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
                //create the NotificationChannelGroup first as the NotificationChannel requires one
                NotificationChannelGroup channelGroup = new NotificationChannelGroup("Notifica", "Notifica");
                String groupDescription = "group description";
                if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.P) {
                    channelGroup.setDescription(groupDescription);
                }
                manager.createNotificationChannelGroup(channelGroup);

                NotificationChannel channel = new NotificationChannel("Notifica", "Notifica", NotificationManager.IMPORTANCE_HIGH);
                channel.setGroup(channelGroup.getId()); //This was "n" but should be "Notifica" (the ID of the group we created)
                manager.createNotificationChannel(channel);
                System.out.println(manager.getNotificationChannel("Notifica"));
            }

            NotificationCompat.Builder notifica = new NotificationCompat.Builder(Main.this, "Notifica")
                    .setPriority(4)
                    .setSmallIcon(R.drawable.ic_launcher_background)
                    .setContentTitle("Circolare")
                    .setContentText("Nome Circolare")
                    .setGroup("n")
                    .setChannelId("Notifica");


            System.out.println(notifica);
            manager.notify(1, notifica.build());
        });

    }
}