I am trying to figure out user to user notification call using FCM in android, I have an example of "subscribeToTopic("TOPIC Name")", but instead of this I am not able to figure out how to apply user to user notification using token id of the users in FCM. I am referring this tutorial https://blog.usejournal.com/send-device-to-device-push-notifications-without-server-side-code-238611c143
please assist, thanks in advance
btnSend.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                TOPIC = "/topics/userABC"; //topic must match with what the receiver subscribed to
                NOTIFICATION_TITLE = edtTitle.getText().toString();
                NOTIFICATION_MESSAGE = edtMessage.getText().toString();
                JSONObject notification = new JSONObject();
                JSONObject notifcationBody = new JSONObject();
                try {
                    notifcationBody.put("title", NOTIFICATION_TITLE);
                    notifcationBody.put("message", NOTIFICATION_MESSAGE);
                    notification.put("to", TOPIC);
                    notification.put("data", notifcationBody);
                } catch (JSONException e) {
                    Log.e(TAG, "onCreate: " + e.getMessage() );
                }
                sendNotification(notification);
            }
        });
btnSend.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                TOPIC = "/topics/userABC"; //topic must match with what the receiver subscribed to
                NOTIFICATION_TITLE = edtTitle.getText().toString();
                NOTIFICATION_MESSAGE = edtMessage.getText().toString();
                JSONObject notification = new JSONObject();
                JSONObject notifcationBody = new JSONObject();
                try {
                    notifcationBody.put("title", NOTIFICATION_TITLE);
                    notifcationBody.put("message", NOTIFICATION_MESSAGE);
                    notification.put("to", TOPIC);
                    notification.put("data", notifcationBody);
                } catch (JSONException e) {
                    Log.e(TAG, "onCreate: " + e.getMessage() );
                }
                sendNotification(notification);
            }
        });