SendNotifiationUsingFirbase.java

final private String FCM_API = "https://fcm.googleapis.com/fcm/send";
final private String serverKey = "key=" + "AAAA.....";
final private String contentType = "application/json";
String TOPIC;



////////////
TOPIC = "/topics/user_" + messageSendToUser.getId(); //topic has to match what the receiver subscribed to

JSONObject notification = new JSONObject();
JSONObject notifcationBody = new JSONObject();
try {
    notifcationBody.put("title", "Message from ahmed bader el din");
    notifcationBody.put("chatID", roomID);
    notifcationBody.put("type", "chat");
    notifcationBody.put("body", "Hi How Are You!!");

    notification.put("to", TOPIC);
    notification.put("notification", notifcationBody);
    notification.put("data", notifcationBody);
} catch (JSONException e) {
    Log.e("onCreate", "onCreate: " + e.getMessage());
}
   sendNotification(notification);
////////////


private void sendNotification(JSONObject notification) {
        JsonObjectRequest jsonObjectRequest = new JsonObjectRequest(FCM_API, notification,
                response -> {
                    Log.i("onResponse", "onResponse: " + response.toString());

                },
                error -> {
                    Log.i("onErrorResponse", "onErrorResponse: Didn't work");
                }) {
            @Override
            public Map<String, String> getHeaders() {
                Map<String, String> params = new HashMap<>();
                params.put("Authorization", serverKey);
                params.put("Content-Type", contentType);
                return params;
            }
        };
        VolleySingleton.getInstance(getApplicationContext()).addToRequestQueue(jsonObjectRequest);
    }