public static final MediaType JSON
        = MediaType.parse("application/json; charset=utf-8");
private void sendNotification(final String reg_token) {
    new AsyncTask<Void,Void,Void>(){
        @Override
        protected Void doInBackground(Void... params) {
            try {
                OkHttpClient client = new OkHttpClient();
                JSONObject json=new JSONObject();
                JSONObject dataJson=new JSONObject();
                dataJson.put("body","Hi this is sent from device to device");
                dataJson.put("title","dummy title");
                json.put("notification",dataJson);
                json.put("to",regToken);
                RequestBody body = RequestBody.create(JSON, json.toString());
                Request request = new Request.Builder()
                        .header("Authorization","key="+Constants.LEGACY_SERVER_KEY)
                        .url("https://fcm.googleapis.com/fcm/send")
                        .post(body)
                        .build();
                Response response = client.newCall(request).execute();
                String finalResponse = response.body().string();
            }catch (Exception e){
                //Log.d(TAG,e+"");
            }
            return null;
        }
    }.execute();

}

json.put("to","/topics/foo-bar")

private void sendFCMPush() {

            String Legacy_SERVER_KEY = YOUR_Legacy_SERVER_KEY;
            String msg = "this is test message,.,,.,.";
            String title = "my title";
            String token = FCM_RECEIVER_TOKEN;

            JSONObject obj = null;
        JSONObject objData = null;
        JSONObject dataobjData = null;

        try {
            obj = new JSONObject();
            objData = new JSONObject();

            objData.put("body", msg);
            objData.put("title", title);
            objData.put("sound", "default");
            objData.put("icon", "icon_name"); //   icon_name image must be there in drawable
            objData.put("tag", token);
            objData.put("priority", "high");

            dataobjData = new JSONObject();
            dataobjData.put("text", msg);
            dataobjData.put("title", title);

            obj.put("to", token);
            //obj.put("priority", "high");

            obj.put("notification", objData);
            obj.put("data", dataobjData);
            Log.e("!_@rj@_@@_PASS:>", obj.toString());
        } catch (JSONException e) {
            e.printStackTrace();
        }

            JsonObjectRequest jsObjRequest = new JsonObjectRequest(Request.Method.POST, Constants.FCM_PUSH_URL, obj,
                    new Response.Listener<JSONObject>() {
                        @Override
                        public void onResponse(JSONObject response) {
                            Log.e("!_@@_SUCESS", response + "");
                        }
                    },
                    new Response.ErrorListener() {
                        @Override
                        public void onErrorResponse(VolleyError error) {
                            Log.e("!_@@_Errors--", error + "");
                        }
                    }) {
                @Override
                public Map<String, String> getHeaders() throws AuthFailureError {
                    Map<String, String> params = new HashMap<String, String>();
                    params.put("Authorization", "key=" + Legacy_SERVER_KEY);
                    params.put("Content-Type", "application/json");
                    return params;
                }
            };
            RequestQueue requestQueue = Volley.newRequestQueue(this);
            int socketTimeout = 1000 * 60;// 60 seconds
            RetryPolicy policy = new DefaultRetryPolicy(socketTimeout, DefaultRetryPolicy.DEFAULT_MAX_RETRIES, DefaultRetryPolicy.DEFAULT_BACKOFF_MULT);
            jsObjRequest.setRetryPolicy(policy);
            requestQueue.add(jsObjRequest);
}

public void onClick(View view) {

    HttpLoggingInterceptor logging = new HttpLoggingInterceptor();
    logging.setLevel(HttpLoggingInterceptor.Level.BODY);

    OkHttpClient.Builder httpClient = new OkHttpClient.Builder();
    httpClient.addInterceptor(new Interceptor() {
        @Override
        public okhttp3.Response intercept(Chain chain) throws IOException {
            Request original = chain.request();

            // Request customization: add request headers
            Request.Builder requestBuilder = original.newBuilder()
                    .header("Authorization", "key=legacy server key from FB console"); // <-- this is the important line
            Request request = requestBuilder.build();
            return chain.proceed(request);
        }
    });

    httpClient.addInterceptor(logging);
    OkHttpClient client = httpClient.build();

    Retrofit retrofit = new Retrofit.Builder()
            .baseUrl("https://fcm.googleapis.com")//url of FCM message server
            .client(client)
            .addConverterFactory(GsonConverterFactory.create())//use for convert JSON file into object
            .build();

    // prepare call in Retrofit 2.0
    FirebaseAPI firebaseAPI = retrofit.create(FirebaseAPI.class);

    //for messaging server
    NotifyData notifydata = new NotifyData("Notification title","Notification body");

Call<Message> call2 = firebaseAPI.sendMessage(new Message("topic or deviceID", notifydata));

    call2.enqueue(new Callback<Message>() {
        @Override
        public void onResponse(Call<Message> call, Response<Message> response) {

            Log.d("Response ", "onResponse");
            t1.setText("Notification sent");

        }

        @Override
        public void onFailure(Call<Message> call, Throwable t) {
            Log.d("Response ", "onFailure");
            t1.setText("Notification failure");
        }
    });
}

public class Message {
String to;
NotifyData notification;

public Message(String to, NotifyData notification) {
    this.to = to;
    this.notification = notification;
}

}

public class NotifyData {
String title;
String body;

public NotifyData(String title, String body ) {

    this.title = title;
    this.body = body;
}

}

public interface FirebaseAPI {

@POST("/fcm/send")
Call<Message> sendMessage(@Body Message message);

}

public class FcmDeviceNotifier {

private final MediaType JSON = MediaType.parse("application/json; charset=utf-8");
private final String TAG = FcmDeviceNotifier.class.getSimpleName();
private String requestType;
private JSONObject payloadJson;

public FcmDeviceNotifier(String requestType /*can be "notification" or "data"*/) {
    this.requestType = requestType;
}

public void setValuePairs(Map<String, String> valuePairs) {
    try {
        payloadJson = new JSONObject();
        Iterator<Map.Entry<String,String>> iterator = valuePairs.entrySet().iterator();
        while (iterator.hasNext()) {
            Map.Entry<String,String> entry = iterator.next();
            payloadJson.put(entry.getKey(), entry.getValue());
        }
    } catch (JSONException e) {
        e.printStackTrace();
    }
}

public void sendNotification(final String regToken /*Token of target device*/) {
    new AsyncTask<Void,Void,Void>(){
        @Override
        protected Void doInBackground(Void... params) {
            try {
                OkHttpClient client = new OkHttpClient();
                JSONObject fcmJson = new JSONObject();
                fcmJson.put(requestType, payloadJson);
                fcmJson.put(TO, regToken);
                RequestBody body = RequestBody.create(JSON, fcmJson.toString());
                Request request = new Request.Builder()
                        .header(AUTHORIZATION, "key=" + Constants.FCM_LEGACY_KEY)
                        .url("https://fcm.googleapis.com/fcm/send")
                        .post(body)
                        .build();
                Response response = client.newCall(request).execute();
                String finalResponse = response.body().string();
                Log.d(TAG, "sendNotification: " + finalResponse);
            }catch (Exception e){
                Log.e(TAG, "sendNotification: " + e.getMessage());
            }
            return null;
        }
    }.execute();
}

private String mTitle;
private String mMessage;
private String mUsername;
private String mUid;
private String mFirebaseToken;
private String mReceiverFirebaseToken;

private FcmNotificationBuilder() {

}

public static FcmNotificationBuilder initialize() {
    return new FcmNotificationBuilder();
}

public FcmNotificationBuilder title(String title) {
    mTitle = title;
    return this;
}

public FcmNotificationBuilder message(String message) {
    mMessage = message;
    return this;
}

public FcmNotificationBuilder username(String username) {
    mUsername = username;
    return this;
}

public FcmNotificationBuilder uid(String uid) {
    mUid = uid;
    return this;
}

public FcmNotificationBuilder firebaseToken(String firebaseToken) {
    mFirebaseToken = firebaseToken;
    return this;
}

public FcmNotificationBuilder receiverFirebaseToken(String receiverFirebaseToken) {
    mReceiverFirebaseToken = receiverFirebaseToken;
    return this;
}

public void send() {
    RequestBody requestBody = null;
    try {
        requestBody = RequestBody.create(MEDIA_TYPE_JSON, getValidJsonBody().toString());
    } catch (JSONException e) {
        e.printStackTrace();
    }

    Request request = new Request.Builder()
            .addHeader(CONTENT_TYPE, APPLICATION_JSON)
            .addHeader(AUTHORIZATION, AUTH_KEY)
            .url(FCM_URL)
            .post(requestBody)
            .build();

    Call call = new OkHttpClient().newCall(request);
    call.enqueue(new Callback() {
        @Override
        public void onFailure(Call call, IOException e) {
            Log.e(TAG, "onGetAllUsersFailure: " + e.getMessage());
        }

        @Override
        public void onResponse(Call call, Response response) throws IOException {
            Log.e(TAG, "onResponse: " + response.body().string());
        }
    });
}

private JSONObject getValidJsonBody() throws JSONException {
    JSONObject jsonObjectBody = new JSONObject();
    jsonObjectBody.put(KEY_TO, mReceiverFirebaseToken);

    JSONObject jsonObjectData = new JSONObject();
    jsonObjectData.put(KEY_TITLE, mTitle);
    jsonObjectData.put(KEY_TEXT, mMessage);
    jsonObjectData.put(KEY_USERNAME, mUsername);
    jsonObjectData.put(KEY_UID, mUid);
    jsonObjectData.put(KEY_FCM_TOKEN, mFirebaseToken);
    jsonObjectBody.put(KEY_DATA, jsonObjectData);
    return jsonObjectBody;
}

void sendFCMPush(String msg,String token) {
    HttpLoggingInterceptor logging = new HttpLoggingInterceptor();
    logging.setLevel(HttpLoggingInterceptor.Level.BODY);

    OkHttpClient.Builder httpClient = new OkHttpClient.Builder();
    httpClient.addInterceptor(new Interceptor() {
        @Override
        public okhttp3.Response intercept(Chain chain) throws IOException {
            Request original = chain.request();

            // Request customization: add request headers
            Request.Builder requestBuilder = original.newBuilder()
                    .header("Authorization", "key="+Const.FIREBASE_LEGACY_SERVER_KEY); // <-- this is the important line
            Request request = requestBuilder.build();
            return chain.proceed(request);
        }
    });

    httpClient.addInterceptor(logging);
    OkHttpClient client = httpClient.build();

    Retrofit retrofit = new Retrofit.Builder()
            .baseUrl("https://fcm.googleapis.com/")//url of FCM message server
            .client(client)
            .addConverterFactory(GsonConverterFactory.create())//use for convert JSON file into object
            .build();

    // prepare call in Retrofit 2.0
    FirebaseAPI firebaseAPI = retrofit.create(FirebaseAPI.class);

    //for messaging server
    NotifyData notifydata = new NotifyData("Chatting", msg);

    Call<Message> call2 = firebaseAPI.sendMessage(new Message(token, notifydata));

    call2.enqueue(new Callback<Message>() {
        @Override
        public void onResponse(Call<Message> call, retrofit2.Response<Message> response) {
            Log.e("#@ SUCCES #E$#", response.body().toString());
        }

        @Override
        public void onFailure(Call<Message> call, Throwable t) {

            Log.e("E$ FAILURE E$#", t.getMessage());
        }
    });
}

public class Message {
String to;
NotifyData data;

public Message(String to, NotifyData data) {
    this.to = to;
    this.data = data;
}
}

public class Notification {
String title;
String message;
enter code here`enter code here`
public Notification(String title, String message) {
    this.title = title;
    this.message = message;
}
}