Using Firebase Admin SDK for Java Push Notification Service

        
            FileInputStream serviceAccount = new FileInputStream("path/to/serviceAccount.json");
            GoogleCredentials credential = GoogleCredentials.fromStream(serviceAccount);
            FirebaseOptions options = FirebaseOptions.builder()
                .setCredentials(credential)
                .build();

            // Initialize the Firebase app
            FirebaseApp.initializeApp(options);

            // Access the Firebase Instance ID Service
            InstanceIdService idService = FirebaseInstanceId.getInstance();
            String token = idService.getToken();
            System.out.println("Device token:" + token);
        
    

        
            public static void main(String[] args) throws Exception {
              String topicName = "/topics/" + "news";

              // Create a new HTTP/HTTPS request
              HttpRequestFactory factory = HTTPTransport.newTrustedTransport().createRequestFactory();

              GenericUrl url = new GenericUrl("https://fcm.googleapis.com/fcm/send");

              JSONObject json = new JSONObject();
              json.put("to", topicName);
              json.put("data", new JSONObject().put("title", "News Update").put("body", "Check out today's news highlights!"));

              HttpContent httpContent = new JsonHttpContent(factory, json);
              httpContent.setType("application/json; charset=utf-8");

              HttpResponse response = factory.buildPostRequest(url, httpContent).execute();
              int statusCode = response.getStatusCode();
              System.out.println("Status Code: " + statusCode);

              if (statusCode == 200) {
                System.out.println("Successfully published message.");
              } else {
                System.err.println("Failed to publish message.");
              }
            }