Java Http POST to send JSON push notification

SendFCMPushMessage.java

import java.io.IOException;
import java.io.OutputStreamWriter;
import javax.net.ssl.HttpsURLConnection;
import java.net.URL;
import org.apache.sling.commons.json.JSONException;
import org.apache.sling.commons.json.JSONObject;

public class SendFCMPushMessage {
      public static void main(String[] args) throws IOException, JSONException {
        URL url = new URL("https://fcm.googleapis.com/fcm/send");
        HttpsURLConnection client;
        client = (HttpsURLConnection) url.openConnection();
        client.setDoOutput(true);
        client.setDoInput(true);
        client.setRequestMethod("POST");
        client.setRequestProperty("Content-Type","application/json");
        client.setRequestProperty("Authorization","key=fcm_server_key_comes_here");
        client.setRequestProperty("Accept", "application/json");

        JSONObject message = new JSONObject();
        JSONObject pushRequest = new JSONObject();
        message.put("title", "message_title_comes_here");
        message.put("body", "message_body_comes_here");
        pushRequest.put("notification", message);
        pushRequest.put("to", "client_token_comes_here");
        OutputStreamWriter wr = new OutputStreamWriter(client.getOutputStream());
        wr.write(pushRequest.toString());
        wr.flush();
        client.getResponseCode();
    }
}