Openvidu-java-client API - openVidu Instance not initializing

Hi,
i am trying to integrate Openvidu-js-java in my Project. whenever i am trying to create session for getting a valid token from openvidu-server by using openvidu-java-client , Null pointer Exception is showing. it shows the openvidu instances is null when its trying to access the create session method ex. Session session = this.openVidu.createSession();

i have already instantiate openvidu in the constructor. By following :
this.openVidu = new OpenVidu(OPENVIDU_URL, SECRET);

My openvidu_url is a Public accessible IP where my instance of OpenVidu Server is up an running. I dont know what could be the problem that the openvidu instance is not intilaizing and showing null.

if anyone give me some closure regarding this issue it would be a great help.
Thank you.

Use a working tutorial app and start modifying it.