Unable to Connect to HP QC using the REST API

I have gone through the login logout example provided by HP (in ALM 11.52). How ever I am unable to use it to connect it to QC.

I know that I have missed some minor thing, but after spending quite an amount of time also, I am unable to figure it out.

I have attached my java code. Please check the attachments.

PFB the below error which I am getting:

Exception in thread "main" java.lang.NullPointerException
at com.HPALM.connection.using.RESTAPI.AuthenticateLoginLogoutExample.isAuthenticated(AuthenticateLoginLogoutExample.java:182)
at com.HPALM.connection.using.RESTAPI.AuthenticateLoginLogoutExample.authenticateLoginLogoutExample(AuthenticateLoginLogoutExample.java:49)
at com.HPALM.connection.using.RESTAPI.AuthenticateLoginLogoutExample.main(AuthenticateLoginLogoutExample.java:25)

I am quite new to REST API.

Any help would be highly appreciated.

Thanks.