带有Oauth和服务帐户的GData

I am in a strange problem. I am writing an application in core java which needs to access its own account at google docs. So after doing a lot of googling, I found that OAuth2.0 with Service Account is something I am looking for. But I am not using App Engine or anything. Its a simple application. I don't know how can I use the OAuth. I have written below code and I dont know how to proceed further. Looking for someone to guide me further:I am in a strange problem. I am writing an appl