Get Userinfo from Oauth2 Google Contacts API

Error which i am getting:

com.google.api.client.googleapis.json.GoogleJsonResponseException: 401 Unauthorized
{
  "code" : 401,
  "errors" : [ {
    "domain" : "global",
    "location" : "Authorization",
    "locationType" : "header",
    "message" : "Invalid Credentials",
    "reason" : "authError"
  } ],
  "message" : "Invalid Credentials"
}

Below code, i am using:

GoogleCredential credential = new GoogleCredential.Builder()
    .setTransport(this.TRANSPORT).setJsonFactory(this.JSON_FACTORY)
    .setClientSecrets(Constants.CLIENT_ID, Constants.CLIENT_SECRET).build();
credential.setAccessToken(tokenResponse.getAccessToken());
credential.setAccessToken(tokenResponse.getRefreshToken());

Till here, i get Refresh token, Access Token, etc

Oauth2 userInfoService = new Oauth2.Builder(this.TRANSPORT,
        this.JSON_FACTORY, credential.getRequestInitializer())
        .setApplicationName(Constants.APPLICATION_NAME).build();

It fails at below line: (Dont know, Why?)

Userinfo userInfo = userInfoService.userinfo().get().execute();

I searched on web, and i get very less examples of it and rare materials. Any body has any idea on it?

What am i doing wrong?