Android : Gmail API: Authentication using token retrieval

I have read this whole article for the authentication using OAuth2.0. But I didn't find a suitable method to do this on an android application. Please suggest a method to get the access token so that I can build a Gmail service object and access the inbox or any other method.

This is the example given by them in this link:

GoogleCredential credential = new GoogleCredential().setAccessToken(accessToken);
Plus plus = new Plus.builder(new NetHttpTransport(), JacksonFactory.getDefaultInstance(), credential)
    .setApplicationName("Google-PlusSample/1.0")
    .build();