code: public static GoogleAccountCredential usingOAuth2(Context context, Collection<String> scopes) {
      Preconditions.checkArgument(scopes != null && scopes.iterator().hasNext());
      String scopesStr = "oauth2: " + Joiner.on(' ').join(scopes);
      return new GoogleAccountCredential(context, scopesStr);
    }
docs: Constructs a new instance using OAuth 2.0 scopes.
  
  @param context context
  @param scopes non empty OAuth 2.0 scope list
  @return new instance
  
  @since 1.15