从存储的AccessToken和RefreshToken创建新的Credentials对象

Question

我正在关注Google Drive SDK文档的Implementing Server Side-Authentication部分，如果留下作为读者的练习，则会在代码中显示以下部分：

static Credential getStoredCredentials(String userId) {
  // TODO: Implement this method to work with your database. Instantiate a new
  // Credential instance with stored accessToken and refreshToken.
  throw new UnsupportedOperationException();
}

考虑到我们将AuthorizationToken和RefreshTokens存储为字符串，我们如何实际执行此操作？