Creating new Credentials object from stored AccessToken and RefreshToken

Question

I'm following the Implementing Server Side-Authentication part of the Google Drive SDK docs, and in the code presented the following part if left as an exercise for the reader:

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();
}

How does one actually do this, given that we have the AuthorizationToken and RefreshTokens stored as strings?