code: static OAuth2Credentials fromJson( JSONObject jsonObj )
      {
          String accessToken = jsonObj.optString( OAuth2Credentials.ACCESS_TOKEN, null );
          Date expiresAt = calculateExpiresAt( jsonObj );
          String refreshToken = jsonObj.optString( OAuth2.REFRESH_TOKEN, null );
          String tokenType = jsonObj.optString( OAuth2Credentials.TOKEN_TYPE, null );
  
          return new OAuth2Credentials( accessToken, expiresAt, refreshToken, tokenType );
      }
docs: package private