JSON field - Parse String to Int

Have a relatively simple (I think) issue, but being a novice to JSON cannot seem to find a neat solution.

I have an Entity object with Id field as type Integer. However, the incoming Json data to be mapped has the id as a string.

With this a straightforward map does not seem to be possible. Is there a way to change the string data in the JSON to an integer, before mapping?

Example Json data

{"Id": "021", "userAge": 99}

Example Entity

@Entity

public class User{

@id
int userId;
int userAge;

}

Many thanks.