Views and contextual filter as user id from url

I want to create some views that shows the content created by the user, content flagged by the user and so on. I also need that when user a views user b then user a can see the content created by user b and so on. So i though i under Contextual filter i passed a default values as User Id from url. However when i visit the page i just see the view heading and no content.Please Help

Regards