Vaadin 8 converter

In Vaadin 7 I used the converters to format the data in my UI and BeanFieldGroup.
In Vaadin 8 “public interface Converter <PRESENTATION, MODEL>” says, “Converters must not have any side effects
(never update UI from inside a Convert)
How can I format the fields in my UI in Vaadin 8?

Check this blog post, specifically point 6. for how to use fluent API with Binder, e.g. withConverter.

Thank you.
But, how could I convert a wrong value entered by a user to a correct value on screen?

In Vaadin 7, my users could enter, in a date field, 3112, 31/12, 31-12, 12/31/17, etc… and with the converters i transformed it on 12/31/2017 on screeen.