I would put this on the line above so that it's:

this.digits = Arrays.stream(string.split(""))
            .map(Integer::parseInt)
            .collect(Collectors.toList());

You might know this already but to make sure formatting is correct you can use ctrl + alt + L if you're using intellij on Windows (it's cmd + alt + L on mac). On eclipse it's ctrl + shift + F (cmd + shift + F on mac). I've found that very useful :)

Also string.split(("")) can be string.split(""). Not your fault at all, I just didn't notice it until now :)

@FridaTveit
thanks for your suggestions, I'll update the PR as soon as get back home.

Is there any industry accepted best practices for code formatting (as you probably mentioned) or it's usually specific to the project (if so, please point me to the page describing this rules)?

The defaults in IntelliJ (and probably also Eclipse) should be fine for formatting :) The google style guide formatting section would probably be useful if you want something for reference. I've mostly learnt best practices from experience and code reviews, so I haven't read the google style guide cover to cover, but it looks sensible and I think many people use it :)

thanks!