To be fair, this doesn't really simulate type classes, it's just allowing you to specify in a method signature that a particular type implements more than one interface. To simulate type classes, you would have to have the ability to declare statically that an existing type, potentially one you have no control over, implements an interface, and to then have the static extension methods apply to that type automatically. This is in effect what Scala's implicits do that allows them to simulate type classes.
Indeed, it really only simulates the function type constraint aspect of Haskell typeclasses. Fortunately most of the time I have direct access to the implementation, and can throw an interface on a type when I need it. This is effectively the expression problem. You can always solve it by using a wrapper class, although it's a bit inconvenient.
