Nothing wrong with a set of static utility methods that belong together in a class. See for example java.util.Collections. If every method in that class that operates on a List would be specified in the List interface itself, they would have to be implemented by all subclasses. As long as they can be implemented by the public List methods, there is no problem.
Of course, as soon as you start adding methods to the interface (or in case of a class, making methods public) only to be able to put functionality in static methods instead of the class itself, then you're on the wrong path.
