[Combinators] Since Java 8 `static` and `default` methods are allowed within `interface`s.

java_combinators.java

interface ArtistValidator extends Function<Artist, Optional<Artist>>{
    static ArtistValidator hasMailWithAtSign(){
        return holds(artist -> artist.email.contains("@"));
    }
    static ArtistValidator nameIsNotEmpty(){
        return holds(artist -> artist.name.trim().length()>0);
    }
    static ArtistValidator holds(Function<Artist, Boolean> p){
        return artist -> p.apply(artist)?Optional.of(artist):Optional.empty();
    }
    default ArtistValidator and(ArtistValidator other){
        return artist -> apply(artist).flatMap(other);
    }
}
ArtistValidator validator = nameIsNotEmpty().and(hasMailWithAtSign());
validator.apply(new Artist("Lenzman", "[email protected]")); // valid 
validator.apply(new Artist("", "[email protected]")); // invalid