String myFunction(Optional<ClassWithIdAsString> instanceWithIdAsString, Optional<ClassWithIdAsLong> instanceWithIdAsLong) {

String id = instanceWithIdAsString.map(ClassWithIdAsString::getId)
                .map(id -> String.valueOf(id))
                .orElseGet(t -> instanceWithIdAsLong
                .map(ClassWithIdAsLong::getId));
return id;
}