Typed subsets of enums

Question

Let's say I have the following enum type:

public enum Country {
    CHINA,
    JAPAN,
    FRANCE,
    // ... and all other countries
    AUSTRIA,
    POLAND;
}

Now I would to create a subset of this enum, conceptually like:

public enum EuropeanUnion constraints Country {
    FRANCE,
    // ... and all other countries within the European Union
    AUSTRIA,
    POLAND;
}

public enum LandlockedCountries constraints Country {
    // ... all landlocked countries
    AUSTRIA;
}

I'd like to create subsets of an enum type so that I can write methods such as

Set<Person> getNationalMembersOfEuropeanParliament(EuropeanUnion euCountry);

Using the subtype EuropeanUnion for parameter euCountry protects the API user from accidentally passing in an invalid country, e.g. the non-EU JAPAN.

Are there any ways of "constraining" the range of valid enum values so that one can benefit from the static type system?

Answer 1

You might want to consider using an EnumSet; this won't give you new types, but it lets you work with collections of enum values in a set-theoretic way:

public enum Country {
    CHINA,
    JAPAN,
    ...;

    public static final EnumSet<Country> EUROPEAN_UNION = EnumSet.of(Country.FRANCE, Country.AUSTRIA, ...);
}

EnumSet implements Set, so you can use e.g. addAll(), removeAll() and retainAll() to produce unions, differences, and intersections (just remember to copy the left-hand-side set first).