Question

I’m writing a program with a lot of enumerations, and I’m having to return the keyset of an EnumMap a lot. But EnumMap.keySet() returns a Set(), so in order to get the EnumSet I want, I have to use a cast:

EnumMap<SomeEnum, Object> myMap = getMap();
EnumSet<SomeEnum> myEnum = (EnumSet<SomeEnum>) myMap.keySet();

If I don’t cast, the compiler will complain of a type mismatch; it cannot convert from Set<SomeEnum> to EnumSet<SomeEnum>. It seems unnecessary to have to cast this, as the keys of an EnumMap will always be an enumeration. Does anyone know why the keySet() method was constructed this way? I’ve thought at times it might have something to do with EnumSet being an abstract class, but surely EnumMap could just return whatever the factory method of EnumSet provides.

Cheers, all!

EDIT: I’m very sorry, the above code throws a CastClassException. You could get the EnumSet by using

EnumSet<SomeEnum> myEnum = EnumSet.copyOf(myMap.keySet());

I really should have checked before posting.

Answer 1

I think its because the keySet is not an EnumSet. 😉

The reason it is not is that the keySet is a view onto the underlying map.

myMap.keySet().removeAll(keysToRemove); // removes multiple keys.