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Hans Wurst
Hans Wurst

Reputation: 384

Why does casting an enum to any interface not cause a compilation error in Java when the enum type contains at least one "extended" enum?

The following code compiles:

    public enum Foo {
        A,
        B{};
        public static void main(String[] args) {
            Foo f = Foo.A;
            List s = (List)f;
        }
    }

This one doesn't:

public enum Foo {
        A,
        B;
        public static void main(String[] args) {
            Foo f = Foo.A;
            List s = (List)f;
        }
    }

I could also replace Foo.A with Foo.B and get the same result. What is going on here? How could Foo.A ever be a List in the first example?

Upvotes: 2

Views: 148

Answers (1)

Ralf Kleberhoff
Ralf Kleberhoff

Reputation: 7290

For this type of casting, the spec on Narrowing Reference Conversion defines the rules. There is no special case for enums, only a distinction between final and non-final classes.

The basic enum falls into the "final class" category, but your extended enum doesn't, as it introduces a subclass via the {} syntax.

Of course, even with the extended enum, there's no way that one of your enum constants could ever implement List, but the current spec simply doesn't cover that situation.

A future revision of the spec might improve that, and then I'd expect compilers to implement the additional checks. But right now, that degree of compile-time safety isn't available.

Upvotes: 3

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