英文:
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?
问题
以下是您要求的翻译内容：
以下代码可以编译：
public enum Foo {
A,
B{};
public static void main(String[] args) {
Foo f = Foo.A;
List s = (List)f;
}
}
而这段代码则不能：
public enum Foo {
A,
B;
public static void main(String[] args) {
Foo f = Foo.A;
List s = (List)f;
}
}
我还可以用 Foo.A 替换 Foo.B 并得到相同的结果。
这里发生了什么？在第一个示例中，Foo.A 如何成为一个 List？
英文:
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?
答案1
得分: 3
针对这种类型的转换，《缩窄引用转换规范》定义了规则。枚举类型没有特殊情况，只有对于final和非final类的区分。
基本枚举属于“final类”类别，但您的扩展枚举不属于该类别，因为它通过{}语法引入了一个子类。
当然，即使是在扩展枚举中，您的枚举常量也无法实现List接口，但是当前的规范简单地没有涵盖那种情况。
规范的未来修订可能会改进这一点，届时我希望编译器会实施额外的检查。但是现在，这种编译时的安全性还不可用。
英文:
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.
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