In Java, are enum types inside a class static?

I can't seem to access instance members of the surrounding class from inside an enum, as I could from inside an inner class. Does that mean enums are static? Is there any access to the scope of the surrounding class's instance, or do I have to pass the instance into the enum's method where I need it?

public class Universe {
public final int theAnswer;


public enum Planet {
// ...
EARTH(...);
// ...


// ... constructor etc.


public int deepThought() {
// -> "No enclosing instance of type 'Universe' is accessible in this scope"
return Universe.this.theAnswer;
}
}


public Universe(int locallyUniversalAnswer) {
this.theAnswer = locallyUniversalAnswer;
}
}

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最佳答案

Yes, nested enums are implicitly static.

From the language specification section 8.9:

Nested enum types are implicitly static. It is permissable to explicitly declare a nested enum type to be static.

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It wouldn't make sense to make an instance-level (non-static) inner enum class - if the enum instances were themselves tied to the outer class they'd break the enum guarantee -

e.g. if you had

public class Foo {
private enum Bar {
A, B, C;
}
}

For the enum values to properly act as constants, (psuedocode, ignoring access restrictions)

Bar b1 = new Foo().A
Bar b2 = new Foo().A

b1 and b2 would have to be the same objects.