field cannot be declared static in a non-static inner type unless initialized with a constant expression

Question

public class Test {

public enum Directions {
        NORTH, WEST, SOUTH, EAST
    }

    static final Directions D1 = Directions.NORTH;

    static class Inner {
        static final Directions D2 = Directions.NORTH;
    }

    class Inner2 {
        static final Directions D3 = Directions.NORTH;
    }

}

I am getting the IDE-Error which is in the title, referring to the variable D3. Can someone explain that to me? Why can I not declare a static variable in an inner class that is not static and why is the enum value not a constant?

Answer 1

JLS §8.1.3 Inner Classes and Enclosing Instances

Inner classes may not declare static members, unless they are constant variables (§4.12.4), or a compile-time error occurs.

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Why is an Enum entry not considered a constant variable?

A variable of primitive type or type String, that is final and initialized with a compile-time constant expression (§15.28), is called a constant variable.