java - 除非用常量表达式初始化，否则不能在非静态内部类型中将字段声明为静态

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;
    }

}

我收到标题中的 IDE 错误，指的是变量 D3。有人可以向我解释一下吗？为什么我不能在非静态的内部类中声明静态变量，为什么枚举值不是常量？

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.