java - java中的枚举-导致编译时错误的变量声明序列

Question

我创建的枚举如下:

public enum Planet {
    MERCURY(3.303e+23, 2.4397e6), VENUS(4.869e+24, 6.0518e6), EARTH(5.976e+24,
            6.37814e6), MARS(6.421e+23, 3.3972e6), JUPITER(1.9e+27, 7.1492e7), SATURN(
            5.688e+26, 6.0268e7), URANUS(8.686e+25, 2.5559e7), NEPTUNE(
            1.024e+26, 2.4746e7);

    private final double mass; // in kilograms
    private final double radius; // in meters

    Planet(double mass, double radius) {
        this.mass = mass;
        this.radius = radius;
    }

但是如果我将变量声明作为枚举中的第一条语句，编译器将不会接受它

public enum Planet {

  private final double mass; // in kilograms
    private final double radius; // in meters
    MERCURY(3.303e+23, 2.4397e6), VENUS(4.869e+24, 6.0518e6), EARTH(5.976e+24,
            6.37814e6), MARS(6.421e+23, 3.3972e6), JUPITER(1.9e+27, 7.1492e7), SATURN(
            5.688e+26, 6.0268e7), URANUS(8.686e+25, 2.5559e7), NEPTUNE(
            1.024e+26, 2.4746e7);

    Planet(double mass, double radius) {
        this.mass = mass;
        this.radius = radius;
    }

知道为什么吗？明白这可能是枚举规范所说的，但我试图从编译器的角度理解变量声明的顺序如何困扰编译器？？

Answer 1

这就是枚举的 java 语法的定义方式 - 枚举值是您希望写下的第一件事，然后才是实例字段方法等。

从JLS的相关部分看:

EnumDeclaration:
    ClassModifiersopt enum Identifier Interfacesopt EnumBody

EnumBody:
    { EnumConstantsopt ,opt EnumBodyDeclarationsopt }

EnumConstants:
    EnumConstant
    EnumConstants , EnumConstant