java - 看来我可以*在定义之前引用一个字段*

Question

public enum MyEnum1 {

    FOO(BAR), BAR(FOO);

    private MyEnum1 other;

    private MyEnum1(MyEnum1 other) {
        this.other = other;
    }

    public MyEnum1 getOther() {
        return other;
    }

}

MyEnum1 生成错误 Cannot reference a field before it is defined，这是可以理解的，因为声明顺序在这里很重要。但是为什么下面的编译？

public enum MyEnum2 {

    FOO { public MyEnum2 getOther() { return BAR; } },
    BAR { public MyEnum2 getOther() { return FOO; } };

    public abstract MyEnum2 getOther();

}

FOO 在定义 BAR 之前引用 BAR，我错了吗？

Answer 1

重要的 JLS 部分是 this和 this

A class or interface type T will be initialized immediately before the first occurrence of any one of the following:

T is a class and an instance of T is created.

T is a class and a static method declared by T is invoked.

A static field declared by T is assigned.

A static field declared by T is used and the field is not a constant variable (§4.12.4).

T is a top level class (§7.6), and an assert statement (§14.10) lexically nested within T (§8.1.3) is executed.

和

The optional class body of an enum constant implicitly defines an anonymous class declaration (§15.9.5) that extends the immediately enclosing enum type.

所以

FOO { public MyEnum2 getOther() { return BAR; } }, 
BAR { public MyEnum2 getOther() { return FOO; } };

您正在创建两个扩展 MyEnum2 的匿名类。

当 BAR 在您调用 Foo.getOther() 时最终被引用，或者某些其他代码段执行 MyEnum2.Bar 时，类型将被初始化。