java - 为什么静态字段自赋值只能使用显式静态语法进行编译？

Question

为什么这段代码在右侧使用显式静态字段符号进行编译，但不是没有？

public class A
{
    static int a = ++A.a;  // compiles
    //static int a = ++a;  // error - cannot reference a field before it is defined

    public static void main(String[] args) {
        System.out.println(a);
    }
}

Answer 1

这就是语言规范的编写方式。具体来说，Sec 8.3.3说:

References to a field are sometimes restricted, even through the field is in scope. The following rules constrain forward references to a field (where the use textually precedes the field declaration) as well as self-reference (where the field is used in its own initializer).

For a reference by simple name to a class variable f declared in class or interface C, it is a compile-time error if:

• ...

强调我的。

A.a 不是简单名称，因此不是编译时错误。