java - 我可以在声明前访问静态 block 中的静态字段吗？

Question

我对下面的代码感到困惑，我预计它会给出一个错误或答案将是 10 但它给出了 20 怎么办？

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

    static{
        x=10;
    }

    static int x=20;
}

Answer 1

在 section 12.4.2 of the JLS 中指定，它给出了类初始化的细节:

Next, execute either the class variable initializers and static initializers of the class, or the field initializers of the interface, in textual order, as though they were a single block.

变量初始值设定项 (x = 20) 在程序文本中出现在静态初始值设定项(包含 x = 10 的 block )之后。因此，初始化结束时的值为 20。

如果您调换轮次顺序，使变量初始值设定项排在第一位，您将看到 10。

不过，我强烈建议您尽可能避免编写依赖于文本排序的代码。

编辑:该变量仍然可以在静态初始值设定项中使用，因为它在范围内——就像您可以在先于变量声明的方法中使用实例变量一样。然而，section 8.3.2.3对此给出了一些限制:

The declaration of a member needs to appear textually before it is used only if the member is an instance (respectively static) field of a class or interface C and all of the following conditions hold:

• The usage occurs in an instance (respectively static) variable initializer of C or in an instance (respectively static) initializer of C.

• The usage is not on the left hand side of an assignment.

• The usage is via a simple name.

• C is the innermost class or interface enclosing the usage.

It is a compile-time error if any of the four requirements above are not met.

因此，如果您将静态初始值设定项更改为:

static {
    System.out.println(x);
}

然后你会得到一个错误。

但是，您现有的静态初始化器以符合所有限制的方式使用 x。