具有继承的静态块的行为 [英] Behavior of static blocks with inheritance

问题描述

我正在尝试使用这样的静态块：

I am trying to use static blocks like this:

我有一个名为的基类Base.java

public class Base {

    static public int myVar;

}

派生类派生。 java ：

public class Derived extends Base {

    static
    {
        Base.myVar = 10;
    }
}

我的主函数是这样的：

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

这会将输出打印为 0 0 我预期的位置 10 0 。有人可以解释这种行为吗？另外，如果我希望我的派生类设置静态变量的值，我该如何实现呢？

This prints the out put as 0 0 where as I expected 10 0. Can somebody explain this behavior? Also, if I want my derived classes to set the values for a static variable how can I achieve that?

推荐答案

据我所知。您不会调用任何派生属性（ myVar 属于 Base ，而不是派生）。并且java没有从 Derived 运行静态块。如果你向 Derived 添加一些静态字段并访问它，那么java会执行所有静态块。

As I understand. You don't call any Derived properties (myVar belongs to Base, not to Derived). And java is not running static block from Derived. If you add some static field to Derived and access it, then java executes all static blocks.

class Base {

    static public int myVar;

}


class Derived extends Base {

    static public int myVar2;

    static
    {
        Base.myVar = 10;
    }
}


public class Main {
    public static void main( String[] args ) throws Exception {
        System.out.println(Derived.myVar2);
        System.out.println(Base.myVar);
    }
}

从java规范中，初始化类时（和静态）块被执行）：

From java specification, when class is initialized (and static block got executed):

12.4.1初始化发生时类或接口类型T将在第一次出现之前立即初始化以下内容：

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

•T是一个类，并且创建了一个T实例。

•T是一个类和一个声明的静态方法调用T。

•指定由T声明的静态字段。

•使用由T声明的静态字段，该字段不是常量变量（§4.12.4 ）。$
•T是顶级类（第7.6节），并且执行在词典内嵌套在T（第8.1.3节）内的断言语句（第14.10节）。

• 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.

这篇关于具有继承的静态块的行为的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！