规定Java中静态变量的继承有哪些规则？ [英] What are the rules dictating the inheritance of static variables in Java?

问题描述

我有一个班级，超级：

public class Super {
    public static String foo = "foo";
}

我还有另一个班级， Sub 扩展超级：

I also have another class, Sub that extends Super:

public class Sub extends Super {
    static {
        foo = "bar";
    }

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

当我运行它时，它打印出 bar 。

我的第三个（也是最后一个）课程是测试：

When I run it, it prints out bar.
My third (and last) class is Testing:

public class Testing {
    public static void main (String[] args) {
        System.out.println(Super.foo);
        System.out.println(Sub.foo);
        System.out.println(Super.foo);
    }
}

打印：

foo
foo
foo

我不明白为什么 foo 的内容会因您访问它的类别而异。任何人都可以解释一下吗？

I don't understand why the contents of foo vary depending on what class you're accessing it from. Can anyone explain?

推荐答案

我不明白为什么foo的内容根据什么而有所不同您正在访问它的类。

I don't understand why the contents of foo vary depending on what class you're accessing it from.

基本上这是类型初始化的问题。当 Sub foo 的值设置为bar >已初始化。但是，在 Testing 类中，对 Sub.foo 的引用实际上被编译为对<$ c $的引用c> Super.foo ，所以它不会最终初始化 Sub ，所以 foo 永远不会变成bar。

Basically it's a matter of type initialization. The value of foo is set to "bar" when Sub is initialized. However, in your Testing class, the reference to Sub.foo is actually compiled into a reference to Super.foo, so it doesn't end up initializing Sub, so foo never becomes "bar".

如果您将测试代码更改为：

If you change your Testing code to:

public class Testing {
    public static void main (String[] args) {
        Sub.main(args);
        System.out.println(Super.foo);
        System.out.println(Sub.foo);
        System.out.println(Super.foo);
    }
}

然后它会打印出bar四次，因为第一个语句会强制初始化 Sub ，这会改变 foo 的值。这根本不在于它的访问位置。

Then it would print out "bar" four times, because the first statement would force Sub to be initialized, which would change the value of foo. It's not a matter of where it's accessed from at all.

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