覆盖Java方法 [英] Overriding a Java Method

问题描述

我是Java的新手，并且已经阅读了一些有关重写方法的教程，但是我正在查看的示例无法按我预期的方式工作.例如，我有以下代码:

I'm new to Java, and I've read over some tutorials on overriding methods, but an example I'm looking at isn't working the way I expect. For example, I have the code:

public class A{
    public void show(){
        System.out.println("A");
    }
    public void run(){
        show();
    }
    public static void main( String[] arg ) {
        new A().run();
    }
}

public class B extends A{
    @Override
    public void show(){
        System.out.println("B");
    }
}

当我实例化并调用B.run()时，我希望看到输出"B".但是，我看到的是"A".我在做什么错了?

When I instantiate and call B.run(), I would expect to see "B" outputted. However, I see "A" instead. What am I doing wrong?

是的，这些类位于两个单独的文件中.为了简洁起见，将它们一起显示.

Yes, the classes are in two separate files. They're shown together for brevity.

我不确定B是如何实例化的，因为它是由第三方程序使用类加载器完成的.

I'm not sure how B is being instantiated, as it's being done by a third-party program using a classloader.

有关第三方程序的更多信息.首先调用A.main()，我最初没有显示(抱歉).我假设我需要制作"new A().run();"更通用的使用当前类的名称.有可能吗?

More info on the third-party program. It starts by calling A.main(), which I didn't initially show (sorry). I'm assuming I need to make "new A().run();" more generic to use the name of the current class. Is that possible?

推荐答案

如果您执行以下操作，该代码将输出B:

That code will output B if you:

(new B()).run();

无论出什么问题，它都不会出现在您引用的代码中.

Whatever the problem is, it's not in the code you've quoted.

已更新(在您修改之后)

如果第三方程序正在调用A.main()，则您无法在B中做任何(合理的事情)将自身注入A的操作.只要A.main在做new A().run()，它将有一个A实例，而不是B实例.没有要使用的当前类名"，或者如果有(取决于您的观点)，则为A，而不是B.

If the third-party program is calling A.main(), there's nothing (reasonable) you can do in B that will inject itself into A. As long as A.main is doing new A().run(), it's going to have an instance of A, not an instance of B. There's no "current class name" to use, or if there is (depends on your point of view), it's A, not B.

您必须让第三方程序以某种方式而不是A来调用B，或者直接修改A(例如，完全摆脱B).您不要修改A以使其使用B；紧紧地将它与后代绑定在一起，使它们之间的分离变得毫无意义.

You'll have to get the third-party program to call B in some way, rather than A, or just modify A directly (e.g., getting rid of B entirely). You do not want to modify A to make it use B; that tightly binds it to a descendant and makes the separation between them largely pointless.

希望有帮助.

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