运行时多态 [英] Runtime polymorphism

问题描述

假设我有A类

public class A
{
    public void method()
    {
        //do stuff
    }
}

还有另一个B类

public class B extends A
{
    public void method()
    {
        //do other stuff
    }
}

现在我有以下陈述:

A a = new B();
a.method();

这是运行时多态性的一个例子吗?如果是，那么在编译时是否不对引用a进行绑定?

Is this an example of run time polymorphism? If yes, then is no binding is done for the reference a at compile time?

推荐答案

编译器将告诉您这是行不通的，因为A和B之间没有关系可以让您编写A a = new B();

B要么必须扩展A，要么都必须实现其中带有void method()的通用接口.

B either has to extend A or both have to implement a common interface with void method() in it.

您可以通过在编译器中尝试快速地回答此问题.成为实验者-比论坛快.

You could have answered this very quickly by trying it with the compiler. Be an experimentalist - it's faster than forums.

更新:

现在，它有效，因为B扩展了A.您关心的绑定(动态绑定)是在运行时完成的.变量"a"的静态编译时间类型为A类；在运行时，它动态绑定到B类型的引用.是的，我认为这是多态的一个例子.

It works, now that B extends A. The binding that you care about, dynamic binding, is done at runtime. The static compile time type for the variable "a" is class A; at runtime it is dynamically bound to a reference of type B. Yes, I would consider this to be an example of polymorphism.

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