为什么我们不能重写方法并定义它以返回原始方法的超类? [英] why we can't override a method and define it to return a superclass of the original method?

问题描述

我目前正在从 java教程oracle <中学习Java类和对象. /a>，并且遇到以下语句和代码.我理解这个概念，但不知道为什么我们不能覆盖一个方法并定义它以返回原始方法的超类?背后的原因是什么?有人可以启发我吗?预先感谢您的帮助！

I'm currently learning Java classes and objects from java tutorial oracle and have encountered the following statements and code. I understand the concept but I do not know why we can't override a method and define it to return a superclass of the original method? What is the reason behind it? Could someone please enlighten me? Thanks in advance for any help!

您可以重写方法并定义它以返回该方法的子类. 原始方法，如下所示:

You can override a method and define it to return a subclass of the original method, like this:

public Number returnANumber() {
    ...
}

覆盖原始方法:

public ImaginaryNumber returnANumber() {
    ...
}

推荐答案

想象一下是否有可能:

public class CarFactory {
    Car giveMeACar() { ... };
}

public class SpecialCarFactory extends CarFactory {
    @Override
    Object giveMeACar() {
        return "hello world";
    }
)

public class Driver {
    void drive() {
        CarFactory carFactory = new SpecialCarFactory();

        Car car = carFactory.giveMeACar();
        // err, wait, sorry, can't do that. 
        // This car factory, despite its name, doesn't produce cars. 
        // It produces objects, and I've heard they're just 
        // "hello world" strings. Good luck driving a "hello world"
        // string on a highway!
    }
}

看，这只是合同中的事情.当您去咖啡店时，您期望它可以出售咖啡.如果不遵守此合同，则不能将其称为咖啡店":咖啡店必须出售咖啡. 它可以出售牛奶咖啡，因为牛奶咖啡仍然是咖啡. (就像汽车制造厂只能生产丰田汽车一样，因为丰田汽车是汽车，而且您可以像其他汽车一样驾驶丰田汽车，而甚至不知道它是丰田汽车:这就是多态性.)

See, it's just a contract thing. When you go to a coffee shop, you expect it to sell coffee. Something can't be called "a coffee shop" if it doesn't comply to this contract: a coffee shop must sell coffee. It can sell milked coffee, because a milked coffee is still a coffee. (just like a car factory can produce Toyota only, because Toyota is a car, and you can drive a Toyota like any other car, without even knowing it's a Toyota: that's polymorphism).

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