为什么我们不能重写一个方法并将其定义为返回原始方法的超类?

Imagine if it was possible:

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，这只是合同的事情.当你去咖啡店时，你希望它卖咖啡.不遵守本合同的东西不能称为咖啡店":咖啡店必须卖咖啡.它可以出售挤奶咖啡，因为挤奶咖啡仍然是咖啡.(就像汽车厂只能生产丰田，因为丰田是一辆汽车，你可以像驾驶其他汽车一样驾驶丰田，甚至不知道它是丰田:这就是多态性.