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! }}瞧，这只是合同问题。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，因为丰田汽车是一辆汽车，您可以像其他任何一辆汽车一样驾驶丰田汽车，
甚至不知道它是丰田汽车 ：这就是多态性。）
