我可以扩展一个类并覆盖附带的枚举类型吗？ [英] Can I extend a class and override enclosed enum type?

问题描述

如果我有一个类包含一个枚举类型，我可以扩展这个类并覆盖枚举类型或者添加更多的常量到这个枚举？
目的是用户将能够调用 getColor（）方法，而不知道动态对象是从基类还是派生类。
例如：

If I have a class that contains an enum type, can I extend this class and override the enum type or add more constants to this enum? The purpose is that user will be able to call getColor() method without knowing if the dynamic object is from base class or derived class. For example:

public class Car {

    private String name;

    private CarColors color;

    public enum CarColors {
        BLUE,
        GREEN
    }

    public Car(CarColors c) {
        color=c;
    }

    public CarColors getColor() {
        return color;
    }
}

public class RacingCar extends Car {

    private CarColors color;

    public RacingCar(CarColors c) {
        color=c;
    }

    public enum CarColors {
        BLUE,
        GREEN,
        RED
    }

    @Override //?????
    public CarColors getColor() {
        return color;
    }
}

和main：

Car racingCar = new RacingCar();
CarColors color = racingCar.getColor();

推荐答案

您不能扩展枚举。枚举本质上是一个 final 类。但是，枚举可以实现，所以结合泛型，你得到：

You can't extend enum. enums are essentially a final class. However, enums can implement, so combined with generics, you get:

interface Color{

}
enum CarColor implements Color{
    BLUE,
    GREEN;
}
enum RacingCarColor implements Color{
    BLUE,
    GREEN,
    YELLOW;
}
class Vehicle<T extends Color> {
    protected T color;
    protected Vehicle(T color){
        this.color = color;
    }
    public T getColor(){
        return color;
    }
}

class Car extends Vehicle<CarColor>{ 
    public Car(CarColor color){
        super(color);
    }
}

class RacingCar extends Vehicle<RacingCarColor>{ 
    public RacingCar(RacingCarColor color){
        super(color);
    }
}

Voila！

如果您想要要求类型为颜色 和成为枚举，使用此绑定：

If you want to require that the type be a Color and be an enum, use this bound:

class Vehicle<T extends Enum<T> & Color> { ...

或者一个好奇的等价物：

Or a curious equivalent:

class Vehicle<T extends Enum<? extends Color>> { ...

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