类型边界: Apple和Orange

Question

class Fruit implements Comparable<Fruit> {

    private final int weigth;

    public Fruit(int weight) {
        this.weigth = weight;
    }

    @Override
    public int compareTo(Fruit other) {
        return Integer.compare(this.weigth, other.weigth);
    }

    public int getWeigth() {
        return this.weigth;
    }
}

class Apple extends Fruit {

    public Apple(int weight) {
        super(weight);
    }

}

class Orange extends Fruit {

    public Orange(int weight) {
        super(weight);
    }
}

任务是重新设计类型系统，并实现水果之间的正确比较(苹果和苹果，橙子和橙子，但不是苹果和橙子，以及橙子和苹果)。

和“苹果与橙子和橙子与苹果的比较应该在编译时受到限制”。

不知道如何正确更改，请给个提示。

Answer 1

我建议在Fruit中引入一个泛型，它代表"SELF type..“

class Fruit<S extends Fruit> implements Comparable<S> {

            private final int weigth;

            public Fruit(int weight) {
                this.weigth = weight;
            }

            @Override
            public int compareTo(S other) {
                return Integer.compare(this.weigth, other.getWeigth());
            }

            public int getWeigth() {
                return this.weigth;
            }
        }

        class Apple extends Fruit<Apple> {

            public Apple(int weight) {
                super(weight);
            }

        }

        class Orange extends Fruit<Orange> {

            public Orange(int weight) {
                super(weight);
            }
        }
    }

证明：

​

Answer 2

class Fruit {

private final int weigth;

public Fruit(int weight) {
    this.weigth = weight;
}


public int getWeigth() {
    return this.weigth;
}

}

class Apple extends Fruit implements Comparable<Apple> {

public Apple(int weight) {
    super(weight);
}

@Override
public int compareTo(Apple o){
    return Integer.compare(this.getWeigth(), o.getWeigth());
}

}

class Orange extends Fruit implements Comparable<Orange>{

public Orange(int weight) {
    super(weight);
}

@Override
public int compareTo(Orange o){
    return Integer.compare(this.getWeigth(), o.getWeigth());
}

}

Answer 3

如果两个结果不能比较，那么为什么要实现可比性呢？应该在具体的类级别定义可比较对象，这样就可以解决您的问题。

或者，如果你想在现有的类中放入check，你可以这样做

@Override
public int compareTo(Fruit other) {
    if(other.getClass().equals(this.getClass())){
        return Integer.compare(this.weigth, other.weigth);
    }
       throw new NotComparableException("Object are of different class, and so can't be compared");
}