java中比较器中super的使用

Question

I understand in

Comparator < ? super T> comp

it returns the maximum element of the given collection, according to the order defined by the specified comparator. But I don't understand the purpose of the

super T

Could anyone possibly explain?

Answer 1

I understand in

Comparator < ? super T> comp

it returns the maximum element of the given collection, according to the order defined by the specified comparator. But I don't understand the purpose of the

super T

Could anyone possibly explain?

Answer 2

I understand in

Comparator < ? super T> comp

it returns the maximum element of the given collection, according to the order defined by the specified comparator. But I don't understand the purpose of the

super T

Could anyone possibly explain?

Answer 3

Lets take an example

interface Fruit {
    int getPrice();
}

class Orange implements Fruit {
    int price;

    public Orange(int price) {
        this.price = price;
    }

    @Override
    public int getPrice() {
        return price;
    }
}

interface Apple extends Fruit {
    int getAppleQuality();
}

class WineSapApple implements Apple {

    public int price, quality;
    public WineSapApple(int price, int quality) {
        this.price = price;
        this.quality = quality;
    }
    @Override
    public int getPrice() {
        return price;
    }
    @Override
    public int getAppleQuality() {
        return quality;
    }
}

class AppleQualityComparator implements Comparator<Apple> {
    public int compare(Apple a1, Apple a2) {
        return Integer.compare(a1.getAppleQuality(), a2.getAppleQuality());
    }
}

class FruitPriceComparator implements Comparator<Fruit> {
    public int compare(Fruit f1, Fruit f2) {
        return Integer.compare(f1.getPrice(), f2.getPrice());
    }
}

Okay now

        List<Fruit> fruits = List.of(new WineSapApple(100, 50), new Orange(10));
        Collections.sort(fruits, new FruitPriceComparator());
//        Collections.sort(fruits, new AppleQualityComparator()); compilation error

Importantly we want to re use the Comparator declared at super class level

    List<Apple> apples = List.of(new WineSapApple(100, 50), new WineSapApple(150, 75));
    Collections.sort(apples, new FruitPriceComparator()); //<--- reason for `? super T`
    Collections.sort(apples, new AppleQualityComparator());

Answer 4

I understand in

Comparator < ? super T> comp

it returns the maximum element of the given collection, according to the order defined by the specified comparator. But I don't understand the purpose of the

super T

Could anyone possibly explain?