如何获取由输入 class arg 参数化的类型引用？

Question

Is there any way to solve the following problem by using parameterized type? I have a parameterized class object and I want to obtain another class object parameterized with the same type. It's easier to check code than explain in words:

public class GenericsCheck {

    static class Fruit {}
    static class Apple extends Fruit {}
    static class Banana extends Fruit {}

    static class FruitBox<T extends Fruit> {}
    static class AppleBox extends FruitBox<Apple> {}
    static class BananaBox extends FruitBox<Banana> {}

    static class Puzzle<T extends Fruit> {

        public Class<FruitBox<T>> getBoxByFruitType(Class<T> fruitType) {
            // COMPILER ERROR
            // Required type: Class<FruitBox<T>>
            // Provided: Class<AppleBox>
            if (fruitType == Apple.class) return AppleBox.class;
            if (fruitType == Banana.class) return BananaBox.class;
            throw new IllegalArgumentException("I don't know");
        }
    }

    public static void main(String[] args) {
        System.out.println(new Puzzle().getBoxByFruitType(Apple.class));
    }
}

I know that generics are invariant. The question is: are there any workarounds? Maybe something that includes some kind of type reference wrapper...

Answer 1

You need to use wildcards :

static class Puzzle<T extends Fruit> {
    public Class<? extends FruitBox<? extends Fruit>> getBoxByFruitType(Class<T> fruitType) {
        if (fruitType == Apple.class) return AppleBox.class;
        if (fruitType == Banana.class) return BananaBox.class;
        throw new IllegalArgumentException("I don't know");
    }
}

Answer 2

What about this?

public class GenericsCheck {

static class Fruit {}
static class Apple extends Fruit {}
static class Banana extends Fruit {}

static class FruitBox<T extends Fruit> {}
static class AppleBox extends FruitBox<Apple> {}
static class BananaBox extends FruitBox<Banana> {}

static class Puzzle<T extends Fruit,D extends FruitBox<T>> {

    @SuppressWarnings("unchecked")
    public  Class<D> getBoxByFruitType(Class<T> fruitType) {
        if (fruitType == Apple.class) 
            return (Class<D>) AppleBox.class;
        if (fruitType == Banana.class) 
            return  (Class<D>) BananaBox.class;
        throw new IllegalArgumentException("I don't know");
    }
}


public static void main(String[] args) {
    System.out.println(new Puzzle().getBoxByFruitType(Apple.class));
}

}

Answer 3

Probably the most correct way to implement this is something like this:

import java.util.Map;
import java.util.HashMap;

public class GenericsCheck {

    static class Fruit {}
    static class Apple extends Fruit {}
    static class Banana extends Fruit {}

    static class FruitBox<T extends Fruit> {}
    static class AppleBox extends FruitBox<Apple> {}
    static class BananaBox extends FruitBox<Banana> {}

    static class Puzzle {

        private Map<Class<? extends Fruit>,
                    Class<? extends FruitBox<? extends Fruit>>> map
            = new HashMap<>();

        public Puzzle() {
            putBoxByFruitType(Apple.class, AppleBox.class);
            putBoxByFruitType(Banana.class, BananaBox.class);
        }

        public <T extends Fruit> void putBoxByFruitType(
                Class<T> fruitType, Class<? extends FruitBox<T>> boxType) {
            map.put(fruitType, boxType);
        }

        @SuppressWarnings("unchecked")
        public <T extends Fruit> Class<? extends FruitBox<T>> getBoxByFruitType(
                Class<T> fruitType) {
            return (Class<? extends FruitBox<T>>) map.get(fruitType);
        }
    }

    public static void main(String[] args) {
        System.out.println(new Puzzle().getBoxByFruitType(Apple.class));
    }
}

And, yes, you cannot assign the result to a variable of type Class<AppleBox> because that isn't safe -- as Konrad Rudolph mentioned, there can be another class SnappleBox extends FruitBox<Apple> , so then the method might return a Class<SnappleBox> .