java - 如何使用具有有界类型参数的抽象工厂模式？

Question

我正在尝试将抽象工厂模式与有界类型参数一起使用，示例如下:

CageAbstractFactory.java

public interface CageAbstractFactory {
    public Cage<?> createCage();
}

CageFactory.java

public class CageFactory {
    public static Cage<?> getCage(CageAbstractFactory factory) {
        return factory.createCage();
    }
}

XxxCageFactory.java (Xxx = Lion, Rat, etc)

public class XxxCageFactory implements CageAbstractFactory {
    @Override
    public Cage<Xxx> createCage() {
        return new XxxCage();
    }
}

Cage.java

public abstract class Cage<T extends Animal> { 
    protected Set<T> cage = new HashSet<T>();
    public abstract void add(T animal);
    public void showAnimals() {
        System.out.println(cage);
    }
}

XxxCage.java (Xxx = Lion, Rat, etc)

public class XxxCage extends Cage<Xxx> {
    @Override
    public void add(Xxx r) {
        cage.add(r);
    }
}

Animal.java

public class Animal {
    public String toString() {
        return getClass().getSimpleName();
    }   

}

class Rat extends Animal {}
class Lion extends Animal {}

AFP.java (Main class)

public class AFP {

    public static void main(String[] args) {
        Cage<?> rc = CageFactory.getCage(new RatCageFactory());
        Cage<?> lc = CageFactory.getCage(new LionCageFactory());
        rc.add(new Rat());
        rc.showAnimals();
        lc.add(new Lion());
    }

}

行号 rc.add(new Rat())或rc.add(new Lion())出现以下错误:

The method add(capture#3-of ?) in the type Cage<capture#3-of ?> is not applicable for the arguments (Rat)

Cage<?> 似乎存在类型转换错误至Cage<Rat>/Cage<Lion>

但问题是 CageFactory.getCage(CageAbstractFactory factory)正在返回Cage<?>这仅在运行时由 CageAbstractFactory 的选择决定作为参数传递的类(即 LionCageFactory 或 RatCageFactory )

Answer 1

不要使用通配符？如

public static Cage<?> getCage(CageAbstractFactory factory) {
    return factory.createCage();
}

但使用通用

public static <T extends Animal> Cage<T> getCage(CageAbstractFactory factory) {
    return factory.createCage();
}