如何使用有界类型参数的抽象工厂模式？

Question

我试图将Abstract Factory Pattern与有界的类型化参数一起使用，示例如下：CageAbstractFactory.javapublic interface CageAbstractFactory {    public Cage<?> createCage();}CageFactory.javapublic class CageFactory {    public static Cage<?> getCage(CageAbstractFactory factory) {        return factory.createCage();    }}XxxCageFactory.java（Xxx = Lion，Rat等）public class XxxCageFactory implements CageAbstractFactory {    @Override    public Cage<Xxx> createCage() {        return new XxxCage();    }}笼子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等）public class XxxCage extends Cage<Xxx> {    @Override    public void add(Xxx r) {        cage.add(r);    }}动物.javapublic class Animal {    public String toString() {        return getClass().getSimpleName();    }   }class Rat extends Animal {}class Lion extends Animal {}AFP.java（主类）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());    }}在第rc.add(new Rat())OR行或rc.add(new Lion())以下错误出现：The method add(capture#3-of ?) in the type Cage<capture#3-of ?> is not applicable for the arguments (Rat)似乎从Cage<?>到Cage<Rat>/的类型转换错误Cage<Lion>但是问题是CageFactory.getCage(CageAbstractFactory factory)返回Cage<?>是仅在运行时通过选择CageAbstractFactory作为参数传递的类（即LionCageFactoryOR RatCageFactory）来决定的。

Answer 1

不要使用wildcard ?作为

public static Cage<?> getCage(CageAbstractFactory factory) {

    return factory.createCage();

}

但是使用通用

public static <T extends Animal> Cage<T> getCage(CageAbstractFactory factory) {

    return factory.createCage();

}

Answer 2

您还需要对进行概括CageAbstractFactory：

public interface CageAbstractFactory<T> {

    public Cage<T> createCage();

}

public class CageFactory {

    public static <T> Cage<T> getCage(CageAbstractFactory<T> factory) {

        return factory.createCage();

    }

}

public class XxxCageFactory implements CageAbstractFactory<Xxx> {

    @Override

    public Cage<Xxx> createCage() {

        return new XxxCage();

    }

}