Java泛型中的接口實現傳播

Question

我有以下代碼（經過簡化，以顯示問題的核心）：

public interface IElement {}


public interface IDataSet<E extends IElement> {}


public interface IPropertyTranslator<D extends IDataSet<? super E>, E extends IElement> {}


public interface IElementTranslator<D extends IDataSet<?>> {}


public class AnimalElement implements IElement {}


public class AnimalDataSet implements IDataSet<AnimalElement> {}


public class AnimalPropertyTranslator implements IPropertyTranslator<AnimalDataSet, AnimalElement> {}


public class UniversalPropertyTranslator implements IPropertyTranslator<IDataSet<IElement>, IElement> {}


public class ElementTranslator<D extends IDataSet<? super E>, E extends IElement> implements IElementTranslator<D> {

    public Collection<IPropertyTranslator<? super D, ? super E>> propertyTranslators = new HashSet<>();

}


public class Demo {

    public static void demo() {
        ElementTranslator<AnimalDataSet, AnimalElement> animalElementTranslator = new ElementTranslator<>();
        animalElementTranslator.propertyTranslators.add(new AnimalPropertyTranslator());
        animalElementTranslator.propertyTranslators.add(new UniversalPropertyTranslator());
    }

}

不幸的是，demo方法的最后一行會產生以下錯誤： The method add(IPropertyTranslator<? super AnimalDataSet,? super AnimalElement>) in the type Collection<IPropertyTranslator<? super AnimalDataSet,? super AnimalElement>> is not applicable for the arguments (UniversalPropertyTranslator) The method add(IPropertyTranslator<? super AnimalDataSet,? super AnimalElement>) in the type Collection<IPropertyTranslator<? super AnimalDataSet,? super AnimalElement>> is not applicable for the arguments (UniversalPropertyTranslator) The method add(IPropertyTranslator<? super AnimalDataSet,? super AnimalElement>) in the type Collection<IPropertyTranslator<? super AnimalDataSet,? super AnimalElement>> is not applicable for the arguments (UniversalPropertyTranslator) 。 通過隨機試驗，我發現問題可能與<D extends IDataSet<? super E>, E extends IElement> <D extends IDataSet<? super E>, E extends IElement>表達式，盡管我仍然不知道如何解決它。

同時，以下代碼變體可以完美運行：

public interface IDataSet {}


public interface IPropertyTranslator<D extends IDataSet> {}


public interface IElementTranslator<D extends IDataSet> {}


public class AnimalDataSet implements IDataSet {}


public class AnimalPropertyTranslator implements IPropertyTranslator<AnimalDataSet> {}


public class UniversalPropertyTranslator implements IPropertyTranslator<IDataSet> {}


public class ElementTranslator<D extends IDataSet> implements IElementTranslator<D> {

    public Collection<IPropertyTranslator<? super D>> propertyTranslators = new HashSet<>();

}


public class Demo {

    public static void demo() {
        ElementTranslator<AnimalDataSet> animalElementTranslator = new ElementTranslator<>();
        animalElementTranslator.propertyTranslators.add(new AnimalPropertyTranslator());
        animalElementTranslator.propertyTranslators.add(new UniversalPropertyTranslator());
    }

}

我不明白為什么接口的第二個通用部分會導致代碼的行為有所不同。

Answer 1

該解決方案與“生產者擴展，超級消費者”規則略有相關。 在問題代碼中，需要將以下標頭更改為以下標頭：

public interface IPropertyTranslator<D extends IDataSet<? extends E>, E extends IElement> {}

public class UniversalPropertyTranslator implements IPropertyTranslator<IDataSet<? extends IElement>, IElement> {}

而且一切都會正常進行。