java - List<List<String>> 泛型问题

Question

我有以下方法:

public static List<List<String>> createObject() {
    List<List<String>> listOfListOfStrings = new LinkedList<List<String>>();
    List<String> listOfStrings = new LinkedList<String>();
    //do some populating on the lists here
    listOfListOfStrings.add(listOfStrings);
    return listOfListOfStrings;
}

现在我希望能够使用 ArrayList/Vector/Stack 而不是 LinkedList(如果需要的话可以进行不同的组合。我阅读了一些关于这个问题的泛型文章，我发现使用工厂模式来创建这些是最合适的(因为我不需要反射，并且因为使用通用 <T extends List<K>, K extends List<String>> 不起作用>.). 所以我想出的解决方案如下:

public class Tester {
public static void main(String[] args){
    checkGenericsOfLists();
}

public static void checkGenericsOfLists(){
    List<List<String>> listOfListOfStrings = createObject(new LinkedListFactory<List<String>>(), new LinkedListFactory<String>());
    print(listOfListOfStrings);
    translate(listOfListOfStrings);
    print(listOfListOfStrings);
}

public static List<List<String>> createObject(ListFactorable mainListFactory, ListFactorable subListsFactory) {
    List<List<String>> listOfListOfStrings = mainListFactory.create();//new LinkedList<List<String>>();
    List<String> listOfStrings = subListsFactory.create();//new LinkedList<String>();
    listOfStrings.add("A");
    listOfListOfStrings.add(listOfStrings);
    return listOfListOfStrings;
}

public static void transform(List<List<String>> listOfListOfStrings){ 
            //do some abuse on the lists here.
}}

此解决方案会发出警告:

ListFactorable is a raw type. References to generic type ListFactorable should be parameterized

这是 createObject 方法签名。并且:

Type safety: The expression of type List needs unchecked conversion to conform to List of List of String>>

这适用于工厂调用 create() 方法的行。

但是如果我使用这个:

public static List<List<String>> createObject(ListFactorable<List<List<String>>, List<String>> mainListFactory, ListFactorable<List<String>, String> subListsFactory) {
    List<List<String>> listOfListOfStrings = mainListFactory.create();//new LinkedList<List<String>>();
    List<String> list = subListsFactory.create();//new LinkedList<String>();
    list.add("A");
    listOfListOfStrings.add(list);
    return listOfListOfStrings;
}

我收到编译器错误:

The method createObject(ListFactorable<List<List<String>>,List<String>>, ListFactorable<List<String>,String>) in the type Tester is not applicable for the arguments (LinkedListFactory<List<String>>, LinkedListFactory<String>)

有什么办法可以让编译器不抛出警告或错误，并在 createObject 方法不知道所使用的 List 实现(在编译时)的情况下实例化这些列表？!

干杯，
暴君

编辑: 请原谅我没有发布其余的类(class)(我很愚蠢:))。我们开始吧:

public interface ListFactorable<T extends List<K>, K> {
T create();}

public class LinkedListFactory<K> implements ListFactorable<LinkedList<K>, K> {
public LinkedList<K> create(){
    return new LinkedList<K>();
}}

EDIT2(尤金提出问题):

public interface EugeneListFactorable<T extends List<?>> {T create();}

public class EugeneLinkedListFactory implements EugeneListFactorable<LinkedList<?>> {
    public LinkedList<?> create(){
        return new LinkedList<List<?>>();
    }
}
public static void checkGenericsOfLists2(){
    List<List<String>> listOfListOfStrings = createObject(new EugeneLinkedListFactory(), new EugeneLinkedListFactory());
    translate(listOfListOfStrings);
}

编译器错误:

    - Type mismatch: cannot convert from LinkedList<?> to List<List<String>>
- Bound mismatch: The generic method createObject(EugeneListFactorable<N>, EugeneListFactorable<M>) of type 
 Tester is not applicable for the arguments (EugeneLinkedListFactory, EugeneLinkedListFactory). The inferred type LinkedList<?> 
 is not a valid substitute for the bounded parameter <N extends List<List<String>>>

请尝试编译并运行我发布的示例和您的解决方案。它是一个测试类，您可以轻松地在 IDE 上运行。谢谢!

Answer 1

以下内容“在我的机器上运行”，没有警告或错误。超出我的认知的是所有大惊小怪的原因 - 请参阅秒代码块。

public class Tester {

    public static void main(String[] args) {
        checkGenericsOfLists();
    }

    public static void checkGenericsOfLists() {
        List<List<String>> listOfListOfStrings = createObject(
                new LinkedListFactory<List<String>>(),
                new LinkedListFactory<String>());
        transform(listOfListOfStrings);
    }

    public static List<List<String>> createObject(
            ListFactorable<List<List<String>>, List<String>> mainListFactory, 
            ListFactorable<List<String>, String> subListsFactory
    ) 
    {
        List<List<String>> listOfListOfStrings = mainListFactory.create();
        List<String> listOfStrings = subListsFactory.create();
        listOfStrings.add("A");
        listOfListOfStrings.add(listOfStrings);
        return listOfListOfStrings;
    }

    public static void transform(List<List<String>> listOfListOfStrings) {
        // do some abuse on the lists here.
        System.out.println(listOfListOfStrings);
    }

    public interface ListFactorable<T extends List<K>, K> {
        T create();
    }

    static public class LinkedListFactory<K> implements ListFactorable<List<K>, K> {
        public LinkedList<K> create() {
            return new LinkedList<K>();
        }
    }
}

这个解决方案更干净一些，消除了一些泛型噪音。

public class Tester2 {
    public static void main(String[] args) {
        checkGenericsOfLists();
    }

    public static void checkGenericsOfLists() {
        List<List<String>> listOfListOfStrings = createObject(
                new LinkedListFactory<List<String>>(),
                new LinkedListFactory<String>());
        transform(listOfListOfStrings);
    }

    public static List<List<String>> createObject(
            ListFactory<List<String>> mainListFactory, 
            ListFactory<String> subListsFactory
    ) 
    {
        List<List<String>> listOfListOfStrings = mainListFactory.create();
        List<String> listOfStrings = subListsFactory.create();
        listOfStrings.add("A");
        listOfListOfStrings.add(listOfStrings);
        return listOfListOfStrings;
    }

    public static void transform(List<List<String>> listOfListOfStrings) {
        // do some abuse on the lists here.
        System.out.println(listOfListOfStrings);
    }

    public interface ListFactory<T> {
        List<T> create();
    }

    static public class LinkedListFactory<T> implements ListFactory<T> {
        public List<T> create() {
            return new LinkedList<T>();
        }
    }
}