在类中实例化泛型类型：V扩展列表

Question

I'm trying to develop a class that works with objects who extend java.util.List, for example, ArrayList. Below is an example of what I think should compile (assuming the necessary import statements):

public class MyClass<V extends List> {

    V myList = new ArrayList();

    MyClass<ArrayList> myinstance = new MyClass<>();
}

While the "myinstace" is no problem, the compiler fails when attempting to instantiate "myList.":

MyClass.java:[17,16] incompatible types: java.util.ArrayList cannot be converted to V

As I understand generics, declaring:

<V extends List>

implies that whatever type "V" is, it must be a subclass of "List." Thus, why cannot I instantiate as above, and is there any correct way to instantiate an object of type "V" within the class?

Furthermore, how might one return an object of type "V" within a method in the class?

public V getSomeData(){
    // How do we create our object of type "V"?
}

Background

I have an interface, PatternElement:

public interface PatternElement<D,W> {

    /**
     * The data that this element contains or signifies.
     * @return 
     */
    public D getData();

    /**
     * The width of this pattern element within a pattern.
     * @return 
     */
    public W getWidth();

}

I'm trying to develop a subclass, PolyPatternElement, which can contain many "data" and "width" items within itself. I thought the following code would accomplish this...

public class PolyPatternElement<V extends List, W extends List> implements PatternElement<V,W>{


    @Override
    public V getData() {
        ArrayList data = new ArrayList();
        return data;
    }

    @Override
    public W getWidth() {
        throw new UnsupportedOperationException("Not supported yet."); //To change body of generated methods, choose Tools | Templates.
    }

} 

Unfortunately, "getData" doesn't compile. Thus, I narrowed down the problem and asked the original question. After the feedback, I think the better PolyPatternElement definition is:

public class PolyPatternElement<V,W> implements PatternElement<List,List>{


    @Override
    public List getData() {
        ArrayList data = new ArrayList();
        return data;
    }

    @Override
    public List getWidth() {
        throw new UnsupportedOperationException("Not supported yet."); //To change body of generated methods, choose Tools | Templates.
    }

}

This allows declaring a PolyPatternElement object whose types are any implementation of List which was my intention.

Answer 1

尝试（未试用）：

public class PolyPatternElement<V, W> implements PatternElement<List<V>, List<W>>

Answer 2

... implies that whatever type "V" is, it must be a subclass of "List."

Within a generic class you cannot simply instantiate an object of the generic parameters class.

public class MyClass<V extends List> {

    V myList = new ArrayList();  // <-- What if 'V' is 'LinkedList'?

}

You either have to provide the class or an object with the constructor:

public class MyClass<V extends List> {

    private final V myList;

    public MyClass(V theList) {
        this.myList = theList;
    }

    public MyClass(Class<V> listType) {
        this.myList = listType.newInstance(); // works only if 'listType' has default constructor.
    }

}