包含扩展类的抽象类列表

Question

So my problem is this. I am using java and I am trying to do something like this (I am working offline so it is harder for me to give code examples but if needed I will) :

Class A - abstract class
Class B - abstract class with list<A> as property
Class C - extends class A
Class D - extends class B

and in this class, in the constructor I am trying to create the list which is in the B properties with a new object from class C.

The error is:  Type mismatch: cannot convert from C to A.

I can't seem to make it work. Any ideas why?

Edit:

abstract public class A {
   public int theInt;
}

abstract public class B {
   public List<A> theList;
}

abstract public class C extends A {
}

abstract public class D extends B {
   public D(){
       this.theList = new ArrayList<C>();
   }
}

This is the code and I have a compiliation error like I mentioned.

Answer 1

Initialize it as

List<A> list= new ArrayList<A>();

That way you would be able to add any subtype of A

---

If you want to assign ArrayList<C> to as it appears from the comment

Do it this way

list.AddAll(arrayListC);

Answer 2

ArrayList<C> does not extend List<A> . Therefore the assignment

this.theList = new ArrayList<C>();

is invalid.

You can fix this by adding a type parameter to B

abstract public class B<T extends A> {
   public List<T> theList
}

abstract public class D extends B<C> {
   public D(){
       this.theList = new ArrayList<C>();
   }
}

You could even add a type parameter to D :

abstract public class D<T extends A> extends B<T> {
   public D(){
       this.theList = new ArrayList<T>();
   }
}

---

Alternative:

Just use ArrayList<A> instead of ArrayList<C> .

---

Please note that lst.add(x); will not compile for the following type combinations:

    type of lst             |     type of x
============================|================================
  List<? extends A>         |  A
----------------------------|--------------------------------
  List<C>                   |  E extends A but not C
----------------------------|--------------------------------
  List<? extends A>         |  E extends A