How do you cast a List of supertypes to a List of subtypes?

Question

For example, lets say you have two classes:

public class TestA {}
public class TestB extends TestA{}

I have a method that returns a L

Answer 1

The best safe way is to implement an AbstractList and cast items in implementation. I created ListUtil helper class:

public class ListUtil
{
    public static  List convert(final List list)
    {
        return new AbstractList() {
            @Override
            public TCastTo get(int i)
            {
                return list.get(i);
            }

            @Override
            public int size()
            {
                return list.size();
            }
        };
    }

    public static  List cast(final List list)
    {
        return new AbstractList() {
            @Override
            public TCastTo get(int i)
            {
                return (TCastTo)list.get(i);
            }

            @Override
            public int size()
            {
                return list.size();
            }
        };
    }
}

You can use cast method to blindly cast objects in list and convert method for safe casting. Example:

void test(List listA, List listB)
{
    List castedB = ListUtil.cast(listA); // all items are blindly casted
    List convertedB = ListUtil.convert(listA); // wrong cause TestA does not extend TestB
    List convertedA = ListUtil.convert(listB); // OK all items are safely casted
}