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

You really can't*:

Example is taken from this Java tutorial

Assume there are two types A and B such that B extends A. Then the following code is correct:

    B b = new B();
    A a = b;

The previous code is valid because B is a subclass of A. Now, what happens with List and List?

It turns out that List is not a subclass of List therefore we cannot write

    List b = new ArrayList<>();
    List a = b; // error, List is not of type List

Furthermore, we can't even write

    List b = new ArrayList<>();
    List a = (List)b; // error, List is not of type List

*: To make the casting possible we need a common parent for both List and List: List for example. The following is valid:

    List b = new ArrayList<>();
    List t = (List)b;
    List a = (List)t;

You will, however, get a warning. You can suppress it by adding @SuppressWarnings("unchecked") to your method.