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

This should would work

List testAList = new ArrayList<>();
List testBList = new ArrayList<>()
testAList.addAll(new ArrayList<>(testBList));