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 only way I know is by copying:

List list = new ArrayList (
    Arrays.asList (
        testAList.toArray(new TestB[0])
    )
);