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

Question

For example, lets say you have two classes:

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

I have a method that returns a List<TestA> and I would like to cast all the objects in that list to TestB so that I end up with a List<TestB>.

Answer 1

Simply casting to List<TestB> almost works; but it doesn't work because you can't cast a generic type of one parameter to another. However, you can cast through an intermediate wildcard type and it will be allowed (since you can cast to and from wildcard types, just with an unchecked warning):

List<TestB> variable = (List<TestB>)(List<?>) collectionOfListA;

Answer 2

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<A> and List<B>?

It turns out that List<B> is not a subclass of List<A> therefore we cannot write

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

Furthermore, we can't even write

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

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

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

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

Answer 3

You cannot cast List<TestB> to List<TestA> as Steve Kuo mentions BUT you can dump the contents of List<TestA> into List<TestB>. Try the following:

List<TestA> result = new List<TestA>();
List<TestB> data = new List<TestB>();
result.addAll(data);

I've not tried this code so there are probably mistakes but the idea is that it should iterate through the data object adding the elements (TestB objects) into the List. I hope that works for you.

Answer 4

With Java 8, you actually can

List<TestB> variable = collectionOfListA
    .stream()
    .map(e -> (TestB) e)
    .collect(Collectors.toList());