How to Cast a List to a List in Java?

Casting a List of Supertypes to a List of Subtypes

In Java, you can encounter scenarios where you have a list of supertype objects and desire to obtain a list of their subtype counterpart. This raises the question: how do you cast a List to a List?

For instance, consider the following two classes:

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

A method returns a List, and you wish to transform all the elements within it to TestB objects, resulting in a List.

The solution involves casting the list through an intermediate wildcard type. Direct casting to List is not feasible due to the inability to cast generic types of different parameters. However, casting to a wildcard type is allowed, with an unchecked warning issued:

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

This approach leverages the ability to cast to and from wildcard types. The unchecked warning is displayed because the compiler cannot guarantee the type safety of the cast.

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