How Can I Cast a List of Supertypes to a List of Subtypes in Java?

Casting a List of Supertypes to a List of Subtypes

You may encounter a situation where you have a list of objects of a supertype and desire to cast them to a specific subtype. To illustrate, consider the following class hierarchy:

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

You may have a method returning a List, and you wish to cast each item in the list to TestB, resulting in a List.

Solution

Directly casting to List will not succeed because it violates Java's generic type safety rules. However, you can employ a trick by casting through an intermediate wildcard type:

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

This cast is permitted because wildcard types can be both cast to and from, albeit with an unchecked warning. By utilizing the wildcard type as an intermediary, you can successfully cast your list of supertypes to your desired list of subtypes.

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