Since String is a final class, how can a wildcard <? extends String> be correct? [duplicate]

Question

In a course on Spring Batch, I was surprised to see ? extends String. Since String is a final class, how is this correct?

            public void write(List<? extends String> items) throws Exception {
                System.out.println(String.format("Received list of size: %s", items.size()));
                items.forEach(System.out::println);
            }

Answer 1

You're correct that there will never be another class that extends String. So the only type that can fill that ? is String itself. However, the extends / super still conveys intent. Namely, a List<? extends String> is a read-only list. You can't add elements to it since ? extends String is a producer. A List<String> is potentially a read-write collection. So by writing List<? extends String>, we're conveying to whoever is reading the code (and to the compiler) that we only intend to read values from this List, not write anything back.