Java Generics wildcard extends final class
generics, java
Solution
Let's say I had a method like this:
public List<? extends T> filterOutNulls(List<T> input) { ...
Granted, not the best signature in the world, but still perfectly legal. What would happen if I passed a `List<String>` to that method? According to the signature, it returns a `List<? extends String>`. If Java disallowed that type, it'd be impossible to use this method for `List<String>` (or at least, it'd be impossible to use the return value).
Secondarily, the `extends` syntax is still useful in this case, since `List<String>` and `List<? extends String>` have different restrictions -- specifically, you can't add anything but a `null` literal to `List<? extends String>`. I'll sometimes use `? extends` to signify that a collection is read-only (since the only `T`s you can pass in are `null`), and `? super` to signify write-only (since you can only get out `T`s as `Object`). This isn't completely fool-proof (you can still call remove methods, pass in `null`s, downcast, etc) but serves as a gentle reminder of how the collection is probably meant to be used.
Problem
Why does Java doesn't throw any warning when compiling my `TestGenerics` `class`, considering that the `String` `class` is `final` and cannot be extended? ``` import java.util.*; public class TestGenerics { public void addStrings(List<? extends String> list) { // some code here } } } ```