I do not understand why casting `this` to `S` is unchecked. Is it a bug or I miss something

# announcements

[Grigorii Yurkov]

I do not understand why casting this to S is unchecked. Is it a bug or I miss something

[nfrankel]

probably because the compiler cannot infer it
regarding how the class uses generics
i can understand why
infinite recursion is not compiler-friendly

[Niklas Gürtler]

looks like what is known in C++ as "CRTP" 🙂

[Vampire]

Why should AbstractBuilder be of type S?

[Niklas Gürtler]

Other way round - S should derive from AbstractBuilder<S>, so an instance of AbstractBuilder<S> can be downcast to S . However, someone could do:

class A : AbstractBuilder<B> {}
class B : AbstractBuilder<A> {}

and then the cast fails, but the JVM can't check it as it's unchecked

[Vampire]

Ah, ugh, that looks like a pretty confusing and error-prone pattern to me.

[Niklas Gürtler]

In C++ it's quite common but you have to be careful, yes. Not sure about Kotlin

[Grigorii Yurkov]

AFAIK this pattern is used in Java streams

[Vampire]

Really? I'm curious, do you know where?

[Grigorii Yurkov]

public interface BaseStream<T, S extends BaseStream<T, S>>

[nfrankel]

yes, this pattern is used
no, it’s not a good idea
😉

[Vampire]

Ah, right

[nfrankel]

all in all, the compiler tells you he cannot be sure about the cast
but if you’re sure, that’s ok
nothing is wrong

[Matteo Mirk]

It’s called “recursive generics” in Java, and it has some purposes:
https://vyazelenko.com/2012/03/02/recursive-generics-to-the-rescue/
https://www.sitepoint.com/self-types-with-javas-generics/

[dmitriy.novozhilov]

Actually this pattern (with recursive generic) may be replaced with new language feature with self types
But it's unknown, would it be added to language or not (but we considering it)