Generics in Java

Question

I would like to understand the following type of syntax.

Example:

public interface A < T extends A < T> > {

}

What is the logic of this interface ?

Dikdik

strongly related: https://mcmap.net/q/136053/-java-enum-definition – Sholeen

also related: https://mcmap.net/q/1021236/-recursive-generic-usage – Sholeen

related to possible uses: #7959835 – Sholeen

possible duplicate of Generics in Java – Celin

Answer 1

This would be used as follows:

class X implements A<X> { /* ... */ }

In other words, you are forced to make the parameter of A the class X itself, and something like class X implements A<Unrelated> is forbidden.

This construction gives the interface access to X through the generic parameter, and the type restriction makes sure that it doesn't get abused. For instance, T can now be assumed to expose all methods that A does.

_{Note that this construction is formally somewhat similar to the curiously recurring template pattern in C++ (although it is technically quite different). In both languages it allows the "base class" to reason about its ultimate derived usage.}

Shererd

This is a great explanation. I'm wondering though, what are some applications of this other than the Enum usage already suggested? A linked list maybe? – Electrodeposit