Add Chapter: Sealed Classes (Java 17+)

[e-reznik]

I would like to propose adding a new chapter on Sealed Classes (introduced in Java 17) to the book.

Sealed classes are an important modern language feature that allow developers to explicitly control class hierarchies. Since this book already covers classes and object-oriented structure, sealed classes would be a good extension of that topic.

I suggest placing the chapter in:

Code Structure II
26. Classes
27. --> Sealed Classes
28. Instance Methods

An example outline for that chapter could be:

1. Motivation
2. Basic Syntax
3. The permits clause
4. final vs. sealed vs. non-sealed
5. Sealed interfaces

If this proposal sounds reasonable, I would be happy to draft the chapter content, following the existing style and structure of the book.

[bowbahdoe]

So I have an idea of how to introduce these and it's probably slightly different than what you're thinking.

Sealed hierarchies have a clear use in emulating ADTs. One of the easiest adts to make is a linked list. Linked lists are one of the easiest ways to teach recursion.

So I think I want to slot the mechanical explanation of sealed-ness in with a much deeper exploration of recursion (stuff like mutual recursion or the accumulator pattern or other things people don't generally learn)

sealed interface LL<T> {
    record Empty<T>() implements LL<T> {}
    record NotEmpty<T>(T first, LL<T> rest)  implements LL<T> {}
}

So while I can put this earlier - explaining what sealed means isn't that much harder than explaining what final means - it would essentially take the place of learning loops. Since you can learn about looping via recursion first and then build up to mutative loops. I consider that often, but I think it'll be easier just to put it later.

[bowbahdoe]

And in general don't draft content. I work on this in bursts including reviewing things and I've yet to have someone actually write main chapter content that I am happy with. It's not an insult to you it's just how I am.

The issues are enough. I will work through this backlog before I die