Self-types and Class tokens

Recursive generics (the CRTP idiom) make fluent APIs return the subclass type:

abstract static class Node<Self extends Node<Self>> {
    private final List<String> labels = new ArrayList<>();
    @SuppressWarnings("unchecked")
    public Self label(String l) { labels.add(l); return (Self) this; }
}
static class Task extends Node<Task> { }

new Task().label("a").label("b") stays a Task — every link in the chain returns Self. The cast (Self) this is safe by convention: the subclass promises Self is itself. Its known cost: self-types do not survive re-parenting — Job extends Task cannot also be Node<Job>; a class has one generic parent.

Class tokens: generics are erased, but a Class<T> object is the runtime type. type.cast(value) and type.isInstance(value) turn erased storage into checked retrieval — the foundation of type-safe registries, DI containers, and JSON binders. Where erasure loses the type, an explicit token carries it.