Can a class extend itself?

Question

I am reading the source code of Spark. I see it seems a class extends itself.

My questions: does it extend itself? If so, what's it called? Why do we do that?

class OneHotEncoderModel private[ml] (
    @Since("2.3.0") override val uid: String,
    @Since("2.3.0") val categorySizes: Array[Int])
  extends Model[OneHotEncoderModel] with OneHotEncoderBase with MLWritable

Answer 1

It's not extending itself. Actually, "extends itself" has no meaning, or one could say all classes extends them-selves.

OneHotEncoderModel(...) extends Model[OneHotEncoderModel] with ...

means that OneHotEncoderModel extends Model. And Model is type-parametrized with OneHotEncoderModel. This construct allows Model to have the actual implementing class as a type-parameter and use it.

This can be used, for example, in an abstract api:

trait Model[A]{
  def join(other: A): A
}

Here, to be a Model sub-class, OneHotEncoderModel will have to implement def join(other: OneHotEncoderModel): OneHotEncoderModel