Scala abstract class method that returns a new corresponding class child object

Question

I have the following class in my mind:

abstract class MyClass (data: MyData) {

  def update(): MyClass = {
    new MyClass(process())
  }

  def process(): MyData = {
    ...
  }

}

However, abstract classes cannot be instantiated so the line new MyClass(process()) is an error. My question is - is there any way to tell the compiler that in case of each of the child classes of MyClass I want to create an object of exactly that child class? It seems an overkill to write this method awhole in all child classes. Playing with type parameters of the class or method I could not acheive that myself.

Answer 1

How about something like this? MyClass is parametrized with the concrete type. Of course, all concrete classes have to implement a method that actually returns a new instance of Self.

trait MyClass[+Self <: MyClass[Self]] {
  def update(): Self = {
    makeNew(process())
  }

  def process(): MyData = {
    // ...
  }

  protected def makeNew(data: MyData): Self
}

class Concrete0 extends MyClass[Concrete0] {
  protected def makeNew(data: MyData) = new Concrete0
}

class RefinedConcrete0 extends Concrete0 with MyClass[RefinedConcrete0] {
  override protected def makeNew(data: MyData) = new RefinedConcrete0
}

Credit: IttayD’s second update to his answer to this question.

Answer 2

To completly avoid implementing almost identical method in all subclasses you would need to use reflection. I guess that would be your last resort if you have chosen Scala. So here is how to minimize the repetitive code:

// additional parameter: a factory function
abstract class MyClass(data: MyData, makeNew: MyData => MyClass) {

  def update(): MyClass = {
    makeNew(process())
  }
  def process(): MyData = {
    ...
  }
}

class Concrete(data: MyData) extends MyClass(data, new Concrete(_))

This way you repeat only the shortest fragment required to instantiate the subclass.