How to specify that an abstract method's return type is the subclass's type

Question

How, in an abstract class, can I specify that a method's return value has the same type as the concrete class that it's a member of?

For example:

abstract class Genotype {
  def makeRandom(): Genotype  // must return subclass type
  def mutate(): Genotype      // must return subclass type
}

I'd like to say that whenever you call mutate() on a concrete Genotype class, you're sure to get back another instance of the same Genotype class.

I'd prefer not to use a type parameter in the manner of Genotype[SpecificGenotype259], since that type parameter is likely to proliferate all over the code (also, it's redundant and confusing). I'd prefer to define concrete Genotype classes by extending from various traits.

Answer 1

You need F-bounded polymorphism:

abstract class Genotype[T <: Genotype[T]] {
  def makeRandom(): T  
  def mutate(): T      
}

class ConcreteGenotype extends Genotype[ConcreteGenotype] {
  def makeRandom(): ConcreteGenotype = ???
  def mutate(): ConcreteGenotype = ???
}