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Animal.kt

// We are modelling different types of animals.
sealed class Animal {

    // Cats are simple. A cat is a cat.
    object Cat : Animal()

    // Dogs are more complicated. They can either be "just a dog", in which case we want
    // to refer to their type as `Dog`, or they can be of a specific breed, so we also
    // want to be able to use the `Dog("labrador")` syntax.
    
    // TODO: the Dog implementation

    // Note that no classes should be `open` in the solution. We want a sealed hierarchy.
    
}

// Example usage
fun main() {

    // All of these lines should work:
    var animal: Animal
    animal = Cat
    animal = Dog
    animal = Dog("husky")

    // Implementation needs to work with an exhaustive when, such as this one:
    val opinion = when (animal) {
        is Cat -> {
            "Cat"
        }
        is Dog -> {
            if (animal.breed == null) "Generic dog" else "Specific dog: ${animal.breed}"
        }
    }

    // This should NOT compile, as for a dog without a breed, we want just the `Dog` syntax.
    Dog()
    
    // This should not compile either, for the same reason.
    Dog(null)
    
    // Finally, different Dogs created are expected to have different breeds when checked later:
    val animal1 = Dog("labrador")
    val animal2 = Dog("husky")
    println(animal1.breed) // labrador
    println(animal2.breed) // husky

}

what about next code?

class Dog : Animal {
      val breed: String?

      constructor() : super() {
        breed = null
      }

      constructor(breed: String) : super() {
        this.breed = breed
      }
    }