Generics, variance problems and bounded types

In scala, generics work for both classes and traits. We can have any number of generic type arguments.

class MyList[A] {
  def add(element: A): MyList[A] = ???
}
val listOfIntegers = new MyList[Int]
val listOfStrings = new MyList[String]

// we can have any number of generic type arguments
class MyMap[K, V]

Variance
It defines inheritance relationships of parameterized types. There are 3 types of variance

class Animal
class Cat extends Animal
class Dog extends Animal

Covariance: List[Cat] can extend from List[Animal]

    class CovariantList[+A] // Observe the symbol +
    val animal: Animal = new Cat
    val animalList: CovariantList[Animal] = new CovariantList[Cat]

Invariance: List[Cat] cannot extend from List[Animal]

  class InvarianceList[A]
  val invariantAnimalList: InvarianceList[Animal] = new InvarianceList[Animal]

Contravariance: List[Animal] can extend from List[Cat]

    class ContraVariantList[-A]
    val contraVariantList: ContraVariantList[Cat] = new ContraVariantList[Animal]
    // this may be confusing, look at the below example
    class Trainer[-A]
    val contraVariantList1: Trainer[Cat] = new Trainer[Animal]
    // Now this makes sense as animal trainer can also train cat

Bounded Types
These allow us to use generic classes only for certain types either subtype/supertype of certain types

SubType
    class Cage[A <: Animal](animal: A) // it says class Cage accepts
       // only of type A that are sub type of Animal
    val cage = new Cage(new Dog)
    class Car
    // val newCage = new Cage(new Car) // compiler does not 
       // report error but it gets runtime exception
SuperType
    // this enforces super type
    class Cage1[A >: Animal](animal: A)