Scala for Java Developers: Generic and Parametrized Types - Part I

Parametrized classes

In its basic use case, generics in scala are very similar to java. Let’s have a look at the example:




class Home[T]{
  private var inside = Set[T]()
  def enter(entering : T) = inside += entering
  def whoIsInside = inside
}

class Creature
class Human extends Creature

var house = new Home[Human]
house enter(new Human)

Parametrized methods

You can parametrize methods the same way as in java:

class Box[T](val content : T)
def wrap[T](present : T) : Box[T] = new Box(present)

Type erasure

Scala parametrized types loose the type information after compilation in the same way as java does; this process is called type erasure.

Parameter type modifiers

Before we'll talk about type modifiers in details let's look briefly at the table below.

Upper bound example

class Home[ T<: Creature ] 
var home = new Home[Human]     // ok; Human is subtype of Creature
var home = new  Home[Object]   // failure!

Lower bound example

class List[A]{
  def append[B >: A](element : B) : List[B] = {...}
  ...
}

scala> var list = new List[Human]
list: List[Human] = List@4587fd42

scala> var list2  = list append(new Dog) // Dog gets downcasted to Creature
list2: List[Creature] = List@4bdc94ea

I In the expression list append(new Dog), the argument of the append method has to be a supertype of Human. The compiler searches for the closest common supertype of a Dog and Human classes, which is Creature, then it downcasts new Dog to Creature. As a result the return type of list append(new Dog) is List[Creature]. This makes a lot of sense because it contains the mixture of Human and Dog objects, which are both subtypes of Creature.

I hope this post was helpful to you. Please let me know what you think about it, and leave a comment.