generic-classes.md

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Generic classes are classes which take a type as a parameter. They are particularly useful for collection classes.

Defining a generic class

Generic classes take a type as a parameter within square brackets []. One convention is to use the letter A as type parameter identifier, though any parameter name may be used.

{% tabs generic-classes-1 class=tabs-scala-version %} {% tab 'Scala 2' for=generic-classes-1 %}

class Stack[A] {
  private var elements: List[A] = Nil
  def push(x: A): Unit =
    elements = x :: elements
  def peek: A = elements.head
  def pop(): A = {
    val currentTop = peek
    elements = elements.tail
    currentTop
  }
}

{% endtab %} {% tab 'Scala 3' for=generic-classes-1 %}

class Stack[A]:
  private var elements: List[A] = Nil
  def push(x: A): Unit =
    elements = x :: elements
  def peek: A = elements.head
  def pop(): A =
    val currentTop = peek
    elements = elements.tail
    currentTop

{% endtab %} {% endtabs %}

This implementation of a Stack class takes any type A as a parameter. This means the underlying list, var elements: List[A] = Nil, can only store elements of type A. The procedure def push only accepts objects of type A (note: elements = x :: elements reassigns elements to a new list created by prepending x to the current elements).

Nil here is an empty List and is not to be confused with null.

Usage

To use a generic class, put the type in the square brackets in place of A.

{% tabs generic-classes-2 class=tabs-scala-version %} {% tab 'Scala 2' for=generic-classes-2 %}

val stack = new Stack[Int]
stack.push(1)
stack.push(2)
println(stack.pop())  // prints 2
println(stack.pop())  // prints 1

{% endtab %} {% tab 'Scala 3' for=generic-classes-2 %}

val stack = Stack[Int]
stack.push(1)
stack.push(2)
println(stack.pop())  // prints 2
println(stack.pop())  // prints 1

{% endtab %} {% endtabs %}

The instance stack can only take Ints. However, if the type argument had subtypes, those could be passed in:

{% tabs generic-classes-3 class=tabs-scala-version %} {% tab 'Scala 2' for=generic-classes-3 %}

class Fruit
class Apple extends Fruit
class Banana extends Fruit

val stack = new Stack[Fruit]
val apple = new Apple
val banana = new Banana

stack.push(apple)
stack.push(banana)

{% endtab %} {% tab 'Scala 3' for=generic-classes-3 %}

class Fruit
class Apple extends Fruit
class Banana extends Fruit

val stack = Stack[Fruit]
val apple = Apple()
val banana = Banana()

stack.push(apple)
stack.push(banana)

{% endtab %} {% endtabs %}

Class Apple and Banana both extend Fruit so we can push instances apple and banana onto the stack of Fruit.

Note: subtyping of generic types is invariant. This means that if we have a stack of characters of type Stack[Char] then it cannot be used as an integer stack of type Stack[Int]. This would be unsound because it would enable us to enter true integers into the character stack. To conclude, Stack[A] is only a subtype of Stack[B] if and only if B = A. Since this can be quite restrictive, Scala offers a type parameter annotation mechanism to control the subtyping behavior of generic types.