Generic Classes

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.

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

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.

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

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

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)

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.