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Generic classes (or traits) take a type as a parameter within square brackets [...]. The Scala convention is to use a single letter (like A) to name those type parameters. The type can then be used inside the class as needed for method instance parameters, or on return types:

// here we declare the type parameter A
//          v
class Stack[A]:
  private var elements: List[A] = Nil
  //                         ^
  //  Here we refer to the type parameter
  //          v
  def push(x: A): Unit = { elements = x :: elements }
  def peek: A = elements.head
  def pop(): A =
    val currentTop = peek
    elements = elements.tail

This implementation of a Stack class takes any type as a parameter. The beauty of generics is that you can now create a Stack[Int], Stack[String], and so on, allowing you to reuse your implementation of a Stack for arbitrary element types.

This is how you create and use a Stack[Int]:

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

See the Variance section for details on how to express variance with generic types.

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