Scala Cookbook

19.1. Creating Classes That Use Generic Types

Problem

You want to create a class (and associated methods) that uses a generic type.

Solution

As a library writer, creating a class (and methods) that takes a generic type is similar to Java. For instance, if Scala didn’t have a linked list class and you wanted to write your own, you could write the basic functionality like this:

class LinkedList[A] {

  private class Node[A] (elem: A) {
    var next: Node[A] = _
    override def toString = elem.toString
  }

  private var head: Node[A] = _

  def add(elem: A) {
    val n = new Node(elem)
    n.next = head
    head = n
  }

  private def printNodes(n: Node[A]) {
    if (n != null) {
      println(n)
      printNodes(n.next)
    }
  }

  def printAll() { printNodes(head) }

}

Notice how the generic type A is sprinkled throughout the class definition. This is similar to Java, but Scala uses [A] everywhere, instead of <T> as Java does. (More on the characters A versus T shortly.)

To create a list of integers with this class, first create an instance of it, declaring its type as Int:

val ints = new LinkedList[Int]()

Then populate it with Int values:

ints.add(1)
ints.add(2)

Because the class uses a generic type, you can also create a LinkedList of String:

val strings = new LinkedList[String]()
strings.add("Nacho")
strings.add("Libre")
strings.printAll()

Or any other type you want to use:

val doubles = new LinkedList[Double]()
val frogs = new LinkedList[Frog]()

At this basic level, creating a generic class in Scala is just like creating a generic class in Java, with ...