Effective Kotlin: Item 29 — Favor generic types

As with Java, there is no excuse in forcing clients to cast types when using your code especially when making a class generic is often trivial. Item 29 of Joshua Bloch’s brilliant Effective Java covers this by showing an example of generifying a Stack class which might look as follows in Kotlin:

class Stack(initialCapacity: Int = 16) {
    private var elements = arrayOfNulls<Any?>(initialCapacity)
    private var size = 0

    val isEmpty = size == 0

    fun push(e: Any) {
        ensureCapacity()
        elements[size++] = e
    }

    fun pop(): Any {
        if (size == 0) throw EmptyStackException()
        return (elements[--size] as Any).also {
            elements[size] = null // Eliminate obsolete reference
        }
    }

    private fun ensureCapacity() {
        if (elements.size == size)
            elements = Arrays.copyOf(elements, 2 * size + 1)
    }
}
val stack = Stack()
stack.push(5)
val value = stack.pop() as Int

The generic version might look as follows where as you cannot use a non reified type in Array we perform an unchecked cast :

class Stack<T : Any>(initialCapacity: Int = 16) {
    @Suppress("UNCHECKED_CAST")
    private var elements = arrayOfNulls<Any?>(initialCapacity) as Array<T?>
    private var size = 0

    val isEmpty = size == 0

    fun push(e: T) {
        ensureCapacity()
        elements[size++] = e
    }

    fun pop(): T {
        if (size == 0) throw EmptyStackException()
        return (elements[--size] as…