Stack in Kotlin

Node.kt

class Node {
    internal var text: String = ""
    internal var nextNode: Node? = null
}

Stack.kt

class Stack {
    var head: Node? = null

    fun pop(): String? {
        val result: String? = head?.text
        head = head?.nextNode
        return result
    }

    fun push(value: String) {
        val newNode = Node()
        newNode.text = value
        newNode.nextNode = head
        head = newNode
    }

    fun size(): Int {
        var size = 0
        var node = head
        while (node != null) {
            size++
            node = node.nextNode
        }

        return size
    }

    fun isEmpty(): Boolean {
        return head == null
    }
}

StackTester.kt

fun main(args: Array<String>) {
    val stack = Stack()

    checkSize(stack)

    // add something
    stack.push("Nick")
//    checkSize(stack)

    // pop something
//    stack.pop()
//    checkSize(stack)
    stack.push("Sue")
    stack.push("Eric")
    stack.push("David")
    checkSize(stack)

    for (i in stack.size() downTo 1) {
        println("pop result = " + stack.pop())
    }
}

fun checkSize(stack: Stack) {
    println("isEmpty? = " + stack.isEmpty())
    println("size = " + stack.size())
    println("")
}