implement queue using linked list in kotlin

Kotlin program for implement queue using linked list. Here more information.

/*
  Kotlin Program for
  Implement queue using linked list
*/
// Create Q node
class QNode
{
	var data: Int;
	var next: QNode ? ;
	constructor(value: Int)
	{
		this.data = value;
		this.next = null;
	}
}
class MyQueue
{
	var head: QNode ? ;
	var tail: QNode ? ;
	var count: Int;
	constructor()
	{
		this.head = null;
		this.tail = null;
		this.count = 0;
	}
	fun size(): Int
	{
		return this.count;
	}
	fun isEmpty(): Boolean
	{
		return this.count == 0;
	}
	// Add new node of queue
	fun enqueue(value: Int): Unit
	{
		// Create a new node
		val node: QNode = QNode(value);
		if (this.head == null)
		{
			// Add first element into queue
			this.head = node;
		}
		else
		{
			// Add node at the end using tail 
			this.tail?.next = node;
		}
		this.count += 1;
		this.tail = node;
	}
	// Delete a element into queue
	fun dequeue(): Int
	{
		if (this.head == null)
		{
			println("Empty Queue");
			return -1;
		}
		// Pointer variable which are storing 
		// the address of deleted node
		val temp: QNode? = this.head;
		// Visit next node 
		this.head = this.head?.next;
		this.count -= 1;
		if (this.head == null)
		{
			// When deleting a last node of linked list
			this.tail = null;
		}
		return temp!!.data;
	}
	// Get front node
	fun peek(): Int
	{
		if (this.head == null)
		{
			println("Empty Queue");
			return -1;
		}
		return this.head!!.data;
	}
}
fun main(args: Array < String > ): Unit
{
	val task: MyQueue = MyQueue();
	// Initially number of element
	println("isEmpty : " + task.isEmpty());
	// Add element into queue
	task.enqueue(10);
	task.enqueue(20);
	task.enqueue(30);
	task.enqueue(40);
	task.enqueue(50);
	// Test other function
	println("size : " + task.size());
	println("peek : " + task.peek());
	println("dequeue : " + task.dequeue());
	println("size : " + task.size());
	println("peek : " + task.peek());
	println("isEmpty : " + task.isEmpty());
}

Output

isEmpty : true
size : 5
peek : 10
dequeue : 10
size : 4
peek : 20
isEmpty : false