Implementation of a string queue in java for tutorial

Queue.java

public class Queue {
	private Node head;
	private Node tail;
	private int size; 
	
	public Queue(){
		head = null;
		tail = null;
		size = 0;
	}
	
	private class Node{
		private String item;
		private Node next;
	}
	
	public boolean isEmpty(){
		if(head == null){ //if(this.size == 0)
			return true;
		}
		else {
			return false;
		}
	}
	
	public void enq(String t){
		//check if queue is empty
		if(isEmpty()){ //or if(this.head == null)
			//add on a node with the string as an item
			head.item = t;
			//current item.next = null 
			head.next = null;
			//increment the number of items in the queue
			size++;
			//set the node tail node equal to head node
			tail = head;
		}
		else {//if its not empty
		 	//create a new node to temp store tail
			Node oldTail = tail;
			Node newNode = new Node();
			// Put the item in the new tail
			newNode.item = t;
			//set current node.next to null 
			newNode.next = null;
			//set oldTail.next to current node
			oldTail.next = newNode;
			tail = newNode;
		}
	}
	
	public String toString(){
		//Instantiate a string to store all the concatenated strings
		String allItems = "";
		//Iterate through the queue from head node to tail node
		Node currentNode = head;
		while(currentNode.next != null){
			//Concanenate current string to the overall string
			allItems = allItems + " " +currentNode.item;
			currentNode = currentNode.next; 
		}
		return allItems;
	}
	
}