LLQueue.java

public class LLQueue <T> implements QueueInterface<T>{
	private class ListNode{
		private T data;
		private ListNode link;
		
		public ListNode(T aData, ListNode aLink) {
			data = aData;
			link = aLink;
		}
	}
	private ListNode head;
	private ListNode tail;
	
	public LLQueue() {
		head = tail = null;
	}
	
	public void enqueue(T aData) {
		ListNode newNode = new ListNode(aData, null);
		if(head==null) {
			head = tail = newNode;
			return;
		}
		tail.link = newNode;
		tail = newNode; //or tail = tail.link
	}
	
	public T dequeue() {
		if(head==null)
			return null;
		T ret = head.data;
		head = head.link;
		return ret;
	}
	
	public T peek() {
		if(head == null)
			return null;
		return head.data;
	}
	
	public void print() {
		ListNode temp = head;
		while(temp!=null) {
			System.out.println(temp.data);
			temp = temp.link;
		}
	}
	
}