Queue implemented by linked-list

public class LinkedListQueueOfStrings{
    public class Node{
        private String item;
        private Node next;
    }
    private Node first=null;
    private Node last=null;
    public boolean isEmpty(){
        return first==null;
    }
    public void enqueue(String s){
            Node oldlast=last;
            last=new Node();
            last.item=s;
            last.next=null;
            if(isEmpty())
                first=last;
            else
                oldlast.next=last;       
    }
    public String dequeue(){ 
        if(!isEmpty()){
            String s=first.item;
            first=first.next;
            return s;
        }
        else{
            last=null;
            return "Queue is empty!";
            }           
    }
}