Stack implemented by linked-list

public class LinkedListStackOfStrings{
    private class Node{
        private String item;
        private Node next;
    }
    private Node first=null;
    public boolean isEmpty(){
        if(first==null)
            return true;
        else
            return false;
    }
    public void push(String s){
        Node oldfirst=first;
        first=new Node();
        first.item=s;
        first.next=oldfirst;
             }
    public String pop(){
        if(!isEmpty()){
        String s=first.item;
        first=first.next;
        return s;
        }
        else 
        return "Stack is empty!";
    }
}