public class NodeStack<E> {

    private Node<E> tos; //reference to the top of stack node
    private int size;    //number of nodes in the stack
	
    public NodeStack(){tos = null; size = 0;}
	
    public int size(){return size;}
	
    public boolean isEmpty(){return tos == null;}
	
    public void push(E element){
	tos = new Node<E>(element,tos); 
	size++;
    }
    //
    // NOTE: no overflow exception!
    //
	
    public E top() throws StackException {
	if (isEmpty()) throw new StackException("Stack empty");
	return tos.getElement();
    }
	
    public E pop() throws StackException {
	if (isEmpty()) throw new StackException("Stack underflow");
	E element = tos.getElement();
	tos = tos.getNext();
	size--;
	return element;
    }

    private String toString(Node<E> node){
	if (node == null) return "";
	return "," + node.getElement() + toString(node.getNext());
    }
	
    public String toString(){
	String s = "";
	if (size > 0) s = s + tos.getElement() + toString(tos.getNext());
	return "[" + s + "]";
    }
}