// Textbook fragment 05.07
public class NodeStack<E> implements Stack<E> {
  protected Node<E> top;        // reference to the head node
  protected int size;           // number of elements in the stack
  public NodeStack() {  // constructs an empty stack
    top = null;
    size = 0;
  }
  public int size() { return size; }
  public boolean isEmpty() {
    if (top == null) return true;
    return false;
  }
  public void push(E elem) {
    Node<E> v = new Node<E>(elem, top); // create and link-in a new node
    top = v;
    size++;
  }
  public E top() throws EmptyStackException {
    if (isEmpty()) throw new EmptyStackException("Stack is empty.");
    return top.getElement();
  }
  public E pop() throws EmptyStackException {
    if (isEmpty()) throw new EmptyStackException("Stack is empty.");
    E temp = top.getElement();
    top = top.getNext();        // link-out the former top node
    size--;
    return temp;
  }
}