1  import java.util.NoSuchElementException;

  2  

  3  /**

  4     An implementation of a stack as a sequence of nodes.

  5  */

  6  public class LinkedListStack

  7  {

  8     private Node first;

  9     

 10     /** 

 11        Constructs an empty stack.

 12     */

 13     public LinkedListStack()

 14     {  

 15        first = null;

 16     }

 17  

 18     /**

 19        Adds an element to the top of the stack.

 20        @param element the element to add

 21     */

 22     public void push(Object element)

 23     {  

 24        Node newNode = new Node();

 25        newNode.data = element;

 26        newNode.next = first;

 27        first = newNode;

 28     }

 29  

 30     /**

 31        Removes the element from the top of the stack.

 32        @return the removed element

 33     */

 34     public Object pop()

 35     {  

 36        if (first == null) { throw new NoSuchElementException(); }

 37        Object element = first.data;

 38        first = first.next;

 39        return element;

 40     }

 41  

 42     /**

 43        Checks whether this stack is empty.

 44        @return true if the stack is empty

 45     */

 46     public boolean empty()

 47     {

 48        return first == null;

 49     }

 50     

 51     class Node

 52     {  

 53        public Object data;

 54        public Node next;

 55     }

 56  }