/* Program 6.12 The Linked Stack Class */

 /*-----------------------------------------*/

  class StackNode {
    Object    item;
    StackNode  link;
  }

 /*-----------------------------------------*/

  class Stack {
 
     private StackNode topNode;

   /*------*/

     // note: Java's default no-arg constructor, new Stack(),
     // works fine for the linked stack representation
     // so there is no need to define one here

   /*------*/

     public boolean empty() {
       return (topNode == null);
     }

   /*------*/

     public void push(Object X) {
       StackNode newNode = new StackNode();
       newNode.item = X;
       newNode.link = topNode;
       topNode = newNode;
     }

   /*------*/

     public Object pop() {
       if (topNode == null) {
         return null;
       } else {
         StackNode tempNode = topNode;
         topNode = topNode.link;
         return tempNode.item;
       }
     }

   /*------*/

     public Object peek() {
       if (topNode == null) {
         return null;
       } else {
         return topNode.item;
       }
     }

   /*------*/

} // end class Stack