/** An instance of this class represents a stack capable of holding items
**  of the specified type T (the generic type parameter).
**  The underlying implementation makes use of a linked structure of
**  objects arising from the Link1 class.
**
**  Author: R. McCloskey
**  Date: September 2018
*/
public class StackViaLink1<T> {

   /*  i n s t a n c e    v a r i a b l e s  */

   private Link1<T> top;  // reference to Link1 object holding top item on stack
   private int numItems;  // # of items on the stack


   /*  c o n s t r u c t o r s  */

   /* Initializes the stack to be empty.
   */
   public StackViaLink1() 
   { 
      top = null;  numItems = 0;
   }


   /*  o b s e r v e r s  */

   
   /* Returns the number of items on the stack.
   */
   public int sizeOf() { return numItems; }

   /* Reports whether or not the stack has no items on it.
   */
   public boolean isEmpty() { return sizeOf() == 0; }

   /* Returns (a reference to) the top item on the stack.
   ** pre: !isEmpty()
   */
   public T topOf() 
   { 
      return top.getItem(); 
   }


   public String toString()
   {
      String result = "";
      if (!isEmpty()) {
         StringBuilder s = new StringBuilder();
         Link1<T> node = top;
         while (node != null) {
            s.append(", " + node.getItem());
            node = node.getNext();
         }
         result = s.toString();
      }
      return result;
   }


   /*  m u t a t o r s  */

   /* Places the specified item onto the top of the stack.
   */
   public void push( T item )  
   { 
      top = new Link1<T>( item, top );
      numItems = numItems + 1;
   }

   /* Removes the top item from the stack.
   ** pre: !isEmpty()
   */
   public void pop()
   { 
      top = top.getNext(); 
      numItems = numItems - 1;
   }

}