/** 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: January 2020
*/
public class StackViaLink1<T> implements Stack<T> {

   // instance variables
   // ------------------

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


   // constructors
   // ------------

   /* Establishes this stack as being empty.
   */
   public StackViaLink1() { clear(); }


   // observers
   // ---------

   public int sizeOf() { return numItems; }

   public boolean isEmpty() { return sizeOf() == 0; }

   public boolean isFull() { return false; }

   public T topOf() { return top.item; }

   public String toString() {
      Link1 pntr = top;
      StringBuilder s = new StringBuilder();
      while (pntr != null) {
         s.append(pntr.item + ",");
         pntr = pntr.next;
      }
      return s.substring(0,Math.max(0, s.length()-1));
   }


   // mutators
   // --------

   public void clear() { 
      top = null; numItems = 0; 
   }

   public void push( T item ) { 
      top = new Link1<T>( item, top );
      numItems = numItems + 1;
   }

   public T pop() { 
      T result = top.item;
      top = top.next; 
      numItems = numItems - 1;
      return result;
   }

}