// a Stack class
//
// This implements StackInterface
// which guarantees the methods
//    isEmpty()
//    peek()
//    pop()    and
//    push()


class Stack implements StackInterface
{

  /* StackCell is an inner class that Stack will
   * use to wrap its data in a linked list */
  class StackCell
  {
    private Object data;
    private StackCell next;

    private StackCell(Object data)
    {
      this.data = data;
      this.next = next;
    }
  }

  /* the one data member of each Stack object
  */ 
  private StackCell front;  // the front of the stack (for pushing/popping)

  /* constructor
   * creates and empty stack
   */
  public Stack()
  {
    front = null;
  }

  /* method: isEmpty
   * inputs: none
   * outputs: a boolean - whether or not this is empty
   */
  public boolean isEmpty()
  {
    return (front == null);
  }

  /* method: push
   * inputs: an object to store at the front of the stack
   * outputs: none
   */
  public void push(Object data)
  {
    StackCell newFront = new StackCell(data);
    if (isEmpty())
    {
      this.front = newFront;
    }
    else
    {
      newFront.next = this.front;
      this.front = newFront;
    }
  }

  /* method: pop
   * inputs: none
   * outputs: the data that was at the front (null if empty)
   *          pop also removes the front StackCell
   */ 
  public Object pop()
  {
    if (isEmpty())
    {
      System.out.println("You can't pop from an empty Stack!");
      return null;
    }
    Object data = front.data;
    front = front.next;
    return data;
  }

  /* method: peek
   * inputs: none
   * outputs: the data that was at the front (null if empty)
   *          peek does not alter the calling stack 
   */ 
  public Object peek()
  {
    if (isEmpty())
    {
      System.out.println("You can't peek at an empty Stack!");
      return null;
    }
    Object data = front.data;
    return data;
  }

  /* method: toString
   * inputs: none
   * outputs: the usual
   */
  public String toString()
  {
    String result = "<FRONT> ";
    StackCell current = this.front;
    while (current != null)
    {
      result += "" + current.data + " ";
      current = current.next;
    } 
    result += "<BACK>";
    return result;
  }

  /* method: main
   * inputs: the usual
   * outputs: none - a place for testing...
   */
  public static void main(String[] args)
  {
    Stack S = new Stack();
    System.out.println("S is " + S);
    S.push("Will");
    System.out.println("S is " + S);
    S.push("this");
    System.out.println("S is " + S);
    S.push("work?");
    System.out.println("S is " + S);
    S.pop();
    System.out.println("S is " + S);
    S.pop();
    System.out.println("S is " + S);
    S.pop();
    System.out.println("S is " + S);
    S.pop();
    System.out.println("S is " + S);
    S.push("Will");
    System.out.println("S is " + S);
    S.push("this");
    System.out.println("S is " + S);
    S.push("work?");
    System.out.println("S is " + S);
  }
}