import Stack;
import Node;

/**
 * An implementation of stacks using nodes.
 *
 * @author Samuel A. Rebelsky
 * @version 1.0 of April 1999
 */
public class NodeBasedStack 
  implements Stack
{
  // +--------+--------------------------------------------------
  // | Fields |
  // +--------+

  /** The top of the stack.  Set to null for empty stacks. */
  protected Node top;

  /** The number of elements in the stack. */
  protected int size;


  // +--------------+--------------------------------------------
  // | Constructors |
  // +--------------+

  /**
   * Create a new empty stack.
   */
  public NodeBasedStack() {
    top = null;
    size = 0;
  } // NodeBasedStack()
  
  // +-----------+-----------------------------------------------
  // | Accessors |
  // +-----------+

  /**
   * Is the stack full?  Node-based stacks are never full.
   */
  public boolean isFull() {
    return false;
  } // isFull()

  /**
   * Is the stack empty?
   */
  public boolean isEmpty() {
    return top == null;
  } // isEmpty()

  /**
   * What's at the top of the stack?
   */
  public Object peek() {
    return top.getContents();
  } // peek()


  // +-----------+-----------------------------------------------
  // | Modifiers |
  // +-----------+

  /**
   * Add an element to the stack.
   */
  public void add(Object elt) {
    top = new Node(elt,top);
  } // add(Object)

  /**
   * Remove and return the top element from the stack.
   */
  public Object get() {
    // Get the element at the top.
    Object tmp = top.getContents();
    // Move on to the next element.
    top = top.getNext();
    // Return the element.
    return tmp;
  } // get()

} // class NodeBasedStack