/**
  * Bloch's Stack example pages 56 (and also 124), 2nd edition
  * Used in SWE 619 to discuss mutable/immutable transform
  */

import java.util.*;

public class Stack {

   private Object[] elements;
   private int size = 0;
   private static final int DEFAULT_INITIAL_CAPACITY = 16;


   public Stack() {
     this.elements = new Object[DEFAULT_INITIAL_CAPACITY];
   }

   public void push (Object e) {
     ensureCapacity();
     elements[size++] = e;
   }

   public Object pop () {
     if (size == 0) throw new IllegalStateException("Stack.pop");
     Object result = elements[--size];
     elements[size] = null;   // Eliminate obsolete reference
     return result;
   }
  
   // Ensure space for at least one more element
   private void ensureCapacity() {
      if (elements.length == size) {
         elements = Arrays.copyOf(elements, 2 * size + 1);
      }
   }
}