// Textbook fragment 05.04
 /**
  * Implementation of the stack ADT using a fixed-length array.  An
  * exception is thrown if a push operation is attempted when the size
  * of the stack is equal to the length of the array.  This class
  * includes the main methods of the built-in class java.util.Stack.
  */
public class ArrayStack<E> implements Stack<E> {
  protected int capacity;       // The actual capacity of the stack array
  public static final int CAPACITY = 1000;      // default array capacity
  protected E S[];              // Generic array used to implement the stack
  protected int top = -1;       // index for the top of the stack
  public ArrayStack() {
    this(CAPACITY); // default capacity
  }
  public ArrayStack(int cap) {
    capacity = cap;
    S = (E[]) new Object[capacity]; // compiler may give warning, but this is ok
  }
  public int size() {
    return (top + 1);
  }
  public boolean isEmpty() {
    return (top < 0);
  }
  public void push(E element) throws FullStackException {
    if (size() == capacity)
      throw new FullStackException("Stack is full.");
    S[++top] = element;
  }
  public E top() throws EmptyStackException {
    if (isEmpty())
      throw new EmptyStackException("Stack is empty.");
    return S[top];
    }
  public E pop() throws EmptyStackException {
    E element;
    if (isEmpty())
      throw new EmptyStackException("Stack is empty.");
    element = S[top];
    S[top--] = null; // dereference S[top] for garbage collection.
    return element;
  }