// from Goodrich and Tamassia Data Structures and Algorithms in Java
// copyright of authors.

public class ArrayStack implements Stack {

   public static final int CAPACITY = 1000 ;
   private int capacity ;
   private Object s[] ;
   private int top = -1 ;

   public ArrayStack() {
      this (CAPACITY) ;
   }

   public ArrayStack(int cap) {
      capacity = cap ;
      s = new Object[capacity] ;
   }

   public int size() { return top + 1 ; } 

   public boolean isEmpty() { return ( top<0 ) ; }

   public void push(Object obj) throws StackFullException {
      if ( size()==capacity ) 
         throw new StackFullException("Stack Overflow.") ;
      s[++top] = obj ;
   }

   public Object top() throws StackEmptyException {
      if ( isEmpty() )
         throw new StackEmptyException("Stack is empty.") ;
      return s[top] ;
   }

   public Object pop() throws StackEmptyException {
      Object elem ;
      if ( isEmpty() )
         throw new StackEmptyException("Stack is empty.") ;
      elem = s[top] ;
      s[top--] = null ;
      return elem ;
   }

}