//import java.util.Stack;


public class stackApplication {

   public static void main(String[] args) {
      xStack s = new xStack();

      if (s.isEmpty()) System.out.println("stack is empty");

      s.push(new Integer(5)); s.push(new Integer(-2));
      s.push(new Integer(1)); s.push(new Integer(99));

      if (s.isEmpty()) System.out.println("stack is empty");

      while (!s.isEmpty()) {
         System.out.println("top of stack value is : "+s.peek());
         s.pop();  /* discard the return value from pop() */
      }

      System.out.println("Stack empty,  exiting ...");

   }

}

interface StackInterface {

   /* returns true if stack is empty, false otherwise */
   public boolean isEmpty() ;

   /* return true if stack is full, false otherwise */
   public boolean isFull();

   /* adds object to stack if stack is not full (let the user beware!) */
   public void push(Object x);

   /* removes and returns object at the top of the stack,
        fails (ungracefully) if the stack is empty  */
   public Object pop();

   /* returns object at the top of the stack,
        fails (ungracefully) if the stack is empty  */
   public Object peek();
  
}

class xStack implements StackInterface {
	private Object[] s;
	private int n;
	
	public xStack() {
		s = new Object[100];
		n = 0;
	}
	
	
	   /* returns true if stack is empty, true otherwise */
   public boolean isEmpty() {return (n == 0);}

   /* return true if stack is full, false otherwise */
   public boolean isFull() { return (n == s.length);}

   /* adds object to stack if stack is not full (let the user beware!) */
   public void push(Object x) {
	   if (! isFull()) {
		   s[n++] = x;
	   }
	   return;}

   /* removes and returns object at the top of the stack,
        fails (ungracefully) if the stack is empty  */
   public Object pop() {
	   if (! isEmpty() ) {
		   Object temp = s[n-1];
		   n--;
		   return temp;
	   }
	   else return null;
	}

   /* returns object at the top of the stack,
        fails (ungracefully) if the stack is empty  */
   public Object peek() {
	   if (! isEmpty() ) {
		   return s[n-1];
	   }
	   else return null;
	}

}