import java.util.*;

public class GenericStack {

    private Node <T> top;

    private class Node<T> {

        private T data;
        private Node<T> next;

        public Node(T item) {
            data = item;
            next = null;
        }

    }

    public GenericStack () {
        top = null;
    }

    public boolean isEmpty() {
        return top = null;
    }

    public T top() {
        return top.data;
    }

    public void push(t item) {
        Node<T> n = new Node<T>(item);

        n.next = top;
        top = n;

    }

    public T pop() throws EmptyStackException{

        if ( top == null )
            throw new EmptyStackException();

         T n = top.data;
        top = top.next;

        return n;
    }
}