LinkedListStack.java

import java.util.NoSuchElementException;
/**
 * An implementation of a stack as a sequence of nodes.
 * */
public class LinkedListStack
{
    private Node first;

    /**
     * Constructs an empty stack.
     */
    public LinkedListStack()
    {
        first = null;
    }

    /**
     * Adds an element to the top of the stack.
     * @param element the element to add
     */
    public void push(Object element)
    {
        Node newNode = new Node();
        newNode.data = element;
        newNode.next = first;
        first = newNode;
    }

    /**
     * Removes the lement from the top of the stack.
     * @return the removed element
     */
    public Object pop()
    {
        if (first == null) 
        {
            throw new NoSuchElementException();
        }
        Object element =  first.data;
        first = first.next;
        return element;
    }

    /**
     * Checks whether this stack is empty
     * @return true if stack is empty
     */
    public boolean empty()
    {
        return first == null;
    }

    public class Node
    {
        public Object data;
        public Node next;
    }

}