Stack

import java.util.Iterator;

public class MyStack<Item> implements Iterable<Item> {

    private class Node{
        Item item;
        Node next;
        Node(Item item) {
            this.item = item;
        }
    }

    private Node head = null;
    private int size = 0;

    public Item pop() {
        Node old = head;
        if (size == 0) {
            throw new NullPointerException();
        } else {
            head = head.next;
            size--;
        }
        return old.item;
    }

    public void push(Item item) {
        Node newNode = new Node(item);
        if (head == null) {
            head = newNode;
        } else {
            newNode.next = head;
            head = newNode;
        }
        size++;
    }

    public int size() {
        return size;
    }

    public boolean isEmpty() {
        return size == 0;
    }

    public Iterator<Item> iterator() {
        return new MyStackIterator();
    }

    private class MyStackIterator implements Iterator<Item> {

        private int i = size;
        private Node  cur = head;
        public boolean hasNext() {
            return i != 0;
        }

        public Item next() {
            Node old = cur;
            cur = cur.next;
            i--;
            return old.item;
        }

        public void remove () {
            throw new UnsupportedOperationException();
        }
    }

    public static void main(String[] args) {
        MyStack<String> q = new MyStack<String>();
        System.out.println("Mystack test");
        q.push("first");
        q.push("second");
        q.push("third");
        for (String s : q) {
            System.out.println(s);
        }
        System.out.println("First test");
        q.pop();
        for (String s : q) {
            System.out.println(s);
        }
        System.out.println("Second test");
        q.push("fourth");
        q.push("fifth");
        q.pop();
        for (String s : q) {
            System.out.println(s);
        }
        System.out.println("Third test");
    }

}