Java Stack Implementation

What is Stack

Code Example of Stack

import java.util.Iterator;
import java.util.NoSuchElementException;
import java.util.Scanner;

class Stack<Item> implements Iterable<Item> {
    private int n;
    private Node first;

    private class Node {
        private Item item;
        private Node next;
    }

    public Stack() {
        first = null;
        n = 0;
    }

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

    public void push(Item item) {
        Node oldFirst = first;
        first = new Node();
        first.item = item;
        first.next = oldFirst;
        n++;
    }

    public Item pop() {
        if (isEmpty()) throw new NoSuchElementException("Stack underflow");
        Item item = first.item;
        first = first.next;
        n--;
        return item;
    }

    public int size() {
        return n;
    }

    public String toString() {
        StringBuilder str = new StringBuilder();
        for (Item item : this) {
            str.append(item);
            str.append(' ');
        }
        return str.toString();
    }

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

    private class ListIterator implements Iterator<Item> {
        private Node current = first;
        public boolean hasNext() {
            return current != null;
        }

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

        public Item next() {
            if (!hasNext()) throw new NoSuchElementException();
            Item item = current.item;
            current = current.next;
            return item;
        }
    }

    public static void main(String[] args) {
        Stack<String> stack = new Stack<String>();

        Scanner scanner = new Scanner(System.in);
        while (scanner.hasNext()) {
            String item = scanner.next();
            if (!item.equals("-")) {
                stack.push(item);
            } else if (!stack.isEmpty()) {
                System.out.println(stack.pop() + " ");
            }
        }
        System.out.println(stack.toString());
        System.out.println("(" + stack.size() + " left on stack)");
    }
}