Stack in Java

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

public class Stack<T> implements Iterable<T> {

// For stack we will utilize a singly linked list style.
private static class Node<T> {
    public T value;
    public Node<T> next;
    
    public Node(T value, Node<T> next){
        this.value = value;
        this.next = next;
    }
}

private Node<T> root;
private int size;

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

public T peek() {
    if(isEmpty()) {
        throw new NoSuchElementException("Stack is empty!");
    }
    return root.value;
}

public void push(T value) {
    if(isEmpty()) {
        root = new Node<T>(value, null);
        size++;
        return;
    }
    Node<T> tmp = new Node<T>(value, root);
    root = tmp;
    size++;
    return;
}

public T pop() {
    if(isEmpty()) {
        throw new NoSuchElementException("Stack is empty!");
    }
    Node<T> tmp = root;
    root = root.next;
    size--;
    return tmp.value;
}

// Notice that poll is identical to pop, except it returns null instead of an error.
public T poll() {
    if(isEmpty()) {
        return null;
    }
    Node<T> tmp = root;
    root = root.next;
    size--;
    return tmp.value;
}

public int size() {
    return size;
}

public String toString() {
    String res = "{";
    Node<T> tmp = root;
    while(tmp != null) {
        res += tmp.value.toString();
        if(tmp.next != null) res += ", ";
        tmp = tmp.next;
    }
    res += "}";
    return res;
}

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

   private class ListIterator implements Iterator<T> {
       private Node<T> tokenNode = root;  

       public boolean hasNext() {
           return tokenNode != null;
       }

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

       public T next() {
           if (!hasNext()) throw new NoSuchElementException();
           T res = tokenNode.value;
           tokenNode = tokenNode.next;
           return res;
       }
   }

}