Coursera_Algorithm class practice: Generic stack using linkedlist implementation

gistfile1.txt

public class MyStack<Item>
{
  private Node first = null;
  
  private class Node 
  {
    Item item;
    Node next;
  }
  
  public boolean isEmpty() {
    return first == null;
  }
  
  public void push(Item item) {
    Node oldFirst = first;
    first = new Node();
    first.item = item;
    first.next = oldFirst;
  }
  
  public Item pop() {
    if(first == null) return null;
    Item tmp = first.item;
    first = first.next;
    return tmp;    
  }
  
    public static void main(String[] args) {
    MyStack<Integer> s1 =  new MyStack<Integer>();
    
    s1.push(1);
    s1.push(2);
    s1.push(3);
    
    System.out.println(s1.pop());
    System.out.println(s1.pop());
    System.out.println(s1.pop());
    System.out.println(s1.pop());
    System.out.println(s1.isEmpty());

    }
  
}