Lost Data in the final node while create Singly Linked List in Java

Question

Hi there. Thank you all for your comments. The problem solved and I also learn something new about editing and formatting. I want to thumb up every comment and close the post but I did not figure out how to do so, so I will do it later.

//My original problem below I cannot print out the data in the last node of the singly linked list when I try to code in the main. Just comment the display method and you will see my original code.

I can only print out all the data when I use the display method outside of the main. Please explain to me why the display method works and the other way doesn't.

Below is my code

package learnSingleLinkedList;

public class SinglyLinkedList {
    private ListNode head;
    private static class ListNode
    {
        private int data;
        private ListNode next;
        
        ListNode(int data)
        {
            this.data = data;
            this.next = null;
        }
    }
public void display()
    {
        ListNode current = head;
        while(current!=null)
        {
            System.out.print(current.data+" --> ");
            current = current.next;
        }
        System.out.print("Null");
    }
    public static void main(String[] args)
    {
        SinglyLinkedList sll = new SinglyLinkedList();
        sll.head = new ListNode(10);
        ListNode second = new ListNode(1);
        ListNode third = new ListNode(8);
        ListNode fourth = new ListNode(11);
        
        sll.head.next = second;
        second.next = third;
        third.next = fourth;
        fourth.next = null;
        
        ListNode current = sll.head;
        while(current.next != null)
        {
            System.out.println(current.data);
            current = current.next;
        }
        
        sll.display();
    }
}