Linked List fundamental concepts [closed]

Question

I am trying to understand the basics of Linked List. The definition of my LinkedList class is as follows:

public class ListNode {
 int val;
 ListNode next;
 ListNode(int x) { val = x; }

Now I am facing a problem. My code is as follows:

ListNode dummy = new ListNode(0);
dummy.next =  head;
ListNode prev = dummy;
ListNode slow = head;
head.next = null;
prev.next = slow;

ListNode temp = slow.next;
prev.next = temp;

System.out.println(dummy.next); //comes out null

Why is it coming out as null? dummy.next was pointing to head and I only changed slow and prev?

Can we use slow and head interchangeably? If yes, then why does this happen?

// head points to a Linked list starting from 1 in 1 -> 2 -> 3
ListNode curr = head;

while(curr.next!= null){
    curr= curr.next;
}

System.out.println(curr);
System.out.println(head); //these are different and head does not change