Question

I am new to java.
The question might not be that clear. Let me explain using code.
The code below is a function that prints out values in a linkedlist.
The first line of the function creates a reference to point to the same object that is pointed by the HeadNode reference. So any changes to currentNode will affect the object that HeadNode is pointing to.
Now, inside the while loop, I am making the change of the currentNode, but I observe the linkedList didn’t change after I exit the function. Why?

static void PrintLinkedList(ListNode HeadNode)
    {
        ListNode currentNode = HeadNode;
        while(currentNode != null)
        {
            System.out.println(currentNode.getData());
            currentNode = currentNode.getNext();
        }
    }

UPDATE:

the reason of me bring this quesiton up is that when I implemented a function to reverse a linkedlist, my tempNode become null after the following code executed:

static ListNode ReverseLinkedList(ListNode headNode)
    {
        ListNode headNodeTemp = headNode;
        headNodeTemp.setNext(null);
        ListNode tempNode = headNode.getNext(); //temp becomes null because headNode is changed. Why??
        ListNode currentNode = headNodeTemp;
        while(tempNode != null)
        {...

Answer 1

So any changes to currentNode will affect the object that HeadNode is pointing to.

No, currentNode is a reference to the object HeadNode also refers to. You never change that object, all you change is what currentNode refers to.

currentNode = currentNode.getNext();

makes currentNode point to the next object, it doesn’t change any object.

To change the object, you need to call a method or set a property,

currentNode.mutate();
currentNode.property = 17;

would change the pointed-to object, and these changes would be visible when the object is queried through HeadNode.

currentNode = currentNode.getNext();

just makes currentNode point to another object.

In

static ListNode ReverseLinkedList(ListNode headNode)
{
    ListNode headNodeTemp = headNode;
    headNodeTemp.setNext(null);

here, we call a method that changes the pointed-to object. Going by the name, it sets the next field to null. And the object headNodeTemp points to is the object headNode points to.

    ListNode tempNode = headNode.getNext(); //temp becomes null because headNode is changed. Why??
    ListNode currentNode = headNodeTemp;
    while(tempNode != null)
    {...

You should move the

ListNode tempNode = headNode.getNext();

line above the setNext(null) line.

static ListNode ReverseLinkedList(ListNode headNode)
{
    ListNode headNodeTemp = headNode;
    ListNode tempNode = headNode.getNext();
    headNodeTemp.setNext(null);
    ListNode currentNode = headNodeTemp;
    while(tempNode != null)
    {...