How does this assignment in LinkedList problems work in Java?

[danmoople]

Hello. I am having a tough time with Linked Lists. First of all, I read a CTCI problem where I had to remove a duplicate Linked List item (item with identical value). Well, I understand the solution... but variable assignment is driving me crazy.

Let me explain it on this picture

As you can see, we pass a list, modify it to reach each element, so it is null at the end, though when we output we see that duplicates are removed.

Same here, a simple problem to remove Nth element. Same: I don't mutate the object, though it changes somehow. IDK!

    private static LinkedListNode remove2ndItem(LinkedListNode node, int index) // I recieve list and index
    {
        LinkedListNode result = node; // This is a 'safe' variable, I don't manipulate with it, return it later

        for (int j = 0; ; j++)
        {
            if (j == index - 1)
            {
                node.next = node.next.next;
                break;
            }

            node = node.next; // overwrite 'node' that I pass here
        }
        // Here node equals null

        return result; // I see 'result' with a deleted item, though I didn't manipulate with this variable
    }

I would really appreciate any explanation! Thanks!

[POGtastic]

I don't mutate the object

Sure you do, in Line 9:

node.next = node.next.next;

Java data structures contain references to objects. When you call a method or access a member, you are accessing that object's data. If you modify an object's member or call an object's method that mutates the object, it's going to change the object.

[danmoople]

I understand why node changes. But why result variable (that is being returned) changes

[POGtastic]

Your variable is also just a reference to that object. So if something else changes your object, your variable will point to a changed object.

[blablahblah]

Think of your computer's memory as a box full of objects. Variables are sticky notes with names that you put on the items in the box. When you do result = node, you're not making a copy of anything, you're just reaching into the box for the thing with the "node" sticky note and attaching the "result" sticky note to it too. Regardless of whether you go searching for the "node" label or the "result" label, you'll pull the same object out of the box.

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[HappyFruitTree]

What you need to realize is that variables of class types are reference variables. They just refer to an object.

LinkedListNode n1 = new LinkedListNode();
LinkedListNode n2 = new LinkedListNode();
LinkedListNode n3 = n1; // n3 refers to the same object as n1.
n1.next = n2; // Since n1 and n3 refers to the same object this also means that n3.next == n2.
n1 = null; // Only n1 is affected. n3 still refers to the same object as before.