Circular Doubly Linked List Java - Explanation

Question

at the moment, I am programming a circular doubly linked list and my task is completed, but I have a slight problem in understanding the code 100%.

This is the code:

if (counter == 0) {
        startpoint = newNode;
        startpoint.next = startpoint;
        startpoint.prev = startpoint;
        head = startpoint;
        currentNode = newNode;

    } else {
        newNode.prev = startpoint.prev;
        newNode.next = startpoint;
        startpoint.prev.next = newNode; <- this is, where I have problems
        startpoint.prev = newNode;
        head = startpoint;
        currentNode = newNode;
    }
    counter++;
}

The first part is completeley clear and just describes, that the first node (if there is no other node) is going to point on itself, when the next or first-method is called. After the else-statement, the first line describes that the first node can point to the second and the second to the previous node, right? The second line after else just describes, that the last node points onto the first one. The fourth line than describes, that the first node points onto the last node, if the prev-method is called and so the circle is closed. But what exactly does the third line describes? The code definitely works.

Thank you for your time!

Greets Dominik

Initially startpoint.prev points to the "last node before startpoint" in the circular list. Thus, startpoint.prev.next = newNode says that the successor of that node is newNode — Federico
You're trying to set a value on a reference that doesn't exist yet (startpoint.prev) by doing startpoint.prev.next = newNode; before startpoint.prev = newNode;. — user1205577

jhamon jhamon · Accepted Answer · 2018-06-11T12:59:00

Your list is circular.

startpoint.previs the element before the startpoint, so it's the last element of the list.

Let's take an example, this is your list: A>B>C, you try to add D.

This line: startpoint.prev.next = newNode; will link the current last element (C as A.prev == C) to your new element. (C.next = D)

this line startpoint.prev = newNode; will set the new element as the last element of the list (A.prev = D).

Circular Doubly Linked List Java - Explanation

2 Answers