Can I use java.util.LinkedList to construct a circular/cyclic linked list?

Question

I would like to create a circular/cyclic linked list where the tail of the list would point back to the head of the list. So can I use java.util.LinkedList and modify the tail node after creation of the list to make it circular/cyclic? If so, can you show me some code on how that would happen?

If I can't use java.util.LinkedList, how should I create my own circular/cyclic linked list implementation? Can you show me the skeletons of how this implementation would look?

Let me know if you need more details and I'll clear up any confusion.

Answer 1

class ListNode {
    public ListNode next;
    public Object data;

    public ListNode(Object data, ListNode next) {
        this.next = next;
        this.data = data;
    }
}

class CircularLinkedList {
    private ListNode head = null;
    private int numberOfElements = 0;
    private ListNode actualElement = null;
    private int index = 0;

    public boolean isEmpty() {
        return (numberOfElements == 0);
    }

    public int getNumberOfElements() {
        return numberOfElements;
    }

    public void insertFirst(Object data) {
        if (!(isEmpty())) {
            index++;
        }
        ListNode listNode = new ListNode(data, head);
        head = listNode;
        numberOfElements++;
    }

    public void insertAfterActual(Object data) {
        ListNode listNode = new ListNode(data, actualElement.next);
        actualElement.next = listNode;
        numberOfElements++;
    }

    public boolean deleteFirst() {
        if (isEmpty())
            return false;
        if (index > 0)
            index--;
        head = head.next;
        numberOfElements--;
        return true;
    }

    public boolean deleteActualElement() {
        if (index > 0) {
            numberOfElements--;
            index--;
            ListNode listNode = head;
            while (listNode.next.equals(actualElement) == false)
                listNode = listNode.next;
            listNode.next = actualElement.next;
            actualElement = listNode;
            return true;
        }
        else {
            actualElement = head.next;
            index = 0;
            return deleteFirst();
        }
    }

    public boolean goToNextElement() {
        if (isEmpty())
            return false;
        index = (index + 1) % numberOfElements;
        if (index == 0)
            actualElement = head;
        else
            actualElement = actualElement.next;
        return true;
    }

    public Object getActualElementData() {
        return actualElement.data;
    }

    public void setActualElementData(Object data) {
        actualElement.data = data;
    }
}

Answer 2

For practical application (e.g. not only playing around or learning) I would personally prefer Guava's Iterables.cycle method - see http://guava-libraries.googlecode.com/svn/trunk/javadoc/com/google/common/collect/Iterables.html#cycle%28java.lang.Iterable%29 .

Answer 3

java.util.LinkedList is one of the Collections datatypes. The purpose of Collections is to provide utility structures, not bothering the programmer to worry about their internal implementation. If you must have internals that work in a certain way, and the java.util ones do not guarantee that is how they work, then they are not for you.

To implement a circular linked list, first create a ListNode class:

class ListNode {
    ListNode next;
    ListNode prev;
    Object data;
}

Then store a ListNode head, and make sure prev of head points to the "end" of the list, and next of the "end" points back to head. Honestly, though, there's little difference between a bidirectionally linked list keeping a tail pointer and a circular linked list.

Answer 4

Refer here for a comprehensive implementation of Singly Linked List in Java. Making it circular is just a tweak in that code.