Java Data Structures - Circular linked lists

Circular Linked List is a variation of Linked list in which the first element points to the last element and the last element points to the first element. Both Singly Linked List and Doubly Linked List can be made into a circular linked list.

Singly Linked List as Circular

In singly linked list, the next pointer of the last node points to the first node.

Doubly Linked List as Circular

In doubly linked list, the next pointer of the last node points to the first node and the previous pointer of the first node points to the last node making the circular in both directions.

As per the above illustration, following are the important points to be considered.

The last link's next points to the first link of the list in both cases of singly as well as doubly linked list.

The first link's previous points to the last of the list in case of doubly linked list.

Example

class Node{	
   int data;
   Node preNode, nextNode, CurrentNode;
   Node() {
      preNode = null;
      nextNode = null; 
   }	   
   Node(int data) {
      this.data = data;
   }		
}
public class CircularLinked {
   Node head, tail;
   int size;
	   
   public void printData() {
      Node node = head;
      if(size<=0) {
         System.out.print("List is empty");
      } else {
         do {
            System.out.print(" " + node.data);
            node = node.nextNode;
         }
         while(node!=head);
      }
   }
   public void insertStart(int data) {
      Node node = new Node();
      node.data = data;
      node.nextNode = head;
      node.preNode = null;
      
      if(size==0) {
         head = node;
         tail = node;
         node.nextNode = head;
      } else {
         Node tempNode = head;
         node.nextNode = tempNode;
         head = node;
         tail.nextNode = node;
      }
      size++;   
   }
   public void insertEnd(int data) {
      if(size==0) {
         insertStart(data);
      } else {
         Node node = new Node();
         tail.nextNode =node;
         tail = node; 
         tail.nextNode = head;
         size++;   
      }
   }
	public static void main(String args[]) {
      CircularLinked dl = new CircularLinked();
      dl.insertStart(10);
      dl.insertStart(20);
      dl.insertStart(30);
      dl.insertStart(1);
      dl.insertStart(56);
      dl.insertStart(40);
      dl.printData();
   }			
}

Output

40 56 1 30 20 10