Reverse Singly Linked List Java, check whether circular

Question

I implemented a singly linked list in Java. I can reverse a singly linked list and check whether a given list is circular. The interesting thing is I can reverse a circular list, too which is strange and interesting. Does it make sense to be able to reverse a circular list?Actually it should get reversed over and over again right? At the moment my below code is able to reverse a circular list and terminates. Is it correct?

public class ListNode{
 int value;
 ListNode next; 
 public ListNode(int value, ListNode next){
 this.value = value;
 this.next = next;
 } 
 public ListNode next(){
 return next;
 }
 public void setNext(ListNode next){
 this.next = next;
 }
 public int value(){
 return value;
 }
}
public class SinglyLinkedList {
 private ListNode head;
 public SinglyLinkedList(ListNode head){
 this.head = head;
 }
 public void reverse(){
 ListNode current = head;
 head = null;
 while (current!=null){
 ListNode temp = current;
 current = current.next;
 temp.next = head;
 head = temp;
 }
 }
 public static boolean isCircular(SinglyLinkedList list){
 ListNode counter1 = list.head;
 ListNode counter2 = list.head;
 while (counter1!=null && counter2!=null){
 counter1 = counter1.next;
 counter2 = counter2.next;
 if (counter2.next!=null){
 counter2 = counter2.next;
 } else 
 return false;
 if (counter1 == counter2)
 return true;
 }
 return false;
 }
 public static void printSinglyLinkedList(ListNode head){
 ListNode temp = head;
 while(temp!=null){
 System.out.print(temp.value + " ");
 temp = temp.next;
 }
 System.out.println();
 }
 public static void main(String[] s){
 ListNode a4 = new ListNode(4, null);
 ListNode a3 = new ListNode(3, a4);
 ListNode a2 = new ListNode(2, a3);
 ListNode a1 = new ListNode(1, a2);
 a4.setNext(a1);
 SinglyLinkedList list1 = new SinglyLinkedList(a1);
 System.out.println(isCircular(list1));
 if (!isCircular(list1))
 printSinglyLinkedList(list1.head);
 list1.reverse();
 if (!isCircular(list1))
 printSinglyLinkedList(list1.head);
 }
}

• Mark Elliot: Given your implementation of reverse, it seems strange that a circular list is reversible, as your stopping condition, that current is eventually null, should never hit in a circular list.
• Bob: @MarkElliot As you can see, list1 is circular but at the same time reversible. My very first question is whether it makes sense to be able to reverse a circular singly linked list? From understanding point of view in my opinion, it makes sense to have two different directions of how I go through a list. Is my thinking correct?
• Tony Hopkinson: I'm having problems with this too. Being able to reverse a circular list is far from strange, in fact direction of search in terms of finding or not finding, is irrelevant, that's one of the properties that identifies a circular list.
• Tony Hopkinson: @Bob it only makes sense if the items are ordered.
• Mark Elliot: @Bob, I'd be interested in seeing some output of a reversed list with loop, I suppose your head-node would be unchanged, and any path before the loop begins would also end up unchanged, but the loop itself would flip, now that I think through it...

Answer 1

a4.setNext(a1);

Remove the above statement. This makes it circular.