Thread: Singly Circular Linked List Error

Hi,

I'm trying to make a singly circular linked list, however i happen to be getting nullpoiterexception in my code. I've never worked with circular linked list, so im not to certain if my approach is even correct. I have a feeling its not a big error and it can be remedied quickly, but i just cant see the solution.

I think i know what the problem is, but i cant figure out a way to solve it:

-basically at line 26 (i've marked it with comments), i have a while loop that should iterate through the linked list and stop just before the 'head' (remember this is a circular list, so the last node points back to the first which is the 'head')
-originally the head is 'null' so it adds the first node, and sets the current node to point to head
-when it attempts to add the second node, im guessing while loop flips out, because it says "current.getNext() != head" but current is equal to head...

so yeah, thats about as far as my debugging has gone. But i know it has something to do with the fact that its circular. because it works as a linear linked list. if you want to try it out as a linear linked list, i've included a few comments to tell you which lines to modify; there are 3 in total.

I would be very grateful if someone could take a look and help me out

package project3;
 
//circular linked list
class Hex
{
	Node head;	
 
	Hex()
	{
		head = null;		
	}
 
	public void insert(int n)	
	{
		Node temp = new Node(n);
		Node current = head;
 
		//if the list is empty, then the new node equal to head
		if(head == null)
		{
			head = temp;
			System.out.println("hit");
		}
		else
		{	
			//Starting from head, move through the list until you get to head again
			while(current.getNext() != head) //<------- ***Error***  (if want linear, change this line to, while(current.getNext() != null)
			{
				current = current.getNext();
			}
			//set the last node's pointer to the new node
			current.setNext(temp);
			//set the pointer of the last node back to the first node
			temp.next = head; //if you want linear, delete this line
		}
	}
 
//node class
	class Node
	{
		int element;
		Node next;
 
		Node()
		{
 
		}
 
		Node(int element)
		{
			this.element = element;
			next = null;
		}
 
		Node(int element, Node next)
		{
			this.element = element;
			this.next = next;
		}
 
		public int getElement() 
		{
			return element;
		}
 
		public void setElement(int element)
		{
			this.element = element;
		}
 
		public Node getNext()
		{
			return next;
		}
 
		public void setNext(Node next) 
		{
			this.next = next;
		}		
	}
 
	public String toString()
	{
		Node current = head;
		String output = "";
		while(current != head) //if want this linear, change this line to while(current.getNext() != null)
		{
			output += "[" + current.getElement() + "]";
			current = current.getNext();
		}
		return output;		
 
	}
 
 
}
 
public class Project3 {
	public static void main(String[] args) 
	{
		Hex h1 = new Hex();		
 
		h1.insert(1);
		h1.insert(2);
		h1.insert(3);
		h1.insert(4);
		h1.insert(5);
		h1.insert(6);		
 
		System.out.println(h1);
	}
 
}

Error:

Exception in thread "main" java.lang.NullPointerException
	at project3.Hex.insert(Project3.java:26)
	at project3.Project3.main(Project3.java:102)

thanks in advance

When you create the head with insert(), to what node should its next reference refer?

Thank you for the prompt reply!

hmmm...

so you mean somethng like this?

if(head == null)
{
	head = temp;
        head.next = head;
	System.out.println("hit");
 
}

if i put that. the error goes away, but it doesnt add anything aside from the first entry i think.

well i tried tracing it out on paper, but i'm not sure i did it correctly. From what i see on my paper, is the issue here the fact that the first node is constantly pointing to itself?

Not sure how to explain it but:

- So we make our first node, and its pointer is null.
- i make that node my head, give it a value of 1, and set its pointer to point to itself
- when i try to add another node, it never hits the while loop because the current is already pointing the head.

Is this correct? How can i remedy this?

Originally Posted by clydefrog

How can i remedy this?

Your code might be working. Your toString() method won't: you set current = head and then while(current != head). Consider changing that so that the loop executes at least once for the head node if it exists