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Thread: Singly Circular Linked List Error

  1. #1
    Member clydefrog's Avatar
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    Default 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


  2. #2
    Super Moderator Sean4u's Avatar
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    Default Re: Singly Circular Linked List Error

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

  3. The Following User Says Thank You to Sean4u For This Useful Post:

    clydefrog (March 6th, 2012)

  4. #3
    Member clydefrog's Avatar
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    Default Re: Singly Circular Linked List Error

    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.

  5. #4
    Administrator copeg's Avatar
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    Default Re: Singly Circular Linked List Error

    Step through your code and write out on paper what is happening. On first insert you create a node:

    Node {next = null}

    When you loop through upon second insertion, you end up setting current to the next value, which is null

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    clydefrog (March 6th, 2012)

  7. #5
    Member clydefrog's Avatar
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    Default Re: Singly Circular Linked List Error

    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?

  8. #6
    Super Moderator Sean4u's Avatar
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    Default Re: Singly Circular Linked List Error

    Quote Originally Posted by clydefrog View Post
    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

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    clydefrog (March 6th, 2012)

  10. #7
    Administrator copeg's Avatar
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    Default Re: Singly Circular Linked List Error

    is the issue here the fact that the first node is constantly pointing to itself?
    I see nowhere in your original code in which you define the first node pointing to itself. See post #3

  11. #8
    Member clydefrog's Avatar
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    Default Re: Singly Circular Linked List Error

    Quote Originally Posted by Sean4u View Post
    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
    Sorry i didnt reply sooner, i had to leave the house for a few hours.

    wtf? Yeah it was the toString() method, it appears to be working correctly now. lol thank you so much.

    Now just another quick question: From what i can tell, it appears as though my circular linked list is implemented correctly. How can test to see if its actually circular?
    Last edited by clydefrog; March 5th, 2012 at 07:54 PM.

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