Question

import javax.swing.JOptionPane;

public class RotateArrayCircularLL
{
    private Node head=null;   

    public void init()
    {

       int choice = 0;

        while (choice != -1){
        choice = Integer.parseInt(JOptionPane.showInputDialog("Enter -1 to stop loop, 1 to continue"));    

        if(choice == -1)
            break;

        inputNum();

      }
      printList();
    }

    public void inputNum()   
    {
        Node n;
        Node temp;
        int k;

        k = Integer.parseInt(JOptionPane.showInputDialog(null,"Enter a number:"));
        n = new Node(k);       

         if (head == null) {
            head = n;            
         } else {            
            temp = head;
            while (temp.getNext() != null)
                temp = temp.getNext();

            temp.setNext(n);                
        }       

    } 

    public void printList()  
    {
        Node temp = head;

        int count =  Integer.parseInt(JOptionPane.showInputDialog("Enter the value to shift to the right"));

        for (int i = 1; i <= count; i++) // Rotates the head
            temp = temp.getNext();

        for (Node c = temp; c != null && c.getNext() != head; c= c.getNext()){ // Prints the new LL
            System.out.print(c.getInfo());   
        }
    }     
}

I get an NPE during the second for loop. I understand that it is giving me a NPE because I reach the end of the list, but how can I stop it from doing this?

Answer 1

It appears from the behavior you are seeing that one of the nodes in your linked list is returning null instead of the next element of the list. At a guess, I’d suggest that the last node of your list is probably not pointing to the first node of your list. Hence as Hovercraft Full Of Eels suggests, you don’t really have a properly circular linked list. If you can post the code showing how temp is populated, it may be possible to give a more concrete solution to your issue. Otherwise, you need to treat the case where getNext() returns null as a special case and ensure that you instead get the first element from the initial list.