Java Circular Linked List

Question

I have to create a method to eliminate numbers in a circular linked list say we have values up to 9

1 2 3 4 5 6 7 8 9

and we want to continually remove every 4th integer going through, it would go as followed

5 6 7 8 9 1 2 3; //  4 is removed
9 1 2 3 5 6 7;   //  8 is removed
5 6 7 9 1 2;     //  3 is removed
1 2 5 6 7;       //  9 is removed
7 1 2 5;         //  6 is removed
7 1 2;           // 5 is removed
1 2;             // 7 is removed
1;               // 2 is removed

I have to create a move to traverse through the elements, and an eliminate to remove the element, but I can do that on my own. I am having a problem with my toString(); method, I am currently not returning any value.

class Digit{ 

    class DigitNode
    {
            public int num=0;           // Digit's position in line
            public DigitNode next=null; // Reference to next digit
            /**
            * Digit constructor, initializes number
            */
            public DigitNode(int number)
            {
                   //
                num = number;
                next = null;
            }
    }

    private int number;
    private DightNode current = null;   // Linked list of digits
    private DigitNode tail = null;  // Tracks end of list as it is constructed

    /**
     * constructs a circular linked list of
     * @param n DigitNodes, current is the first DigitNode and tail is the last DigitNode(next of tail is current)
     */

    public Digit(int n)
    {
        //
        number = n;
        current = null;
        tail = null;
     }

     /*
     * prints all Digits starting from current until tail
     */
     @Override
     public String toString()
     {
    //
         String strVal = "";
         DigitNode position = current;
         while (position != null) {
             strVal = strVal + position + " ";
             position = current.next;
         }
         return strVal;
     }

To me, I understand that I'm assigning position as the current value which should be 1, thus while position is not null, strVal be the position [1] + " " for spacing. then I call position to be the next value which is [2], and I continue until null which is after 9. Thus strVal should be 1 2 3 4 5 6 7 8 9. But I'm not return anything unfortunately, I tried debugging, and placing some System.out.prinln(); markers to see if I was returning anything, but I wasnt.

Answer 1

First of all, you need to fill your Digit with DigitNode's objects. I don't see the code which does that from snapshot you posted.
Presumably you could do this in constructor of Digit, or create a method Digit.add(DigitNode node). You need this, otherwise your current will always be null.

Next, you need to add toString in DigitNode as I said earlier in the comment, or you could change Digit.toString() to have :

strVal = strVal + position.num + " "; // note position.num to get the number

Answer 2

You have no toString() in DigitNode so when you call

strVal = strVal + position + " ";

you are just appending to strVal whatever the default toString() method is for position, which is a DigitNode. This is because adding an object to a String with '+' calls it's toString() to get something to add to the String (in this case strVal).