Why my Circular Singly Linked List Java code not working? It says that Node is null can't put data on it

Question

For the meantime, I'm trying to figure out the basics of how Circular Singly Linked List works. In this case, I'm creating a program in which, user can input from 1 to 3, the number of linked nodes. However, I'm stuck for over 5 hours and can't find a solution elsewhere on this problem. It says that I cannot assign data on my Node because it is null, but I already assigned it to a newnode.

After figuring this 1-3 node circular singly linked list. I'm planning on increasing the nodes.

The IDE says this:

Exception in thread "main" java.lang.NullPointerException: Cannot assign field "data" because "DS2.prac2.head" is null
at DS2.prac2.inc(prac2.java:47)
at DS2.prac2.Start(prac2.java:34)
at DS2.prac2.main(prac2.java:104)

My code:

package DS2;
import java.util.Scanner;

public class prac2{

    
    static class Node {  
        int data;  
        Node next;  
      
      
    }
  
   
    static Node head;  
    static Node tail;  
    static Node current;  
    public static int num;

    public prac2(){


        num=0;
    }


 
    static void Start(int n){ //Starts the program

   
    
    for(int i=0; i<n;i++){ //A loop to create nodes
        inc();
    }

    }

    static void inc(){  //the method for each nodes created

        Node newnode = new Node();
        newnode.data=num;num++;
        newnode.next=head;
        
        if(num==1){
            newnode=null;
            head.data=num;num++;
            head.next=head;
            

        }

        else if(num==2){
       
        tail=newnode;
        head.next=tail;

        }
        else{
         
            tail.next=newnode;
            tail=newnode;

        }



    }

    static void Display(){ //For Displaying the Circular List
    int temp1=0;
    current=head;

    do{
    if(current==head)  //If current passed head twice it means that it passed through all the nodes
        temp1++;

    System.out.println(current.data);
    current=current.next;


    }
    while(temp1==1);

    

    }
  
   
    
    public static void main(String[] args) {
        

      

        int M,N;

        Scanner scan = new Scanner(System.in);

       // M=scan.nextInt();
       System.out.print("Input how many nodes (1-3): ");
        N=scan.nextInt();
       
        Start(N);

        scan.close();



        
        
    }
}``` 


   

Answer 1

The error is on this line:

head.data=num;num++;

It is quite clear that head was never assigned a node reference. It is still null . In fact, you did create a node and assigned it to newnode , but then with newnode=null you cut the reference to it. Instead you should have assigned it to head , with head=newnode .

In the case the node is the first node, you should also initialise tail -- to be the same node as the head node.

Here is corrected code for inc :

static void inc() {
    Node newnode = new Node();
    newnode.data = num++;
        
    if (head == null){
        head = newnode;
    } else {
        tail.next = newnode;
    }

    tail = newnode;
    tail.next = head;
}

Not related, but you could get rid of head all together, as it can be derived from tail . When tail is null then you know the head is null too. If tail is not null you know that the head is at tail.next . So there really is no reason to waste memory on storing head . Of course, this means you will have to rewrite the display function a bit too.