Getting a linked list null pointer and I don't understand why

Question

This program is supposed to allow the user to insert integers into a Linked list and keep them always sorted. I am stuck on why I am getting a null pointer exception at this point after I return the value back my other method. It feels like I am going in circles at this point. I have dummy print statements to try and figure out the problem.

Class Node:

public class Node {
  Comparable data;
  Node next;

  Node(Node n, Comparable a) {
    this.data = a;
    this.next = n;
  }
}

Class SortedLinkedList:

public class SortedLinkedList {
  Node head = null;
  private Comparable SortedLinkedList;


  public SortedLinkedList() {
    this.head = null;
    this.SortedLinkedList = SortedLinkedList ;
  }

  public Node insert(Node head, Comparable a){
        if (head == null || (head.data.compareTo(a)> 0))
        { 
            System.out.println("In insert first if");
            head = new Node( head, a);
            //head = new Node(head, 22);
            System.out.println("Head = " + head.data + " before     return");
            return head;
        }
        Node pointer = head;
            while (pointer.next != null)
            {
                if (pointer.next.data.compareTo(a) > 0){
                    System.out.println("In insert, in while, in if");
                    break;
                }
                pointer = pointer.next;
            }
        return head;
  }

  public void print(Node head){
    System.out.println("In print outside of for" + head);
    for (Node pointer = head; pointer != null ; pointer = pointer.next)
    {
        System.out.println("In print");
        System.out.print(pointer.data);
    }
  }
}

Class TestInteger

public class TestInteger implements Comparable{
    // This is the User Interface for manipulating the List

    static SortedLinkedList sll = new SortedLinkedList ();

    public static void nodeMenu() {
        Node head = sll.head;
        System.out.println(head);
        int option;

        while(true){
            System.out.println();
            System.out.println("**** Integer Node Menu ****");
            System.out.println("****************************");
            System.out.println("** 1. Insert              **");
            System.out.println("** 2. Delete              **");
            System.out.println("** 3. Clear               **");
            System.out.println("** 4. Smallest            **");
            System.out.println("** 5. Largest             **");
            System.out.println("** 6. Return to Main Menu **");
            System.out.println("****************************");

            Scanner sc = new Scanner(System.in);
        try{

            option = sc.nextInt();
            switch (option){
            case 1:{
                try{
                    System.out.println("Type an integer to insert: ");
                    int x = sc.nextInt();
                    Integer insertItem = new Integer(x);
                    sll.insert(head, insertItem);

                    System.out.println("After insert back in case1 head = " + head.data);
                    sll.print(head);
                }catch(InputMismatchException e){
                    System.out.println("Enter only integers");
                    sc.nextLine();
                }
                nodeMenu();
            }

It prints correctly in the actual insert method within class SortedLinkedList but gets a null pointer in class TestInteger. Here is output:

1
Type an integer to insert: 
5
In insert first if
Head = 5 before return
Exception in thread "main" java.lang.NullPointerException
at CS_240_HW_2.TestInteger.nodeMenu(TestInteger.java:58)
at CS_240_HW_2.Main.mainMenu(Main.java:52)
at CS_240_HW_2.Main.main(Main.java:30)

Answer 1

head = sll.insert(head, insertItem);

instead of

sll.insert(head, insertItem);

?

Answer 2

There's a lot to change with your code. For example, you only need one head and you only need the one Node class for data. The SortedLinkedList class can be a utility class that only has some methods used to fool with the nodes in a particular way.

So I suggest a change to Node . This class holds all the data, all but the head itself.

public class Node {
  Comparable data;
  Node next;

  Node(Comparable a) {
    this.data = a;
  }
}

Then these changes to the inserter class. This class is just some helpful methods that are used to do handy things with your linked list and/or its nodes.

public class SortedLinkedList {

    public Node insert(Node head, Comparable a){

        Node curr = head;
        Node prev = null;
        while (curr != null && curr.data.compareTo(a) > 0) {
            prev = curr;
            curr = curr.next;
        }
        if (prev = null) {
            return new Node(a);
        }
        prev.next = new Node(a);
        prev.next.next = curr;
        return head;
    }

    // print doesn't need changing
}

And for the the test class not too many things to change:

public class TestInteger implements Comparable{
    // This is the User Interface for manipulating the List

    static SortedLinkedList sll = new SortedLinkedList ();
    Node head = null;

    public static void nodeMenu() {

        // no changes in this part ...

                    Integer insertItem = new Integer(x);
                    head = sll.insert(head, insertItem);
                }catch(InputMismatchException e){
                    System.out.println("Enter only integers");
                    sc.nextLine();
                }
            }

Be sure to take out the recursive call to nodeMenu()

Answer 3

You initialize a list:

static SortedLinkedList sll = new SortedLinkedList ();

In this constructor, the head of the list is set to null:

this.head = null;

Then you initialize a variable with the head of the list:

Node head = sll.head;

So head is null.

Then you try to print the value of head.data :

System.out.println("After insert back in case1 head = " + head.data);

And since head is null, you get a NullPointerException.