Initializing Singly Linked List Java

Question

I am trying to implement an abstract data type of NumList in class NumLinkedList as a singly linked list.

public class NumLinkedList implements NumList
{

Node head;
int nItem;

private class Node
{
    public Node next;
    public double value;
    public Node(double i, Node j)
    {
        value = i;
        next = j;

    }

}

public NumLinkedList()
{
    head = null;
    nItem = 0;

}

is my initialization and I am having trouble with the following methods.

public void print()
{
    Node currNode = head;
    while(currNode.next != null)
    {
        currNode = currNode.next;
        System.out.println(currNode.value);
    }

}

public int size()
{
    int size = 0;
    Node currNode = head;
    while(currNode.next != null)
    {
        currNode = currNode.next;
        size++;
        nItem++;

    }
    return size;

}

public void insert(int i, double value)
{
    if( i < 0)
    {
        System.out.println("Error. out of bounds.");
    }
    if ( i > size())
    {
        Node currNode = head;
        for(int j = 0; j < i; j++)
        {
            currNode = currNode.next;
        }
        currNode.next = new Node(value,null);
    }

    if ( i <= size())
    {
        Node currNode = head;
        for(int j = 0; j < i-1; j++) // this moves CurrNode to the i-1th node
        {
            currNode = currNode.next;
        }
        currNode.next = new Node(value,currNode.next);
    }

    nItem++;
}

when I run my testing code,

public static void main (String[] args)
{
    NumLinkedList test;
    test = new NumLinkedList();

    //System.out.println("this is how many initial items the initialized list has");
    //System.out.println(test.size());
    test.insert(1, 0.1);
    System.out.println("have tried to insert value 0.1 @ position 1, that is the first element in list.");
    test.print();
    System.out.println("tried print(). did it work?");
    test.insert(4, -1);

I get an error in the

test.insert(1, 0.1);

, referring to

if ( i > size())

and

while(currNode.next != null)

as I failed initializing my array ADT as well, I believe that my list ADT is also mis-initialized. It's hard to find proper examples on Google, any references for ADT initialization?

Answer 1

Your problem isn't in the initialization, which is fine, but in the methods themselves.

The problem is that you're initializing your head to null , but then, both in print and in size , you're doing currNode = head , which also makes it null , and then looping on currNode.next . That'll throw a NullPointerException right there. You need to go to all your methods and add special code for the limit case:

For print :

if ( head == null )
    return;

For size :

if ( head == null )
    return 0;

Alternatively, you can simply replace your entire size method with a simple return nItem; statement.

For insert :

if ( head == null )
{
    head = new Node(value,null);
    return;
}

As an aside, in your insert , you also need a return inside your first if , and you need to replace your third if with an else .