Linked List of Strings

Question

I'm trying to make a StringBuilder like class using linked lists, and I think I'm messing up somewhere in my constructor. Can anyone find an issue here? I believe the issue is with how I'm moving to the next node.

Node class:

private class CNode
{
    private char data;
    private CNode next;

    private CNode(char c)
    {
        data = c;
        next = null;
    }

    private CNode(char c, CNode nextNode)
    {
        data = c;
        next = nextNode;
    }
}

Constructor:

private CNode firstNode;
private int length;

public MyString(String s)
{
    if(s == null)
    {
        this.length = 0;
        this.firstNode = null;
    }
    else if(s.length() == 1)
    {
        this.length = 1;
        this.firstNode.data = s.charAt(0);
        this.firstNode.next = null;
    }
    else
    {
        this.length = s.length();
        CNode node = null;
        CNode nextNode = null;

        this.firstNode = new CNode(s.charAt(0), node);

        for(int i = 1; i < s.length(); i++)
        {
            node = new CNode(s.charAt(i), nextNode);
            node = node.next;
        }
    }
}

Answer 1

One problem I see is at

this.firstNode = new CNode(s.charAt(0), node);

When that line executes, node is null, so your firstNode ends up linked to nothing. Further, in the for loop where you're trying to build the links, you never assign nextNode, but you try to use it to link one node to the next. Therefore, all the nodes end up linking to null, the initial value of nextNode.

Another issue:

this.firstNode.data = s.charAt(0);
this.firstNode.next = null;

That should be creating a new CNode instead because this.firstNode is still null when that code executes, which will cause a NullPointerException.

Answer 2

There may be other issues here, but consider this block of code:

else if(s.length() == 1)
{
    this.length = 1;
    this.firstNode.data = s.charAt(0);
    this.firstNode.next = null;
}

Notice that you've never allocated this.firstNode, which means that you're going to get a NullPointerException when executing the second line of code. Try allocating a node before writing to it.

Hope this helps!