Adding values to LinkedList class

Question

Main Class:

ArrayList<LinkedList> my_lists = new ArrayList<LinkedList>();

    try {
        Scanner sc = new Scanner(file);
        while (sc.hasNextLine()) {

            String line = sc.nextLine();
            LinkedList the_list = new LinkedList();
            String[] templist = line.split("\\s*,\\s*");

            for(int i=0; i<templist.length; i++){
                temp = templist[i];
                the_list.add(temp);
                System.out.println(templist[i]);
            }

            my_lists.add(the_list);
            System.out.println(line);
        }
        sc.close();
    }

Add method from my LinkedList class:

    public void add (Object newData){
    Node cache = head;
    Node current = null;
    while ((current = cache.next) != null)
        cache = cache.next;

    cache.next = new Node(newData,null);
}

It's giving me an error everytime I run this for the line: the_list.add(temp); Any ideas on what's going on?

What is the specific error/stack trace that's returned?

Steven Mastandrea
– Steven Mastandrea

2014-03-20 13:37:44 +00:00
Commented Mar 20, 2014 at 13:37 — Steven Mastandrea
– Steven Mastandrea, Commented Mar 20, 2014 at 13:37

Steven Mastandrea · Accepted Answer · 2014-03-20 13:47:20Z

2

If you're getting a NullPointerException, it probably because you haven't initialized the head variable in your class. The first time you go to add an Object to your LInkedLIst, you call the add method and head is null; Thus, cache = null and you then try to reference cache.next in the while loop, which throws an Exception.

Try adding this to the beginning of your add method to handle the special case.

if (head == null)
head = new Node(newData, null);
else {
  .. rest of method 
}

edited Mar 20, 2014 at 13:47

answered Mar 20, 2014 at 13:42

Steven Mastandrea

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4 Comments

mickdeez Over a year ago

I had that at the top of my code, just not copied to stackexchange. I moved it to the beginning of my loop and same error still. I'm pretty sure it's something within the the_list.add(temp; line

Steven Mastandrea Over a year ago

What is the full Exception/stack trace?

Steven Mastandrea Over a year ago

Yep. Please remember to accept answers as well if they best answered your question.

mickdeez Over a year ago

Done :) Just had to wait on the timer

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