Question

I am having trouble with java.util.LinkedList. I am getting a null pointer exception upon using the poll() on the list even after adding an element to the list. I am not using threads or anything similar to threads.

Any help is appreciated. Below is the myMethod code which is called from a main method of another class:

  void myMethod(Node start, int startRow){
    LinkedList<Node> queue = new LinkedList<Node>();
    LinkedList<Integer> rowQueue = new LinkedList<Integer>();

    queue.addFirst(start);
    rowQueue.addFirst((Integer)startRow);
    System.out.println( rowQueue.size() );      

    while (queue.size()!=0){            
        Node n = queue.poll();
        int row = rowQueue.poll().intValue(); //This is the line 33 in the error!
            /*Some remaining code which uses variables n and row. The thread of control does not reach here */

    }
  }

Below is the output:

Exception in thread "main" java.lang.NullPointerException
    at BFS.isConnected(LinkedListTest.java:33)
    at GraphsMain.main(GraphsMain.java:36)
1
ZERO

I am confused because the print statements execute after the error, and clearly I have written them the other way around. Is this a threading issue at all? I know that LinkedLists are not synchronized, but is that the problem here? should I be worried about it just for a simple implementation?

Answer 1

Assume that you’ve some elements to your queue and nothing in your rowQueue. Then your logic in the while loop would work as follows:

• Node n = queue.poll(); will get the first and only element from the queue and store in the n variable.

• It would check if the size of rowQueue is zero or not. If it is, it would print a zero

• However, and your problem lies in here, it would still try to poll() again, get a null this time and call intValue() on the returned value, resulting in the NullPointerException

The issue as such lies with the following logic in your code:

        if(rowQueue.size() == 0){
            System.out.println("ZERO");
        }
        int row = rowQueue.poll().intValue(); 

While you’re checking if rowQueue.size() is 0, and if true, you should not only print Zero but also ensure that you do not perform int row = rowQueue.poll().intValue();. So you should consider breaking out of the loop or something along those lines.

So you should try something like this:

        if(rowQueue.size() == 0){
            System.out.println("ZERO");
            break; //This ensures that you come out of the loop
        }
        int row = rowQueue.poll().intValue();