Nullpointer exception in LinkedList while using for-each loop

Question

I came across very odd Java behaviour, and I don\'t know if it is a bug, or am I missing something.

The code simply goes through the stateStack (LinkedList) list and

Answer 1

The piece of code below generates the same exception almost every time I run it - the idea is to modify the list while iterating from another thread. With (un-)lucky timing, the modification happens after checkForComodification but before next = next.next; in the ListItr#next method, causing a NPE.

Exception in thread "main" java.lang.NullPointerException at java.util.LinkedList$ListItr.next(LinkedList.java:891) at javaapplication4.Test1.main(Test1.java:74)

public class Test {
    public static void main(String[] args) {
        final int SIZE = 100000;
        final Random rand = new Random();
        final List<Integer> list = new LinkedList<>();
        for (int i = 0; i < SIZE; i++) {
            list.add(i);
        }

        Runnable remove = new Runnable() {

            @Override
            public void run() {
                while (true) {
                    int i = rand.nextInt(SIZE);
                    list.remove(i);
                    try {
                        Thread.sleep(10);
                    } catch (InterruptedException ex) {
                        break;
                    }
                    list.add(i);
                }
            }
        };
        Thread t = new Thread(remove);
        t.start();
        for (int i = 0; i < 100; i++) {
            try {
                for (Integer j: list) {
                    ///whatever
                }
            } catch (ConcurrentModificationException e) {
            } catch (NullPointerException e) {
                e.printStackTrace();
            }
        }
        t.interrupt();
    }
}

Answer 2

This is the internal implementation of LinkedList.ListItr.next():

public E next() {
    checkForComodification();
    if (!hasNext())
        throw new NoSuchElementException();

    lastReturned = next;
    next = next.next;   // your stacktrace says the NullPointerException happens here
    nextIndex++;
    return lastReturned.item;
}

The NullPointerException happens because the internal variable next is null; however, it seems that hasNext() is validating that there is a next element.

It seems to me that:

• you have more than one thread modifying your list, OR
• you are modifying your list in the implementation of destroy() while iterating over the list.

If you update your answer with your implementation of destroy() as sugested by @mthmulders, I either update, correct or delete my answer.