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 destroy all the states.

public void clearStates()
{
    LogFactory.getLog(StateController.class.getName())
      .info( "Clearing states. #ofstates="+stateStack.size());
    for (State state : stateStack)  // Line 132 (see exception)
    {
        state.destroy();
    }

    // ...
} 

The following exception was trowed:

INFO  controllers.StateController : Clearing states. #ofstates=1
java.lang.NullPointerException\
    at java.util.LinkedList$ListItr.next(LinkedList.java:891)
    at *.controllers.StateController.clearStates(StateController.java:132)
    // ... //

This code usually works without a problem and has been in the production for more than a year.

Is it possible that this is Java bug?

/* Update */

destroy() call does not modify stateStack. If it would I guess Java would throw ConcurrentModificationException.

stateStack was populated with 1 state, which overrides destroy, but only does local modifications. The super implementation than prints additional log ("Destroying state..."), which was not in the log file, so I guess the exception was thrown at the beginning of iteration.

public void destroy()
{
    destroyed = true;
    LogFactory.getLog(State.class.getName()).info( "Destorying state : "+getClass().getName());
    propertyChangeSupport.firePropertyChange(PROP_DESTROYED, null, this);
}

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();
    }
}