ConcurrentModificationException on Iterator.next()

Question

I am getting a ConcurrentModificationException on the following code:

private static List<Task> tasks = new LinkedList<Task>();
...
public void doTasks(){
    synchronized(tasks){
        Iterator<Task> it = tasks.iterator();

        while(it.hasNext()){
            Task t = it.next(); < Exception is always thrown on this line.

            if(t.isDone()){
                it.remove();
            } else {
                t.run();
            }
        }
    }
}
...
public void addTask(Task t){
    synchronized(tasks){
        tasks.add(t);
    }
}
...
public void clearTasks(){
    synchronized(tasks){
        tasks.clear();
    }
}

The Object "tasks" is not used anywhere else in the class. I'm not sure why I'm getting the exception. Any help would be greatly appreciated.

Answer 1

This is your issue:

if(t.isDone()){
    ...
} else {
    t.run(); // probably changing the task, so consequently the list tasks
}

EDIT: you can't change the tasks list in the loop. Check out the ConcurrentModificationException documentation for more details.

Cheers!

Answer 2

Found the bug! I forgot the scenario where the task ran in doTask() can actually call addTask(). However I am a bit confused why this can happen as I thought the "tasks" object would be locked by the doTask() function.