Question

I know this is a daft questions but cant work out how to fix this, I have not had much experience with using threads before.

Below should create first of all a timer which will execute the command output.write(mylist) every 10 seconds which will simply output the contents of mylist.

Secondly it loops through about 3 lists I have and for each them creates a thread which will continue to loop through getting the next word in the list.
Please note: this is stripped down version and not complete so please don’t comment on the arraylist / list but rather the error itself.

There is a concurrent modification exception happening frequently but not all the time when it tries to do output.write(). I am guessing this is because one of the other threads are currently saving something to mylist? How would I go about fixing this?

    Runnable timer = new Runnable() {
        public void run() {
            try {
                while (true) {
                    Thread.sleep(10000);
                    output.write(mylist);
                }
            } catch (InterruptedException iex) {}
        }
    };
    Thread timerThread = new Thread(timer);
    timerThread.start();

    for (final ValueList item : list) {

        Runnable r = new Runnable() {
            public void run() {
                try {
                    while (true) {

                        char curr = item.getNext();

                         mylist.addWord(curr);
                    }
                } catch (InterruptedException iex) {}
            }
        };

        Thread thr = new Thread(r);
        thr.start();
    }
}

Answer 1

There is a concurrent modification exception happening frequently but not all the time when it tries to do output.write…

The problem is (I assume) that the output.write(...) method is iterating through your myList at the same time as another thread is calling myList.addWord(curr);. This is not allowed unless myList is a concurrent collection.

How would I go about fixing this?

You will need to synchronize on myList every time you are accessing it — in this case when you are outputting it or adding a word to it.

  synchronized (myList) {
      output.write(mylist);
  }
  ...

  synchronized (myList) {
      myList.addWord(curr);
 }

In this case, because output.write(mylist) is probably iterating through the list, you cannot use the Collections.synchronizedList(...) method because the iterators need to be synchronized by the caller.

If this is some high-performance method that is called tons of times then you could also use a ConcurrentLinkedQueue but that is a queue, not a list obviously. ConcurrentSkipList is another option although that is a heavier data structure.