Thread: What's wrong with my method?

 
/**
     * Remove from the club's collection all members who
     * joined in the given month, and return them stored
     * in a seperate collection object.
     * @param month The month of the membership.
     * @param year The year of the membership.
     * @return The members who joined in the given month.
     */
    public ArrayList<Membership> purge(int month)
    {
        ArrayList<Membership> purgeMembers = new ArrayList<Membership>();
        if(month < 1 || month > 12) {
            System.out.println("Error: Month " + month + 
                " out of range. Must be in the range 1 ... 12");
        }
        else {
            for(Membership membership : members) {
                if(membership.getMonth() == month) {
                    members.remove(members.indexOf(membership));
                    purgeMembers.add(membership);
                }
            }
 
        }
        return purgeMembers;
    }

When I call this method with a valid parameter, it throws a java.util.ConcurrentModificationException.
I read in the API documentation for the ArrayList class what it is, but I dont fully understand it and I dont know how to get over it. Can I use the Collections.synchronizedList method here and if yes how I do it? Can someone explain me a little bit about this staff? Or should I just separate these threads so that thay dont access the arrayList at the same time and achieve my goal like that? Nevertheless it is an opportunity to learn about something that I accidentally came across..

You get this exception because you are modifying (add / remove) a collection while you are iterating over it at the same time.
The standard iterators are written in a way to always throw these exceptions if the underlying collection has been changed after they have been created.

There are several solutions to your problem. First, you could use the Iterators "remove" method to safely remove an element from a collection while you are iterating over it.
Other then that you could use a second, temporary collection in which you remember which elements you want to delete after your iteration, then you delete them at the apropriate time.

Just to expand on this a bit more in case you're still unsure of the documentation at ArrayList (Java Platform SE 7 ), there are two possible causes of ConcurrentModificationException:

1. "If multiple threads access an ArrayList instance concurrently, and at least one of the threads modifies the list structurally [...] (A structural modification is any operation that adds or deletes one or more elements, or explicitly resizes the backing array; merely setting the value of an element is not a structural modification.)"
2. "If the list is structurally modified at any time after the iterator is created, in any way except through the iterator's own remove or add methods", i.e., the "fail-fast behaviour"

#1 only occurs if you have 2 or more threads using (and structurally modifying) the ArrayList at the same time. The use of "List list = Collections.synchronizedList(new ArrayList(...));" is applicable for this scenario. (If you're not sure what I mean by "thread", see Lesson: Concurrency and Processes and Threads.)

#2 occurs even when you have just one thread using the ArrayList. The word "concurrent" in the exception in this case does not refer to concurrent access of the ArrayList by multiple threads, but to performing structural modification after having created the iterator for the ArrayList. Collections.synchronizedList() will not help in this scenario.

In your case, it is #2, as the good members above have helpfully posted.