ConcurrentModificationException in unmodifiable collection

Question

I have this code below, and I'm getting a ConcurrentModificationException by executing the following line:

filterCardsToDevice(getCollection());

the code:

private List<MyClass> filterCardsToDevice(Collection<MyClass> col) {
    final List<MyClass> newList = new ArrayList<MyClass>();

    for (MyClass myObj : col) {
        long id = myObj.getId();
        if (id < 0 || id > 0xFFFFFFFFl) {
            // just a log here
        } else {
            newList.add(myObj);
        }
    }

    return newList;
}

private final Map<Long, MyClass> map = new HashMap<Long, MyClass>();

public Collection<MyClass> getCollection() {
    synchronized (map) {
        return Collections.unmodifiableCollection(map.values());
    }
}

The stack is:

at java.util.HashMap$HashIterator.nextEntry(HashMap.java:841)                 
at java.util.HashMap$ValueIterator.next(HashMap.java:871)                 
at java.util.Collections$UnmodifiableCollection$1.next(Collections.java:1010)

Exactly on foreach line:

for (MyClass myObj : col) {

I don't see why this error occurs, because I'm not modifying the list.

Answer 1

Be aware, that Collections.unmodifiable* is not copying collection data, but only wrapping the original collection in a special wrapper. So if you modify the original collection, you can get this error.

---

If you want to create really independent unmodifiable collection instance:

Collections.unmodifiableCollection(new ArrayList<>(map.values()));

Answer 2

You must be updating the map in another thread while you are iterating through col. Both map#values and Collections.unmodifiableCollection return views of existing data structures, so what you are iterating over (and this is witnessed by your stacktrace) is the entry set of your map.