Why is there a ConcurrentModificationException even when list is synchronized?

Question

I have Android multi-threading application.

There is some probability that two or more triggers might run the same part of code.

I have a list of objects.

I made it to be synchronized by Collections.synchronizedList

private List<WmGroupItemSample> mGroupItemSampleList;

mGroupItemSampleList = new ArrayList<WmGroupItemSample>();
mGroupItemSampleList = Collections.synchronizedList(mGroupItemSampleList);

However sometimes I get Exception on line:

Collections.sort(mGroupItemSampleList, new GroupItemSampleComparator());

---

java.util.ConcurrentModificationException
       at java.util.AbstractList$SimpleListIterator.next(AbstractList.java:62)
       at java.util.Collections.sort(Collections.java:1895)

• Is this flow legal?
• do I need create the copy and run sort on copy?
• why Collections.synchronizedList doesn't prevent this Exception?

[EDIT]

GroupItemSampleComparator

public class GroupItemSampleComparator implements java.util.Comparator<WmGroupItemSample> {

    public GroupItemSampleComparator() {
        super();        
    }

    public int compare(WmGroupItemSample s1, WmGroupItemSample s2) {
       return ( (s2.getStartDate() - s1.getStartDate()) > 0 ) ? (-1) : (1);
    }
}

Thanks,

Answer 1

Collections.synchronizedList(list) returns a synchronized list which means that the methods of the list will be synchronizd (only one of them can be running at the same time).

This however does not mean you can't call a method of the list while someone else (or maybe you) is iterating over the list using its iterator (iterators returned by iterator() are not synchronized). synchronizedList() does not protect you from getting a ConcurrentModificationException if someone is iterating over the list and it is modified in any other way than the iterator's methods.

Edit:

Also your GroupItemSampleComparator is bad, it must return 0 if the passed 2 objects are considered to be equal by their equals() method. Try this (assuming getStartDate() returns long):

public int compare(WmGroupItemSample s1, WmGroupItemSample s2) {
    long diff = s2.getStartDate() - s1.getStartDate();
    return diff > 0 ? -1 : diff < 0 ? 1 : 0;
}