… While locking can prevent iterators from throwing ConcurrentMofdificationException, You have to remember to use locking everywhere a shared collection might be iterated. This is trickier than it sounds …

Brian Goetz p.83-84

Question: The following code could throw ConcurrentMofdificationException, even though add() is synchronized, why?

public class HiddenIterator  {
     private final Set set = new HashSet();

public synchronized void add(Integer i) {
     set.add(i);
}

public synchronized void remove(Integer i) {
     set.remove(i);
}

public void addTenThings() {
   Random r = new Random();
   for (int index = 0; index < 10;  index++;) {
       add(r.nextInt());
   }

   System.out.println("Added ten elements to set: " + set);
}
}

Looking forward for your answers dear readers