When using Java Iterator with a Java collection component in thread-safe way, we have to synchronize that component, not the iterator itslef. example: Map m = Collections.synchronizedMap(new HashMap()); ... Set s = m.keySet(); // Needn't be in synchronized block ... synchronized(m) { // Synchronizing on m, not s! Iterator i = s.iterator(); // Must be in synchronized block while (i.hasNext()) foo(i.next()); }