Well I thought Vectors were synchronized. The Object is, but Iterators are not.
From the Java Doc
” The Iterators returned by Vector’s iterator and listIterator methods are fail-fast: if the Vector is structurally modified at any time after the Iterator is created, in any way except through the Iterator’s own remove or add methods, the Iterator will throw a ConcurrentModificationException. Thus, in the face of concurrent modification, the Iterator fails quickly and cleanly, rather than risking arbitrary, non-deterministic behavior at an undetermined time in the future. The Enumerations returned by Vector’s elements method are not fail-fast. “
Hence, create an iterator on a vector and you will get a ConcurrentModificationException on the iterator if the vector is modified, even though the Vector itself is Synchronized.