@Override
 public void clear() {
   write.lock();
   try {
     super.clear();
   } finally {
     write.unlock();
   }
 }
 @Override
 public void putAll(Map<? extends K, ? extends C> m) {
   write.lock();
   try {
     super.putAll(m);
   } finally {
     write.unlock();
   }
 }