/** Disable/enable the move-to-newer/older buttons. */
 @Override
 protected void updateNavigationArrows() {
   final MessageOrderManager orderManager = getMessageOrderManager();
   if (orderManager == null) {
     // shouldn't happen, but just in case
     mMessageCommandButtons.enableNavigationButtons(false, false, 0, 0);
   } else {
     mMessageCommandButtons.enableNavigationButtons(
         orderManager.canMoveToNewer(), orderManager.canMoveToOlder(),
         orderManager.getCurrentPosition(), orderManager.getTotalMessageCount());
   }
 }