private void errorStateChange(final FragmentState current, final FragmentState target) {
   final String msg = "%s: Invalid state transition %s --> %s";
   throw new StateTransitionException(
       String.format(msg, fragmentName, current.name(), target.name()));
 }
 private void warnStateChange(final FragmentState current, final FragmentState target) {
   logger.warn(
       fragmentName + ": Ignoring unexpected state transition {} --> {}",
       current.name(),
       target.name());
 }