private void validateCompletableReturnType ( Method commandMethod , Class < ? > callbackReturnType ) { if ( Void . TYPE == callbackReturnType ) { throw new FallbackDefinitionException ( createErrorMsg ( commandMethod , method , "fallback cannot return 'void' if command return type is " + Completable . class . getSimpleName ( ) ) ) ; } }
everything can be wrapped into completable except void