Handling the end of persistence

Main7.java

persistSales(sales, t -> {
    if(t != null){
        LOGGER.error("failed to persist sales: " + sales, t);
    }else{
        LOGGER.info("persisting completed, notifying involved parties...");
        sales.stream().forEach(sale -> {
            if (sale.getBuyer().listener != null)
                sale.getBuyer().listener.onEvent(EventType.PURCHASE, sale);
            if (sale.getSeller().listener != null)
                sale.getSeller().listener.onEvent(EventType.SALE, sale);
        });
        ...
    }
    listener.onEvent(EventType.STOPPED, null);
});