@Test public void test_findPriceInUSD() { Shop s1 = shops.get(0); String product = "MacBook 11"; Future<Double> priceInUSD = CompletableFuture.supplyAsync(() -> s.getPrice(product)) .thenCombine( CompletableFuture.supplyAsync(() -> ExchangeService.getRate(USD, FRANC)), (price, rate) -> price * rate); }