### Question:
PaymentService { public Flux<Payment> findPayments(Flux<String> userIds) { return userIds.flatMap(repository::findAllByUserId); } PaymentService(); Flux<Payment> findPayments(Flux<String> userIds);  }### Answer:
@Test public void findPayments() { PaymentService service = new PaymentService(); StepVerifier.create(service.findPayments(Flux.range(1, 100) .map(String::valueOf)) .then()) .expectSubscription() .verifyComplete(); }