Verifying Interactions

For methods that return nothing (sending email, publishing an event), check that the mock was called correctly with verify.

@Test
void sendsConfirmationEmailOnce() {
    when(productRepository.findById(1L)).thenReturn(Optional.of(phone));
    when(paymentGateway.charge(any(), any())).thenReturn(PaymentStatus.SUCCESS);

    orderService.placeOrder("ravi", 1L, 1);

    verify(emailSender, times(1)).send(eq("ravi"), contains("Order confirmed"));
    verify(productRepository).save(phone);                 // times(1) is the default
    verifyNoMoreInteractions(emailSender);
}

@Test
void noEmailWhenPaymentFails() {
    when(productRepository.findById(1L)).thenReturn(Optional.of(phone));
    when(paymentGateway.charge(any(), any())).thenReturn(PaymentStatus.DECLINED);

    orderService.placeOrder("ravi", 1L, 1);

    verify(emailSender, never()).send(any(), any());
}

@Captor ArgumentCaptor<OrderPlacedEvent> eventCaptor;

@Test
void publishesEventWithCorrectTotal() {
    // ... arrange stubs ...
    orderService.placeOrder("ravi", 1L, 2);

    verify(eventPublisher).publish(eventCaptor.capture());
    OrderPlacedEvent event = eventCaptor.getValue();
    assertEquals(new BigDecimal("31998"), event.total());
    assertEquals("ravi", event.customer());
}