Stubbing Void Methods

Direct Answer

Void methods use Mockito's do...when family because when(...).thenReturn(...) needs a value expression. An unstubbed void method on a normal mock already does nothing. Use doThrow to simulate failure, doAnswer for controlled behavior, and doNothing mainly to suppress a real spy method or make a consecutive sequence explicit.

Code Example

@ExtendWith(MockitoExtension.class)
class NotificationServiceTest {
    @Mock
    private EmailSender emailSender;
    @InjectMocks
    private NotificationService notificationService;

    @Test
    void testSuccessfulNotification() {
        // Stub void method to do nothing
        doNothing().when(emailSender).sendEmail(anyString(), anyString());
        
        notificationService.notifyUser("user@example.com", "Welcome!");
        
        verify(emailSender).sendEmail("user@example.com", "Welcome!");
    }

    @Test
    void testNotificationFailure() {
        // Stub void method to throw exception
        doThrow(new EmailServiceException("SMTP failure"))
            .when(emailSender).sendEmail(anyString(), anyString());
        
        assertThrows(EmailServiceException.class, () -> 
            notificationService.notifyUser("user@example.com", "Alert"));
    }
}

Common Mistakes & Pitfalls

• Using when().thenReturn() on void methods: Freshers often try when(mock.voidMethod()).thenReturn() which won't compile because void has no return type. The compiler error "void type not allowed here" confuses beginners who don't realize void methods need the do-family API.
• Forgetting doNothing() is the default: Many write doNothing() explicitly when it's unnecessary — mocked void methods do nothing by default. You only need doNothing() for readability or when chaining multiple stubbings.
• Not using doThrow() for exception testing: Freshers sometimes try to make void methods throw exceptions using when-syntax, missing that doThrow() is the correct approach for testing error handling in void method callers.