Can ArgumentCaptor be used for both stubbing and verification in Mockito

Verification

`ArgumentCaptor` is most effectively used during the verification phase of a test. It allows you to capture the arguments passed to a mocked method and then assert that these arguments match your expectations. This is particularly useful when you need to verify that a method was called with specific arguments.

Here's an example of using `ArgumentCaptor` for verification:

java
@Test
public void shouldCapture() {
    // Create a mock object
    PersonRepository personRepository = mock(PersonRepository.class);
    
    // Create an ArgumentCaptor
    ArgumentCaptor captor = ArgumentCaptor.forClass(Person.class);
    
    // Call the method on the service that uses the mock
    PersonService personService = new PersonService(personRepository);
    personService.delete(new Person("John"));
    
    // Verify that the method was called with the captured argument
    verify(personRepository).delete(captor.capture());
    
    // Assert the captured argument
    Person captured = captor.getValue();
    assertEquals("John", captured.getName());
}

Stubbing

Stubbing involves setting up a mock object to return a specific value when a method is called. While you can technically use `ArgumentCaptor` with stubbing, it is not recommended because it can decrease test readability and may lead to unclear intent in your test code.

For example, using `ArgumentCaptor` with stubbing might look like this:

java
// Not recommended approach
@Test
public void shouldStub() {
    UserRegistrationService userRegistrationService = mock(UserRegistrationService.class);
    ArgumentCaptor captor = ArgumentCaptor.forClass(UserRegistration.class);
    
    // Stubbing with ArgumentCaptor
    when(userRegistrationService.registerUser(captor.capture())).thenReturn(new UserRegistration("Jammie", "[email protected]", 23));
    
    // Call the method
    UserRegistration registration = new UserRegistration("Jammie", "[email protected]", 23);
    userRegistrationService.registerUser(registration);
    
    // Assertion
    assertEquals(registration, captor.getValue());
}

However, this approach is less clear and less efficient than using matchers directly in the stubbing setup. For instance, you could use `Mockito.eq()` to match the exact argument:

java
// Preferred approach for stubbing
@Test
public void shouldStubPreferred() {
    UserRegistrationService userRegistrationService = mock(UserRegistrationService.class);
    UserRegistration registration = new UserRegistration("Jammie", "[email protected]", 23);
    
    // Stubbing with Mockito.eq()
    when(userRegistrationService.registerUser(Mockito.eq(registration))).thenReturn(registration);
    
    // Call the method
    userRegistrationService.registerUser(registration);
}

Conclusion

While `ArgumentCaptor` can be used for both stubbing and verification, its primary and recommended use is for verification. Using it for stubbing can lead to less readable tests and is generally discouraged in favor of using matchers like `Mockito.eq()` or `Mockito.any()` for stubbing purposes.