Is it possible to use ArgumentCaptor with Mockito's when() method

Overview of ArgumentCaptor

`ArgumentCaptor` is a feature in Mockito that allows you to capture arguments passed to a method call on a mock object. It is primarily used during the verification phase of a test, where you want to check if a method was called with specific arguments. This is typically done using `Mockito.verify()`.

Using ArgumentCaptor with Mockito.verify()

When using `ArgumentCaptor` with `Mockito.verify()`, you capture the arguments passed to a method after it has been called. This allows you to make assertions about those arguments. Here's an example:

java
@Mock
private PersonRepository personRepository;

@Captor
private ArgumentCaptor captor;

@Test
public void shouldCapture() {
    Person person = new Person("test");
    personService.delete(person);
    Mockito.verify(personRepository).delete(captor.capture());
    Person captured = captor.getValue();
    Assertions.assertThat(captured.getName()).isEqualTo("deleted");
}

Attempting to Use ArgumentCaptor with Mockito.when()

While it's technically possible to use `ArgumentCaptor` with `Mockito.when()`, it's not recommended because it doesn't provide any additional value over using matchers like `Mockito.any()` or `Mockito.eq()`. Here's why:

1. Lack of Clarity: Using `ArgumentCaptor.capture()` in stubbing makes the test less readable. It's unclear what arguments are being matched without additional assertions.

2. Undefined Behavior: During stubbing, there are no arguments to capture yet, so using `ArgumentCaptor.capture()` can lead to undefined behavior.

3. Extra Assertions: You would need additional assertions to verify the captured arguments, which is unnecessary compared to using matchers directly.

Here's an example of how you might attempt to use it, though it's not recommended:

java
// Not recommended
Mockito.when(personRepository.delete(captor.capture())).thenReturn(null);

This approach is not practical because you cannot directly return a value based on the captured argument without additional logic, which defeats the purpose of using `when()` for stubbing.

Conclusion

While technically possible, using `ArgumentCaptor` with `Mockito.when()` is not recommended due to its lack of clarity and potential for undefined behavior. Instead, use matchers like `Mockito.any()` or `Mockito.eq()` for stubbing, and reserve `ArgumentCaptor` for verification phases where you need to capture and assert arguments passed to methods[2][5].