How do you assert with delete method void - spring-boot

Question

I'm unit testing a Spring boot web app with Mockito. One of my methods is returning a void, but if I try to test it, I get compilation errors.
This is the test I wrote:
public void testDeleteActor()throws NetflixException {
when(actorRepository.findById(1L)).thenReturn(Optional.of(Mockito.any(Actor.class)));
assertEquals(null, service.deleteActorById(Mockito.anyLong());
}
And this is the method I'm trying to test:
#Override
public void deleteActorById(Long id) throws NetflixException {
Actor actor = actorRepository
.findById(id)
.orElseThrow(() -> new NotFoundException("Actor id not found - " + id));
actorRepository.delete(actor);
}
As you can see in the following screenshot, I'm getting an error with my assertEquals() statement:

Answer 1

In your code, your method isn't actually returning null, it's returning nothing (void). That means that you can't write assertions based on what the method returns. This is the reason why the assertEquals() statement is giving you an error.
In stead of testing what the method returns, you can test the expected behaviour of the method. In this example, there are three things we expect:
The method should retrieve the actor by its ID.
The method should throw an exception if no actor was found with the given ID.
The method should delete the actor if it was found.
To implement this these tests, you can use Mockito's verify() and AssertJ's assertThatExceptionOfType(). For example:
#Test
void deleteActorById_retrievesActorByID() {
Actor actor = new Actor();
when(actorRepository.findById(1L)).thenReturn(Optional.of(actor));
service.deleteActorById(1L);
verify(repository).findById(1L);
}
#Test
void deleteActorById_throwsExceptionIfIDNotFound() {
assertThatExceptionOfType(NotFoundException.class)
.isThrownBy(() -> service.deleteActorById(1L))
.withMessage("Actor id not found - 1");
}
#Test
void deleteActorById_deletesActorIfIDFound() {
Actor actor = new Actor();
when(actorRepository.findById(1L)).thenReturn(Optional.of(actor));
service.deleteActorById(1L);
verify(actorRepository).delete(actor);
}