Mockito mock same method calls with different collection-arguments

I try to mock same method calls with different collection-arguments.

My problem is that im not getting the correct mocked-answer from Mocked-Call for the input.

Test-Class:

@ExtendWith(SpringExtension.class)
public class CollectionTest {

	@MockBean
	private Controller c;
	
	@BeforeEach
	public void init() {
		Collection<String> a = Mockito.anyCollection();
		a.add("a");
		Mockito.when(c.run(a)).thenReturn("a");
		
		Collection<String> b = Mockito.anyCollection();
		b.add("b");
		Mockito.when(c.run(b)).thenReturn("b");
	}

	@Test
	public void test() {
		assertEquals("a", c.run(Lists.newArrayList("a"))); // DOESNT'WORK!!! Returns "b" but should "a"
		assertEquals("b", c.run(Lists.newArrayList("b"))); // 
	}
}

Controller-Class:

@Service
public class Controller{
	public String run(Collection<String> c) {
		return "not-mocked";
	}	
}

I'v got no idea why it doesn't return "a". I tried to change the collection to string but same behaviour.

What are the Steps to do, to get the following behaviour?

@Test
public void test() {
	assertEquals("a", c.run(Lists.newArrayList("a"))); // should return "a"
	assertEquals("b", c.run(Lists.newArrayList("b"))); // should return "b"
}

Im using Java Mockito "3.1" and Spring, but I think Mockito is the important information here.

Your second call - Mockito.when(c.run(b)).thenReturn("b");
is overruling our first call so Mockito will therefore always return "b".
If you need multiple answers from the same call, you can use the varags variant:

when(c.run(anyCollection())).thenReturn("a", "b");

Now the first call to the controller's run method will return "a" and all subsequent calls will return "b". You can provide as many return results as you want and the last one will be repeated from then on as the answer.

Write two tests will show you the results you are expecting.
You are adding to the same Controller two different results so you get only the last one : Mockito.when(c.run(b)).thenReturn("b");
Normal. The last mocked expected result in your setUp() will stay in memory.

Previous answer was :
You can use something like junit and mockito to test your spring-web-mvc application.
It looks like that :

@WebMvcTest(controllers = UserController.class)
@ActiveProfiles("test")
class UserControllerTest {
   
    @Autowired                           
    private MockMvc mockMvc;  
                                                 
    @MockBean                           
    private UserService userService; 
                                               
    private List<User> userList;       
                                            
    @BeforeEach                           
    void setUp() {                               
       this.userList = new ArrayList<>();
       this.userList.add(new User(1L, "user1@gmail.com", "pwd1","User1"));
       this.userList.add(new User(2L, "user2@gmail.com", "pwd2","User2"));
       this.userList.add(new User(3L, "user3@gmail.com", "pwd3","User3"));                                                       
    }
}

And as an example :

@Test
void shouldFetchAllUsers() throws Exception {
	given(userService.findAllUsers()).willReturn(userList);
	this.mockMvc.perform(get("/api/users"))
		.andExpect(status().isOk())
		.andExpect(jsonPath("$.size()", is(userList.size() )));
}

Example from @see https://medium.com/backend-habit/integrate-junit-and-mockito-unit-testing-for-controller-layer-91bb4099c2a5