MockistTDD.java

class Endpoint {
    private final UserInformationService userInformationService;
    private final ResponseFactory responseFactory;

    Endpoint(UserInformationService userInformationService, ResponseFactory responseFactory) {
        this.userInformationService = userInformationService;
        this.responseFactory = responseFactory;
    }

    public Response getUsername(int id) {
        try {
            String username = userInformationService.getUsername(id);
            return responseFactory.createOk(username);
        }
        catch (NotFoundException ex) {
            return responseFactory.createNotFound();
        }
    }
}

// This test is written using mockito, and the creation of mock object was omitted to keep it brief.

private final String A_USERNAME = "Bobby";
private final int USER_ID = 1337;

@Test
public void getUsername_delegatesToUserInformationService() {
    endpoint.getUsername(USER_ID);
    
    verify(userInformationService).getUsername(USER_ID);
}

@Test
public void getUsername_returnsOkWhenUserIsFound() {
    Response OK_RESPONSE = new OkResponse();
    given(responseFactory.createOk(A_USERNAME)).willReturn(OK_RESPONSE);
    given(userInformationService.getUsername(USER_ID)).willReturn(A_USERNAME);
    
    Response result = endpoint.getUsername(USER_ID);
    
    assertThat(result).isSameAs(OK_RESPONSE);
}

@Test
public void getUser_returnsNotFoundWhenUserIsNotFound() {
    Response NOT_FOUND_RESPONSE = new NotFoundResponse();
    given(responseFactory.createNotFound()).willReturn(NOT_FOUND_RESPONSE);
    given(userInformationService.getUsername(USER_ID)).willThrow(NotFoundException.class);
    
    Response result = endpoint.getUsername(USER_ID);
    
    assertThat(result).isSameAs(NOT_FOUND_RESPONSE);
}

@olafleur can you fix this snippet and put the fields of Endpoint private final

Also, I wonder if the test code would be cleaner if we'd group the given parts together. Making a clear given/when/then separation

-- thanks

Done.