Unit testing method that uses already tested method

Question

Assume we have class Handler and class Validator. The Handler uses Validator to validate incoming requests.

The Validator class is unit tested, whether returns appropriate error etc.

Later, we want to create unit tests for Handler. The Validator has already unit tests, so how would the test for Handler look?

Will we write the same tests as for Validator whether Handler returns the appropriate error (retrieved from Validator class)? It doesn't make sense to me.

So how would the unit test for Handler class look in this case?

Answer 1

You shouldn't re-test the Validator's logic in the Handler's unit test.

The idea of unit (as opposed to component, integration, or end-to-end tests) is to isolate the logic of the minimal unit (usually a class) you're testing.

The textbook solution for this would be to mock the Validator and then test that the Handler properly reacts to different behaviors the Validator may have.

You didn't tag the question with any programming language, but any half-decent language should have some library to allow mocking. E.g., here's how I'd do it in java using mockito and junit:

// Imports snipped for brevity's sake
@ExtendWith(MockitoExtension.class)
public class HandlerTest {
    // Create a mock Validator
    @Mock
    private Validator validator;

    private Handler handler;

    // Set up the handler being tested with said validator
    @BeforeEach
    public void setUp() {
        handler = new Handler(validator);
    }

    @Test
    void handlePassedValidation() {
        // Mock the validator with a "passing" validation
        when(validator.isValid()).thenReturn(true);
      
        handler.handle();

        // Assert the handler did what it should when the validation passed
    }

    @Test
    void handleFailedValidation() {
        // Mock the validator with a "failing" validation
        when(validator.isValid()).thenReturn(false);
      
        handler.handle();

        // Assert the handler did what it should when the validation failed
    }
}