Creating Mocks and Stubbing

Mockito in a JUnit test

Mockito creates mock objects of interfaces and classes. Create them with mock(Type.class), or declaratively with @Mock fields in a test class annotated @ExtendWith(MockitoExtension.class), which also enables strict stubs. @InjectMocks creates the class under test and injects the mocks through its constructor (or setters and fields), although many teams prefer creating the object explicitly with new for clarity. Unstubbed methods return default values: null for objects, 0 for numbers, false, empty collections and Optional.empty(). Stub behaviour with when(mock.method(args)).thenReturn(value), chain several values (thenReturn(a, b)) for consecutive calls, or thenThrow(exception) to simulate failures. Since Mockito 5, the inline mock maker is the default, so final classes and methods can be mocked too. With MockitoExtension's strictness, a stub that the test never uses raises UnnecessaryStubbingException, keeping tests clean and catching copy-paste mistakes.

A service test with mocks

Mocks supply data and simulate a failure; the test checks the result.

@ExtendWith(MockitoExtension.class)
class PriceQuoteServiceTest {

    @Mock ProductCatalog catalog;
    @Mock TaxService tax;

    PriceQuoteService service;

    @BeforeEach
    void setUp() {
        service = new PriceQuoteService(catalog, tax);    // explicit construction
    }

    @Test
    void quoteAddsTax() {
        when(catalog.priceOf("pen")).thenReturn(new BigDecimal("100.00"));
        when(tax.rateFor("pen")).thenReturn(new BigDecimal("0.18"));

        Quote q = service.quote("pen", 2);

        assertThat(q.total()).isEqualByComparingTo("236.00");
    }

    @Test
    void unknownProductIsReported() {
        when(catalog.priceOf("ghost")).thenThrow(new ProductNotFoundException("ghost"));

        assertThatThrownBy(() -> service.quote("ghost", 1))
            .isInstanceOf(ProductNotFoundException.class);
    }
}

An actor reading lines

A mock is an understudy actor who knows only the lines you hand them: ask about the price of a pen and they say "100". Ask anything else and they say nothing useful, which is exactly what you want in a controlled rehearsal.

त्वरित जाँच: What does an unstubbed Mockito mock method returning List<String> return by default?

• null
• An empty list
• A list with one element
• It throws an exception