Mockito Argument Matchers | Java Fresher Interview

Direct Answer

Argument matchers let Mockito accept a group of argument values instead of one exact value. any() matches any reference value, including null; type-specific matchers such as anyString() and any(Order.class) match non-null values of that type. eq(value) matches one value. If one parameter uses a matcher, every parameter in that invocation must use a matcher.

Code Example

@ExtendWith(MockitoExtension.class)
class OrderServiceTest {
    @Mock
    private OrderRepository repository;
    @InjectMocks
    private OrderService service;

    @Test
    void shouldCreateOrderForAnyValidUser() {
        // Stub: accept any Long userId and any Order object
        when(repository.save(any(Order.class)))
            .thenReturn(new Order(1L, "Laptop", 1200.0));

        var result = service.createOrder(42L, "Laptop", 1200.0);

        assertThat(result.id()).isEqualTo(1L);
        verify(repository).save(any(Order.class)); // Verify with matcher
    }

    @Test
    void shouldApplyDiscountForSpecificCategory() {
        // Mix matchers: both arguments must use matchers
        when(repository.findByCategory(eq("Electronics")))
            .thenReturn(List.of(new Order(1L, "Phone", 800.0)));

        var orders = service.getOrdersByCategory("Electronics");

        assertThat(orders).hasSize(1);
    }
}

Common Mistakes & Pitfalls

• Mixing matchers and literals: Writing when(service.process(any(), "ACTIVE")) throws InvalidUseOfMatchersException because the second argument is a bare String literal. Fix: when(service.process(any(), eq("ACTIVE"))).
• Overusing any() in verify: Writing verify(emailService).sendEmail(any(), any(), any()) passes even if wrong values are sent—use specific matchers or argThat when arguments matter.
• Forgetting anyString() versus any(): Using any() when you mean anyString() loses type safety—if the method signature changes to accept Integer, any() still passes but anyString() fails fast, catching the bug.