How do you combine JUnit and Mockito to write comprehensive unit tests?

Question

Accepted Answer

JUnit and Mockito work together by letting JUnit drive test structure and lifecycle while Mockito supplies and controls the mock dependencies each test needs.

Key Points:
• Annotate the test class with @ExtendWith(MockitoExtension.class) so JUnit 5 initializes Mockito annotations automatically.
• Use @Mock to create dependency mocks and @InjectMocks to wire them into the class under test.
• Stub behavior with when()/thenReturn() before invoking the method under test.
• Use JUnit's assertions (assertEquals, assertThrows, etc.) to verify return values, and Mockito's verify() to verify interactions.
• Keep each @Test method focused on one scenario - one stubbing setup, one action, one set of assertions.

Example:
A test for OrderService.placeOrder() might mock PaymentGateway and InventoryClient, stub them to simulate a successful payment, call placeOrder(), then assert the returned order status with JUnit and verify the payment was charged with Mockito.

Code Example:
```
@ExtendWith(MockitoExtension.class)
class OrderServiceTest {

@Mock private PaymentGateway gateway;
    @InjectMocks private OrderService orderService;

@Test
    void confirmsOrderWhenPaymentSucceeds() {
        when(gateway.charge(50.0)).thenReturn(true);

assertTrue(orderService.placeOrder(50.0));
        verify(gateway).charge(50.0);
    }
}
```

Interview Tip:
A concise interview answer is:

"JUnit provides the test runner, lifecycle annotations, and assertions, while Mockito supplies mock dependencies via @Mock and @InjectMocks. I stub behavior with when()/thenReturn(), exercise the method under test, then combine JUnit assertions on the result with Mockito's verify() on interactions."