How do you mock objects in Mockito when using constructor injection?

Question

Accepted Answer

Constructor injection makes mocking straightforward in Mockito: you create mocks for each dependency, then pass them directly into the constructor of the class under test.

Key Points:
• Use mock(Dependency.class) for each collaborator the class needs.
• Instantiate the class under test by calling its constructor with the mocks, e.g. new OrderService(mockRepo, mockMailer).
• With @Mock and @InjectMocks plus @ExtendWith(MockitoExtension.class), Mockito can wire constructor mocks automatically.
• This approach forces the class to depend on abstractions passed in, which is what makes it easily testable in isolation.
• No reflection or field hacking is needed, unlike mocking dependencies injected via field or setter injection.

Example:
For a class OrderService(PaymentGateway gateway, InventoryClient inventory), a test simply builds mock(PaymentGateway.class) and mock(InventoryClient.class) and constructs OrderService with them before stubbing behavior.

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

@Mock
    private PaymentGateway gateway;

@Mock
    private InventoryClient inventory;

@InjectMocks
    private OrderService orderService;

@Test
    void placesOrderWhenPaymentSucceeds() {
        when(gateway.charge(anyDouble())).thenReturn(true);
        assertTrue(orderService.placeOrder("SKU-1", 2));
    }
}
```

Interview Tip:
A concise interview answer is:

"With constructor injection I mock each dependency with mock(), then construct the class under test with those mocks directly, or let @Mock plus @InjectMocks wire them automatically. It's the cleanest form of dependency injection to test because there's no field reflection involved."