Question 1

What is the difference between @Mock and @MockBean in Spring context?

Accepted Answer

The 30-Second Answer: @Mock is a Mockito annotation that creates a mock object without Spring involvement, used in lightweight unit tests. @MockBean is a Spring Boot annotation that creates a mock and registers it in the Spring application context, replacing any existing bean. The key difference is Spring context integration: @Mock works independently, while @MockBean integrates with Spring's dependency injection.

Question 2

What is the difference between a mock and a stub?

Accepted Answer

The 30-Second Answer: A stub provides predefined responses to method calls during tests, while a mock goes further by also recording interactions and allowing you to verify that specific methods were called with expected arguments. Stubs focus on state verification, mocks focus on behavior verification. The 2-Minute Answer (If They Want More): The distinction between mocks and stubs comes from Martin Fowler's classification of test doubles.

Question 3

What is the difference between a mock, stub, spy, and fake?

Accepted Answer

The 30-Second Answer: These are all types of test doubles: a stub returns predefined values, a mock records interactions for verification, a spy wraps a real object allowing partial mocking, and a fake is a lightweight working implementation. Each serves different testing needs based on whether you need behavior verification, partial real functionality, or simplified implementations.

Question 4

What is the difference between mock() and @Mock annotation?

Accepted Answer

The 30-Second Answer: Both mock() and @Mock create mock objects, but mock() is a method call that creates mocks programmatically within the test method, while @Mock is a declarative annotation on fields that requires initialization via MockitoExtension or MockitoAnnotations.openMocks() . The annotation approach is cleaner for multiple mocks and better suited for modern testing practices.

Question 5

What is the difference between thenReturn() and thenAnswer()?

Accepted Answer

The 30-Second Answer: thenReturn() returns a fixed value immediately, while thenAnswer() executes custom logic each time the method is called. Use thenReturn() for simple static values and thenAnswer() when you need dynamic behavior, access to method arguments, or need to compute the return value based on the invocation context. The 2-Minute Answer (If They Want More): The key difference between thenReturn() and thenAnswer() lies in when and how the return value is determined.

Question 6

What is the difference between doReturn(), doThrow(), doAnswer(), and doNothing()?

Accepted Answer

The 30-Second Answer: These are the "do-family" methods in Mockito that provide alternative stubbing syntax: doReturn() stubs a return value, doThrow() stubs an exception, doAnswer() executes custom logic, and doNothing() explicitly stubs void methods to do nothing. All use the syntax doX().when(mock).method() and are particularly useful for spies and void methods.

Question 7

What is the difference between @Mock and @Spy?

Accepted Answer

The 30-Second Answer: @Mock creates a bare mock with no real behavior - all methods return default values unless stubbed. @Spy wraps a real instance and calls actual methods by default unless specifically stubbed. Use @Mock when you want complete control over behavior; use @Spy when you want mostly real behavior with selective overrides. The 2-Minute Answer (If They Want More): The fundamental difference lies in their default behavior.

Question 8

What is the mockito-extensions/org.mockito.plugins.MockMaker file?

Accepted Answer

The 30-Second Answer: The mockito-extensions/org.mockito.plugins.MockMaker file is a configuration file placed in src/test/resources/ that tells Mockito which mock creation strategy to use. Setting its content to mock-maker-inline enables mocking of final classes and methods without adding the mockito-inline dependency.

Question 9

What is @MockBean and how does it differ from @Mock?

Accepted Answer

The 30-Second Answer: @MockBean is a Spring Boot annotation that creates a Mockito mock and registers it in the Spring application context, replacing any existing bean of the same type. @Mock is a pure Mockito annotation that creates a mock but doesn't interact with the Spring context. I use @MockBean in Spring integration tests and @Mock in plain unit tests.

Question 10

What is @SpyBean and when would you use it?

Accepted Answer

The 30-Second Answer: @SpyBean creates a Mockito spy of an existing Spring bean, allowing me to use the real implementation while selectively stubbing or verifying specific methods. I use it when I need to test with mostly real behavior but want to override certain methods, verify method calls, or avoid side effects like sending emails or external API calls.

Mocking Final Classes and Methods(4 questions)

How do you mock final classes and methods in Mockito?

What is the mockito-extensions/org.mockito.plugins.MockMaker file?

What are the limitations of mocking final classes?

When should you avoid mocking final classes?

Testing with Spring and Mockito(6 questions)

How do you use Mockito with Spring Boot tests?

What is @MockBean and how does it differ from @Mock?

What is @SpyBean and when would you use it?

How do you use @WebMvcTest with mocked services?

What is the difference between @Mock and @MockBean in Spring context?

How do you test Spring components with constructor injection and mocks?

Advanced Mockito Features(8 questions)

What is BDDMockito and given().willReturn() syntax?

How do you reset mocks with Mockito.reset()?

What is Mockito.lenient() and when would you use it?

How do you handle deep stubs with RETURNS_DEEP_STUBS?

What is RETURNS_SMART_NULLS?

How do you mock methods with generic return types?

What is Answer interface and how do you implement custom answers?

How do you use Mockito with Kotlin?

Mockito Fundamentals(10 questions)

What is Mockito and why is it used for testing?

What is the difference between a mock and a stub?

What is the difference between a mock, stub, spy, and fake?

How do you create a mock object with Mockito?

What is the @Mock annotation and how do you use it?

What is @InjectMocks and how does it work?

How do you initialize mocks with MockitoAnnotations or @ExtendWith(MockitoExtension.class)?

What is the difference between mock() and @Mock annotation?

How do you verify that a method was called on a mock?

What is Mockito.verify() and its verification modes?

Stubbing with Mockito(12 questions)

What is stubbing in Mockito?

How do you use when().thenReturn() for stubbing?

What is the difference between thenReturn() and thenAnswer()?

How do you stub methods that throw exceptions with thenThrow()?

What is doReturn() and when would you use it instead of when().thenReturn()?

What is the difference between doReturn(), doThrow(), doAnswer(), and doNothing()?

How do you stub void methods in Mockito?

How do you stub consecutive calls with different return values?

What is argument matching in Mockito?

How do you use argument matchers like any(), eq(), anyString(), anyInt()?

What is the rule for mixing argument matchers with raw values?

How do you create custom argument matchers with argThat()?

Mockito Spies(7 questions)

What is a spy in Mockito?

What is the difference between @Mock and @Spy?

When would you use a spy instead of a mock?

How do you create a spy with Mockito.spy()?

How do you stub specific methods on a spy while keeping real behavior for others?

What are the pitfalls of using spies?

How do you use doReturn() with spies?

Mockito Verification(8 questions)

How do you verify method invocations with verify()?

What are the verification modes (times(), never(), atLeast(), atMost())?

How do you use verifyNoMoreInteractions()?

What is verifyNoInteractions() and when would you use it?

How do you verify the order of method calls with InOrder?

How do you capture arguments passed to mock methods with ArgumentCaptor?

What is @Captor annotation?

How do you verify timeout for async operations?

Mocking Static Methods and Constructors(6 questions)

How do you mock static methods in Mockito 3.4+?

What is MockedStatic and how do you use it?

How do you mock constructors with Mockito?

What is MockedConstruction and when would you use it?

What are the alternatives to mocking static methods (refactoring approaches)?

Why should you avoid mocking static methods when possible?