@Mock annotation in JUnit - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a mock object using the @Mock annotation.

JUnit

public class UserServiceTest {
    @[1]
    private UserRepository userRepository;
}

Drag options to blanks, or click blank then click option'

AMock

BInjectMocks

CAutowired

DTest

Attempts:

3 left

💡 Hint

Common Mistakes

Using @InjectMocks instead of @Mock

Forgetting the '@' symbol

Using @Test annotation here

The @Mock annotation is used to create a mock object for the UserRepository in the test class.

2fill in blank

medium

Complete the code to initialize mocks before running tests.

JUnit

@BeforeEach
public void setup() {
    Mockito[1](this);
}

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AcreateMocks

BinitMocks

CstartMocks

DopenMocks

Attempts:

3 left

💡 Hint

Common Mistakes

Using deprecated initMocks method

Misspelling the method name

Not calling any initialization method

Mockito.openMocks(this) initializes the mock objects annotated with @Mock before each test.

3fill in blank

hard

Fix the error in the test method to use the mock correctly.

JUnit

@Test
public void testFindUser() {
    when(userRepository.findById([1])).thenReturn(Optional.of(new User()));
    User user = userService.findUserById(1);
    assertNotNull(user);
}

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A1

BanyInt()

C"1"

Dnull

Attempts:

3 left

💡 Hint

Common Mistakes

Using a raw value '1' causing mismatch if argument differs

Using a string instead of integer

Passing null as argument

Using anyInt() matcher allows the mock to respond to any integer argument, avoiding strict matching errors.

4fill in blank

hard

Fill both blanks to correctly verify the mock interaction.

JUnit

@Test
public void testSaveUser() {
    User user = new User();
    userService.saveUser(user);
    verify(userRepository, [1]).save([2]);
}

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Atimes(1)

Bany()

Cnever()

Deq(user)

Attempts:

3 left

💡 Hint

Common Mistakes

Using never() instead of times(1)

Using any() instead of eq(user)

Omitting verify call

verify(userRepository, times(1)).save(eq(user)) checks that save was called once with the exact user object.

5fill in blank

hard

Fill all three blanks to create a mock and inject it into the service under test.

JUnit

public class OrderServiceTest {
    @[1]
    private OrderRepository orderRepository;

    @[2]
    private OrderService orderService;

    @BeforeEach
    public void init() {
        Mockito.[3](this);
    }
}

Drag options to blanks, or click blank then click option'

AMock

BInjectMocks

CopenMocks

DBefore

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing @Mock and @InjectMocks annotations

Using deprecated initMocks method

Forgetting to initialize mocks

Use @Mock for the repository, @InjectMocks for the service, and Mockito.openMocks(this) to initialize mocks.

Practice

1. What is the main purpose of the @Mock annotation in JUnit testing?

easy

A. To create fake objects that simulate real dependencies for testing

B. To run tests in parallel threads

C. To generate test reports automatically

D. To connect to a real database during tests

Solution

1. Step 1: Understand the role of @Mock

   The @Mock annotation is used to create fake objects that imitate real dependencies in tests.

2. Step 2: Identify the purpose in testing context

   This helps isolate the code being tested from external systems or complex objects.

3. Final Answer:

   To create fake objects that simulate real dependencies for testing -> Option A

4. Quick Check:

   @Mock creates fake objects [OK]

Hint: Mocks create fake objects to isolate tests [OK]

Common Mistakes:

• Thinking @Mock runs tests in parallel
• Confusing @Mock with test report generation
• Assuming @Mock connects to real databases

2. Which of the following is the correct way to declare a mock object using @Mock in a JUnit 5 test class?

easy

A. @Mock private MyService service;

B. @Mock MyService service = new MyService();

C. @Mock public void service();

D. @Mock MyService service = null;

Solution

1. Step 1: Check correct syntax for @Mock field declaration

   The mock object should be declared as a private field with the @Mock annotation and no initialization.

2. Step 2: Identify the correct option

   @Mock private MyService service; correctly declares: @Mock private MyService service; without initializing it.

3. Final Answer:

   @Mock private MyService service; -> Option A

4. Quick Check:

   Declare mock as private field without initialization [OK]

Hint: Declare mocks as private fields without new keyword [OK]

Common Mistakes:

• Initializing mock objects manually
• Using @Mock on methods instead of fields
• Declaring mocks as public or with invalid syntax

3. Given the following JUnit 5 test code snippet, what will be the output of service.getData() when called inside the test?

import static org.mockito.Mockito.*;
import org.junit.jupiter.api.Test;
import org.mockito.Mock;
import org.mockito.MockitoAnnotations;

class MyServiceTest {
  @Mock
  MyService service;

  @Test
  void testGetData() {
    MockitoAnnotations.openMocks(this);
    when(service.getData()).thenReturn("mocked data");
    System.out.println(service.getData());
  }
}

medium

A. Original data from MyService

B. null

C. Throws NullPointerException

D. mocked data

Solution

1. Step 1: Understand mock initialization

   The call to MockitoAnnotations.openMocks(this) initializes the @Mock annotated fields.

2. Step 2: Analyze the stubbed method behavior

   The line when(service.getData()).thenReturn("mocked data") sets the mock to return "mocked data" when getData() is called.

3. Final Answer:

   mocked data -> Option D

4. Quick Check:

   Stubbed mock method returns set value [OK]

Hint: Initialize mocks and stub methods to control output [OK]

Common Mistakes:

• Forgetting to initialize mocks before use
• Expecting real method output instead of stubbed
• Confusing null return with stubbed return

4. Identify the error in the following JUnit 5 test code using @Mock:

import org.junit.jupiter.api.Test;
import org.mockito.Mock;

class UserServiceTest {
  @Mock
  UserRepository repo;

  @Test
  void testFindUser() {
    when(repo.findUserById(1)).thenReturn(new User(1, "Alice"));
    UserService service = new UserService(repo);
    User user = service.findUser(1);
    assertEquals("Alice", user.getName());
  }
}

medium

A. The test method is missing the @Mock annotation

B. The when method is used incorrectly

C. Mocks are not initialized before use

D. UserService constructor cannot accept mocks

Solution

1. Step 1: Check mock initialization

   The code declares @Mock UserRepository repo; but never initializes mocks with MockitoAnnotations.openMocks(this) or similar.

2. Step 2: Understand consequence of missing initialization

   Without initialization, repo is null, so calling when(repo.findUserById(1)) causes a NullPointerException.

3. Final Answer:

   Mocks are not initialized before use -> Option C

4. Quick Check:

   Always initialize mocks before using them [OK]

Hint: Always initialize mocks before calling methods on them [OK]

Common Mistakes:

• Forgetting MockitoAnnotations.openMocks(this)
• Assuming @Mock auto-initializes mocks
• Misusing when() without initialized mocks

5. You want to test a service method that depends on two different collaborators. You use @Mock to create fake objects for both. Which of the following is the best practice to ensure your mocks work correctly in the test?

hard

A. Declare mocks as local variables inside test methods

B. Initialize mocks with MockitoAnnotations.openMocks(this) in a setup method

C. Use real instances instead of mocks for better accuracy

D. Manually instantiate mocks with new keyword

Solution

1. Step 1: Understand mock initialization best practice

   Mocks annotated with @Mock must be initialized before use, typically by calling MockitoAnnotations.openMocks(this) in a setup method.

2. Step 2: Evaluate other options

   Manually creating mocks with new is incorrect; real instances break isolation; local variables lose scope and initialization benefits.

3. Final Answer:

   Initialize mocks with MockitoAnnotations.openMocks(this) in a setup method -> Option B

4. Quick Check:

   Initialize mocks in setup for reliable tests [OK]

Hint: Initialize all @Mock fields in setup method [OK]

Common Mistakes:

• Creating mocks manually with new keyword
• Using real objects instead of mocks
• Declaring mocks inside test methods without initialization