Why advanced mocking handles complex dependencies in JUnit - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a mock object using Mockito.

JUnit

MyService service = Mockito.[1](MyService.class);

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

Amock

Bspy

Cverify

Dwhen

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'spy' instead of 'mock' creates a partial mock, not a full mock.

Using 'verify' or 'when' does not create mocks.

Mockito.mock() creates a mock object for the given class.

2fill in blank

medium

Complete the code to stub a method call to return a specific value.

JUnit

Mockito.[1](service.getData()).thenReturn("mocked data");

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

Aspy

Bverify

Cmock

Dwhen

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'verify' instead of 'when' is for checking calls, not stubbing.

Using 'mock' or 'spy' here is incorrect.

Mockito.when() is used to stub a method call to return a desired value.

3fill in blank

hard

Fix the error in verifying that a method was called exactly once.

JUnit

Mockito.verify(service, Mockito.[1]()).process();

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

Atimes(2)

Btimes(1)

Conce()

Dnever()

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'once()' causes a compilation error because it is not a valid Mockito method.

Using 'times(2)' checks for two calls, not one.

Mockito.times(1) verifies the method was called exactly once. 'once()' does not exist.

4fill in blank

hard

Fill both blanks to mock a complex dependency and stub its method.

JUnit

Dependency dep = Mockito.[1](Dependency.class);
Mockito.[2](dep.getValue()).thenReturn(42);

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

Amock

Bwhen

Cspy

Dverify

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'spy' instead of 'mock' creates a partial mock, which may not be desired.

Using 'verify' instead of 'when' is incorrect for stubbing.

First, create a mock with mock(). Then stub the method with when().

5fill in blank

hard

Fill all three blanks to verify a method call with argument matcher and times.

JUnit

Mockito.verify(service, Mockito.[1](3)).execute(Mockito.[2]("test"), Mockito.[3]());

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

Atimes

Beq

Cany

Dnever

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'never' instead of 'times' causes verification to expect zero calls.

Using 'any' instead of 'eq' loses exact argument matching.

Use times(3) to check call count, eq("test") to match exact argument, and any() for any argument.

Practice

1. Why do we use advanced mocking in JUnit tests when dealing with complex dependencies?

easy

A. To automatically generate user interfaces

B. To isolate the code under test and control the behavior of its dependencies

C. To avoid writing any test cases

D. To make the code run faster in production

Solution

1. Step 1: Understand the purpose of mocking

   Mocking replaces real dependencies with fake ones to isolate the code being tested.

2. Step 2: Recognize the benefit of advanced mocking

   Advanced mocking allows control over complex dependencies, making tests reliable and focused.

3. Final Answer:

   To isolate the code under test and control the behavior of its dependencies -> Option B

4. Quick Check:

   Mocking isolates dependencies [OK]

Hint: Mocking isolates dependencies to test code alone [OK]

Common Mistakes:

• Thinking mocking speeds up production code
• Believing mocking removes the need for tests
• Confusing mocking with UI generation

2. Which of the following is the correct syntax to create a mock object using Mockito in a JUnit test?

easy

A. MyClass mock = Mockito.mock(MyClass.class);

B. MyClass mock = new MyClass();

C. MyClass mock = mockito.create(MyClass);

D. MyClass mock = Mockito.createMock(MyClass);

Solution

1. Step 1: Recall Mockito mock creation syntax

   Mockito uses the static method mock(Class) to create mock objects.

2. Step 2: Verify the correct syntax

   The correct syntax is: MyClass mock = Mockito.mock(MyClass.class);

3. Final Answer:

   MyClass mock = Mockito.mock(MyClass.class); -> Option A

4. Quick Check:

   Mockito.mock(Class) creates mocks [OK]

Hint: Use Mockito.mock(Class.class) to create mocks [OK]

Common Mistakes:

• Using new keyword instead of Mockito.mock
• Calling non-existent Mockito methods
• Incorrect class reference syntax

3. Given the following JUnit test code using Mockito:

@ExtendWith(MockitoExtension.class)
class ServiceTest {
  @Mock
  Repository repo;

  @InjectMocks
  Service service;

  @Test
  void testGetData() {
    when(repo.fetch()).thenReturn("data");
    String result = service.getData();
    assertEquals("data", result);
  }
}

What will be the test result if repo.fetch() is called inside service.getData()?

medium

A. Test fails because repo.fetch() returns null

B. Test fails due to syntax error in mocking

C. Test throws NullPointerException due to uninitialized repo

D. Test passes because repo.fetch() returns mocked "data"

Solution

1. Step 1: Understand Mockito annotations usage

   @Mock creates a mock for Repository, and @InjectMocks injects it into Service.

2. Step 2: Analyze the when-thenReturn setup

   Calling repo.fetch() returns "data" as stubbed, so service.getData() returns "data".

3. Final Answer:

   Test passes because repo.fetch() returns mocked "data" -> Option D

4. Quick Check:

   Mock returns stubbed value [OK]

Hint: Stub methods with when().thenReturn() to control outputs [OK]

Common Mistakes:

• Assuming mocks return null by default
• Forgetting to initialize mocks with @ExtendWith or MockitoAnnotations
• Confusing mock injection with real object creation

4. Consider this JUnit test snippet using Mockito:

@ExtendWith(MockitoExtension.class)
class TestClass {
  @Mock
  Database db;

  @Test
  void testQuery() {
    when(db.query("SELECT * FROM users")).thenReturn(null);
    String result = db.query("SELECT * FROM users");
    assertNotNull(result);
  }
}

What is the problem with this test?

medium

A. The assertion will fail because the mock returns null

B. The syntax of when() is incorrect

C. The mock object db is not created properly

D. The test will pass because null is acceptable

Solution

1. Step 1: Analyze the stubbed return value

   The mock is set to return null for the query method call.

2. Step 2: Check the assertion logic

   The test asserts that the result is not null, but the stub returns null, so assertion fails.

3. Final Answer:

   The assertion will fail because the mock returns null -> Option A

4. Quick Check:

   Assertion contradicts stubbed null [OK]

Hint: Match stubbed return values with assertions [OK]

Common Mistakes:

• Ignoring that stub returns null
• Assuming syntax error without checking code
• Believing test passes with failing assertions

5. You have a service class that depends on two complex external APIs. You want to write a JUnit test that verifies your service logic without calling these APIs. Which advanced mocking approach best handles this scenario?

hard

A. Manually create fake API responses inside the service code

B. Write integration tests that call the real APIs directly

C. Use Mockito to create mocks for both API clients and inject them into the service

D. Skip testing the service since APIs are complex

Solution

1. Step 1: Identify the need to isolate service from external APIs

   Testing should avoid calling real APIs to keep tests fast and reliable.

2. Step 2: Choose advanced mocking to simulate API behavior

   Mockito mocks for API clients allow controlling responses and injecting them into the service.

3. Final Answer:

   Use Mockito to create mocks for both API clients and inject them into the service -> Option C

4. Quick Check:

   Mock external APIs to isolate service [OK]

Hint: Mock external dependencies to isolate service logic [OK]

Common Mistakes:

• Testing with real APIs causing slow or flaky tests
• Hardcoding fake responses inside service code
• Avoiding tests due to complexity