Choosing the right test double in JUnit - Interactive Code Practice

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'

Aspy

Bmock

Cverify

Dwhen

Attempts:

3 left

💡 Hint

Common Mistakes

Using spy instead of mock when a full mock is needed.

Using verify to create objects, which is for checking calls.

The mock method creates a mock object for the given class.

2fill in blank

medium

Complete the code to verify that a method was called once on a mock.

JUnit

Mockito.[1](mockObject).someMethod();

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

Aspy

Bmock

Cverify

Dwhen

Attempts:

3 left

💡 Hint

Common Mistakes

Using mock() instead of verify() for checking calls.

Confusing spy with verify.

The verify method checks if a method was called on a mock.

3fill in blank

hard

Fix the error in the code to stub a method call on a spy object.

JUnit

Mockito.[1](spyObject.someMethod()).thenReturn("value");

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

Amock

Bspy

Cverify

Dwhen

Attempts:

3 left

💡 Hint

Common Mistakes

Using verify instead of when for stubbing.

Using mock instead of when for stubbing.

The when method is used to stub method calls on mocks or spies.

4fill in blank

hard

Fill both blanks to create a spy and stub a method to return a value.

JUnit

MyService spyService = Mockito.[1](new MyService());
Mockito.[2](spyService.someMethod()).thenReturn("Hello");

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

Aspy

Bmock

Cwhen

Dverify

Attempts:

3 left

💡 Hint

Common Mistakes

Using mock instead of spy to wrap a real object.

Using verify instead of when for stubbing.

First, create a spy with spy(). Then stub a method call with when().

5fill in blank

hard

Fill all three blanks to mock a repository, stub a method, and verify the call.

JUnit

Repository repo = Mockito.[1](Repository.class);
Mockito.[2](repo.findById(1)).thenReturn(Optional.of(new Item()));
Mockito.[3](repo).findById(1);

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

Amock

Bwhen

Cverify

Dspy

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing spy and mock incorrectly.

Using verify before when.

Create a mock with mock(), stub a method with when(), and verify the call with verify().

Practice

1. Which test double should you use in JUnit when you want to replace a real object and verify that certain methods were called during the test?

easy

A. Mock

B. Stub

C. Fake

D. Spy

Solution

1. Step 1: Understand the purpose of mocks

   Mocks are used to fake behavior and verify interactions, such as method calls.

2. Step 2: Compare with other test doubles

   Stubs provide fixed responses, fakes have working implementations, spies record calls but mocks are designed for verification.

3. Final Answer:

   Mock -> Option A

4. Quick Check:

   Verify calls = Mock [OK]

Hint: Mocks verify calls; stubs just return data [OK]

Common Mistakes:

• Confusing stubs with mocks
• Using fake when verification is needed
• Thinking spies are the same as mocks

2. Which of the following is the correct way to create a stub in JUnit using Mockito?

easy

A. spy(new Service());

B. when(service.getData()).thenReturn("data");

C. mock(Service.class);

D. verify(service).getData();

Solution

1. Step 1: Identify stub creation syntax

   Stubs return fixed data; in Mockito, use when(...).thenReturn(...) to stub a method.

2. Step 2: Check other options

   Verify checks calls, mock creates mocks, spy wraps real objects; only when().thenReturn() stubs.

3. Final Answer:

   when(service.getData()).thenReturn("data"); -> Option B

4. Quick Check:

   Stub method = when().thenReturn() [OK]

Hint: Stubs use when().thenReturn() syntax [OK]

Common Mistakes:

• Using verify() to stub methods
• Confusing mock() with stub setup
• Using spy() instead of stub

3. Given the following JUnit test code snippet using Mockito, what will be the output of the assertion?

List<String> list = mock(List.class);
when(list.size()).thenReturn(5);
assertEquals(5, list.size());

medium

A. Test passes because size() returns 5

B. Test fails because size() returns 0 by default

C. Test throws NullPointerException

D. Test fails due to syntax error

Solution

1. Step 1: Analyze mock behavior

   The list is mocked, and size() is stubbed to return 5.

2. Step 2: Check assertion

   assertEquals(5, list.size()) compares 5 with stubbed return 5, so it passes.

3. Final Answer:

   Test passes because size() returns 5 -> Option A

4. Quick Check:

   Stubbed method returns set value [OK]

Hint: Stubbed methods return set values, so assertion passes [OK]

Common Mistakes:

• Assuming default mock returns 0
• Expecting real method call on mock
• Confusing mock with spy behavior

4. Identify the error in this JUnit test using Mockito:

@Test
void testServiceCall() {
  Service service = mock(Service.class);
  service.process();
  verify(service.process());
}

medium

A. Mock creation syntax is wrong

B. Missing stub for process() method

C. process() method cannot be mocked

D. Incorrect use of verify; should be verify(service).process();

Solution

1. Step 1: Check verify syntax

   verify() expects the mock object, then method call separately: verify(service).process();

2. Step 2: Identify error in code

   Current code calls verify(service.process()), which is incorrect syntax and causes compile error.

3. Final Answer:

   Incorrect use of verify; should be verify(service).process(); -> Option D

4. Quick Check:

   verify syntax = verify(mock).method() [OK]

Hint: verify() needs mock object, then method call separately [OK]

Common Mistakes:

• Calling verify with method call inside
• Forgetting to mock the object
• Confusing stub and verify usage

5. You want to test a payment processing system. The payment gateway is an external service that charges real money. Which test double should you use in your JUnit tests to avoid real charges but still verify that the payment method was called correctly?

hard

A. Use a stub to return a fixed success response

B. Use a fake with a simple in-memory implementation

C. Use a mock to verify the payment method call

D. Use a spy to wrap the real payment gateway

Solution

1. Step 1: Understand test goal

   We want to avoid real charges but verify the payment method was called.

2. Step 2: Choose test double type

   Mocks fake behavior and verify calls, perfect for checking method calls without real effects.

3. Step 3: Compare other options

   Stubs return fixed data but don't verify calls; fakes have real logic; spies call real methods, risking real charges.

4. Final Answer:

   Use a mock to verify the payment method call -> Option C

5. Quick Check:

   Verify calls without real effect = Mock [OK]

Hint: Mocks verify calls without real side effects [OK]

Common Mistakes:

• Using spy risks real external calls
• Using stub misses call verification
• Choosing fake adds unnecessary complexity