MockK for mocking in Android Kotlin - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a mock of the UserRepository class using MockK.

Android Kotlin

val userRepository = [1]<UserRepository>()

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

Amockk

Bspyk

Cevery

Dverify

Attempts:

3 left

💡 Hint

Common Mistakes

Using spyk instead of mockk when a pure mock is needed.

Trying to use every or verify to create a mock.

Use mockk to create a mock object in MockK.

2fill in blank

medium

Complete the code to stub the getUserName() method to return "Alice".

Android Kotlin

every { userRepository.[1]() } returns "Alice"

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

AgetUserId

BgetUserName

CfetchUser

DloadName

Attempts:

3 left

💡 Hint

Common Mistakes

Using a method name that does not exist in the class.

Confusing getUserId with getUserName.

The method to stub is getUserName as it returns the user name.

3fill in blank

hard

Fix the error in verifying that getUserName() was called exactly once.

Android Kotlin

verify(exactly = [1]) { userRepository.getUserName() }

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

A1

B2

Cany

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using zero which means never called.

Using 'any' which is not a valid number.

Use exactly = 1 to verify the method was called once.

4fill in blank

hard

Fill both blanks to create a spy of UserService and stub fetchData() to return "Data".

Android Kotlin

val userService = [1]<UserService>()
every { userService.[2]() } returns "Data"

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

Aspyk

Bmockk

CfetchData

DgetData

Attempts:

3 left

💡 Hint

Common Mistakes

Using mockk instead of spyk for spying.

Stubbing a method name that does not exist.

Use spyk to create a spy and stub the fetchData method.

5fill in blank

hard

Fill all three blanks to verify that saveUser() was called with any User object exactly twice.

Android Kotlin

verify(exactly = [1]) { userRepository.saveUser([2]) }

val slot = slot<User>()
verify { userRepository.saveUser(capture(slot)) }

assert(slot.captured.name == [3])

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

A2

Bany()

C"Alice"

D"Bob"

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong number for calls.

Not using any() matcher.

Asserting wrong user name.

Verify called twice, with any User object, and assert the captured user's name is "Alice".

Practice

1. What is the main purpose of using MockK in Android Kotlin testing?

easy

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

B. To speed up the app's UI animations

C. To generate random data for the app

D. To compile Kotlin code faster

Solution

1. Step 1: Understand MockK's role

   MockK is a library used to create fake objects that imitate real objects in tests.

2. Step 2: Identify the main use

   This helps test parts of the app separately without relying on real implementations.

3. Final Answer:

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

4. Quick Check:

   MockK purpose = Fake objects for testing [OK]

Hint: MockK fakes objects to isolate tests quickly [OK]

Common Mistakes:

• Thinking MockK speeds up UI animations
• Confusing MockK with data generators
• Believing MockK affects compilation speed

2. Which of the following is the correct syntax to create a mock of a class MyClass using MockK?

easy

A. val mock = MockK<MyClass>()

B. val mock = mockk<MyClass>()

C. val mock = mock<MyClass>()

D. val mock = createMock<MyClass>()

Solution

1. Step 1: Recall MockK mock creation syntax

   MockK uses the function mockk<T>() to create mocks of type T.

2. Step 2: Match syntax to options

   val mock = mockk<MyClass>() matches the correct syntax exactly.

3. Final Answer:

   val mock = mockk<MyClass>() -> Option B

4. Quick Check:

   MockK mock creation = mockk<Type>() [OK]

Hint: Use mockk<Type>() to create mocks in MockK [OK]

Common Mistakes:

• Using capitalized MockK instead of mockk
• Using mock() which is from other libraries
• Trying createMock() which is not in MockK

3. Given the code snippet:

val mock = mockk<Calculator>()
every { mock.add(2, 3) } returns 5
val result = mock.add(2, 3)

What is the value of result?

medium

A. Throws an exception

B. 0

C. 5

D. 2

Solution

1. Step 1: Understand the mocking setup

   The line every { mock.add(2, 3) } returns 5 tells MockK to return 5 when add(2, 3) is called.

2. Step 2: Check the method call

   The call mock.add(2, 3) matches the mocked behavior, so it returns 5.

3. Final Answer:

   5 -> Option C

4. Quick Check:

   Mocked method returns set value = 5 [OK]

Hint: Mocked calls return specified values, not real logic [OK]

Common Mistakes:

• Expecting real method logic to run
• Assuming default 0 return for Int
• Thinking mock throws error without setup

4. What is wrong with this MockK code snippet?

val mock = mockk<UserService>()
verify { mock.getUser() }

medium

A. mockk<UserService>() is invalid syntax

B. getUser() method cannot be verified

C. verify requires a return value setup

D. verify is called before any method call on mock

Solution

1. Step 1: Understand verify usage

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

2. Step 2: Check method call timing

   Here, verify is called before mock.getUser() is ever called, so verification fails.

3. Final Answer:

   verify is called before any method call on mock -> Option D

4. Quick Check:

   Verify needs method call before checking [OK]

Hint: Call method before verify to avoid errors [OK]

Common Mistakes:

• Calling verify before method invocation
• Thinking verify needs return setup
• Believing all methods are unverifiable

5. You want to test a function that calls fetchData() on a mock and then processes the result. Which MockK feature lets you simulate fetchData() returning different values on consecutive calls?

hard

A. Using every with returnsMany

B. Using slot to capture arguments

C. Using andThen to chain return values

D. Using relaxed mocks to auto-return defaults

Solution

1. Step 1: Identify feature for multiple returns

   MockK's returnsMany allows specifying a list of return values for consecutive calls.

2. Step 2: Match feature to options

   Using every with returnsMany correctly pairs every with returnsMany to simulate different returns on calls.

3. Final Answer:

   Using every with returnsMany -> Option A

4. Quick Check:

   Multiple returns = every + returnsMany [OK]

Hint: Use returnsMany to mock multiple call results [OK]

Common Mistakes:

• Confusing andThen with returnsMany
• Using slot for return values instead of capturing
• Relying on relaxed mocks for varied returns