MockK for mocking in Android Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ ui_behavior

intermediate

MockK: What happens when you mock a class without specifying behavior?

Consider you mock a Kotlin class using MockK but do not specify any behavior for its functions. What will happen when you call a function on this mock?

A The function returns a random value.

B The function returns the default value for its return type (e.g., 0 for Int, null for objects).

C The function executes the real implementation of the class.

D The function throws a NullPointerException immediately.

💡 Hint

Think about what MockK returns when no behavior is defined.

By default, MockK returns default values for functions when no behavior is specified. For example, Int returns 0, Boolean returns false, and objects return null.

📝 Syntax

intermediate

MockK syntax: How to mock a function to return a specific value?

Which of the following MockK code snippets correctly mocks a function 'getData()' to return the string "Hello"?

Android Kotlin

class MyClass {
  fun getData(): String = "Real Data"
}

A every { myClass.getData() } returns "Hello"

B mockk { getData() returns "Hello" }

C when(myClass.getData()).thenReturn("Hello")

D stub { myClass.getData() returns "Hello" }

💡 Hint

Look for the MockK function that sets behavior on mocks.

In MockK, 'every { ... } returns ...' is the syntax to define behavior for a mocked function.

❓ lifecycle

advanced

MockK: What happens if you forget to unmockkAll() after tests?

In a test suite using MockK, what is the likely effect of not calling unmockkAll() after tests?

A The app crashes immediately after tests.

B Tests run faster because mocks are reused.

C Mocks may leak and affect other tests, causing unexpected behavior.

D There is no effect; MockK cleans up automatically.

💡 Hint

Think about resource cleanup and test isolation.

Not calling unmockkAll() can cause mocks to persist and interfere with other tests, breaking test isolation.

🔧 Debug

advanced

MockK: Why does this test fail with 'no answer found' error?

Given the code snippet below, why does the test fail with a 'no answer found' error when calling myClass.getData(42)? class MyClass { fun getData(id: Int): String = "Real Data" } val myClass = mockk() every { myClass.getData(1) } returns "Data1" // Behavior defined only for specific argument val result = myClass.getData(42)

A Because the mockk function was called incorrectly.

B Because getData() is a final function and cannot be mocked without extra setup.

C Because getData() is not mocked and returns Unit by default, causing a type mismatch.

D Because the mock was not created with relaxed = true, so no default answer is provided.

💡 Hint

Consider MockK's relaxed mocks feature.

By default, MockK mocks are strict and throw an exception if no behavior is defined. Using relaxed = true allows default values to be returned.

🧠 Conceptual

expert

MockK: How to verify a function was called exactly twice with specific arguments?

Using MockK, which code snippet correctly verifies that the function 'sendMessage("Hi")' was called exactly two times?

A verify(exactly = 2) { myClass.sendMessage("Hi") }

B verify(times = 2) { myClass.sendMessage("Hi") }

C verify { myClass.sendMessage("Hi") twice }

D verify(exactly = 2) { myClass.sendMessage(any()) }

💡 Hint

Check the correct parameter name for specifying call count in verify.

In MockK, 'verify(exactly = 2) { ... }' checks that the function was called exactly two times with the specified arguments.

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