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.