Mocking with MockK in Kotlin - Step-by-Step Execution

Concept Flow - Mocking with MockK

Create Mock Object

↓

Define Behavior with every {}

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Call Mocked Method

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Verify Calls with verify {}

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Assert Results

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End

This flow shows how to create a mock, define its behavior, call it, verify interactions, and assert results using MockK.

Execution Sample

Kotlin

val mockList = mockk<MutableList<String>>()
every { mockList.size } returns 3
println(mockList.size)
verify { mockList.size }

This code mocks a MutableList, sets size to return 3, prints size, and verifies the size property was accessed.

Execution Table

Step	Action	Evaluation	Result
1	Create mock object mockList	mockList is a mock of MutableList<String>	mockList created
2	Define behavior: every { mockList.size } returns 3	When mockList.size is called, return 3	Behavior set
3	Call mockList.size	mockList.size called	Returns 3
4	Print mockList.size	Prints 3	Output: 3
5	Verify mockList.size was called	Check if mockList.size was accessed	Verification passed
6	End of test	No more actions	Test complete

💡 All steps executed; mock behavior defined and verified successfully.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	Final
mockList	null	mock object	mock object with size=3 behavior	mock object with size=3 behavior	mock object with size=3 behavior	mock object with size=3 behavior
mockList.size	N/A	N/A	returns 3	3	3	3

Key Moments - 3 Insights

Why does mockList.size return 3 even though the list is empty?

Because we defined the behavior with every { mockList.size } returns 3 in step 2, so the mock returns 3 regardless of actual data.

What happens if we call a method on mockList without defining behavior?

MockK will throw an exception or return default values unless behavior is defined; here, size was defined to return 3 (see step 2).

Why do we use verify { mockList.size } after calling it?

To check that the mocked method was actually called during the test, ensuring interaction correctness (step 5).

Visual Quiz - 3 Questions

Look at the execution table, what value does mockList.size return at step 3?

A 3

B 0

C null

D Throws exception

💡 Hint

Check the 'Result' column at step 3 in the execution_table.

At which step is the behavior of mockList.size defined?

A Step 1

B Step 2

C Step 4

D Step 5

💡 Hint

Look at the 'Action' column describing 'Define behavior' in the execution_table.

If we remove the verify { mockList.size } call, what changes in the execution?

A The mockList.size would not return 3

B The mockList would not be created

C The test would not check if size was called

D The println would print null

💡 Hint

Refer to step 5 in the execution_table about verification.

Concept Snapshot

Mocking with MockK:
- Create mock with mockk<T>()
- Define behavior with every { ... } returns ...
- Call mocked methods normally
- Verify calls with verify { ... }
- Use to isolate and test code parts

Full Transcript

This example shows how to use MockK in Kotlin to create a mock object of MutableList<String>. We define that when the size property is accessed, it returns 3. Then we call mockList.size and print the result, which outputs 3. Finally, we verify that the size property was accessed during the test. This helps test code that depends on list size without needing a real list.