Kotlin Testing Tutorial: Unit Testing, JUnit, MockK, and Code Coverage Best Practices

This Kotlin Testing tutorial covers unit testing fundamentals using JUnit and MockK, along with code coverage concepts and best practices. It explains how to write reliable, maintainable, and testable Kotlin code for Android and backend applications, preparing developers for production-ready projects and technical interviews.

Testing in Kotlin (Complete Tutorial)

Unit Testing Concepts

Unit testing verifies that individual units of code behave as expected.

Key Principles

  1. Tests should be fast and isolated
  2. Each test validates one behavior
  3. No external dependencies (DB, network)

Example Function


fun add(a: Int, b: Int): Int = a + b

Simple Test Case


@Test
fun testAddition() {
assertEquals(5, add(2, 3))
}

JUnit in Kotlin

JUnit is the most widely used testing framework.

Basic Annotations

  1. @Test
  2. @BeforeEach
  3. @AfterEach
  4. @BeforeAll
  5. @AfterAll

Example


class CalculatorTest {

@BeforeEach
fun setup() {
println("Setup before test")
}

@Test
fun testSum() {
assertEquals(10, 5 + 5)
}
}

Best Practices

  1. Name tests clearly
  2. Follow Arrange-Act-Assert pattern
  3. Avoid logic inside tests

MockK

MockK is a mocking library designed for Kotlin.

Why MockK?

  1. Works well with final classes
  2. Coroutine-friendly
  3. Clean Kotlin syntax

Mocking Example


class UserService(private val repository: UserRepository) {
fun getUserName(): String = repository.getUser().name
}

class UserServiceTest {

private val repository = mockk<UserRepository>()
private val service = UserService(repository)

@Test
fun testGetUserName() {
every { repository.getUser() } returns User("Muni")
assertEquals("Muni", service.getUserName())
}
}

Best Practices

  1. Mock dependencies, not logic
  2. Use verify to confirm behavior
  3. Avoid over-mocking

Testing Coroutines

Coroutine Test Example


@Test
fun testSuspendFunction() = runBlocking {
val result = fetchData()
assertEquals("Data", result)
}

Best Practices

  1. Use runTest from kotlinx-coroutines-test
  2. Control dispatchers
  3. Avoid real delays

Code Coverage

Code coverage measures how much code is tested.

Popular Tools

  1. JaCoCo
  2. IntelliJ built-in coverage

Coverage Goals

  1. Aim for 70–80% coverage
  2. Focus on business logic
  3. Do not chase 100% blindly

Gradle Example


plugins {
jacoco
}

Best Practices

  1. Review uncovered code
  2. Cover edge cases
  3. Integrate coverage in CI/CD

Testing Best Practices Summary

  1. Write tests alongside production code
  2. Test business logic, not UI
  3. Keep tests independent
  4. Use mocks wisely
  5. Automate tests in pipelines

Chapter Summary

This chapter explained Kotlin testing fundamentals, including unit testing concepts, JUnit, MockK, and code coverage. Strong testing skills are essential for building reliable Kotlin applications and succeeding in professional development roles.