Test code using JUnit in JVM – tutorial | Kotlin (original) (raw)

This tutorial shows you how to write a simple unit test in a Kotlin/JVM project and run it with the Gradle build tool.

In this project, you'll use the kotlin.test library and run the test using JUnit. If you're working on a multiplatform app, see the Kotlin Multiplatform tutorial.

To get started, first download and install the latest version of IntelliJ IDEA.

Add dependencies

1. Open a Kotlin project in IntelliJ IDEA. If you don't have a project, create one.
2. Open the build.gradle(.kts) file and check that the testImplementation dependency is present. This dependency allows you to work with kotlin.test and JUnit:
   dependencies { // Other dependencies. testImplementation(kotlin("test")) }
   dependencies { // Other dependencies. testImplementation 'org.jetbrains.kotlin:kotlin-test' }
3. Add the test task to the build.gradle(.kts) file:
   tasks.test { useJUnitPlatform() }
   test { useJUnitPlatform() }

Here's a complete code for the build.gradle.kts:

plugins { kotlin("jvm") version "2.1.20" } group = "org.example" version = "1.0-SNAPSHOT" repositories { mavenCentral() } dependencies { testImplementation(kotlin("test")) } tasks.test { useJUnitPlatform() }

Add the code to test it

1. Open the Main.kt file in src/main/kotlin.
   The src directory contains Kotlin source files and resources. The Main.kt file contains sample code that prints Hello, World!.
2. Create the Sample class with the sum() function that adds two integers together:
   class Sample() { fun sum(a: Int, b: Int): Int { return a + b } }

Create a test

1. In IntelliJ IDEA, select Code | Generate | Test... for the Sample class:
   
2. Specify the name of the test class. For example, SampleTest:
   
   IntelliJ IDEA creates the SampleTest.kt file in the test directory. This directory contains Kotlin test source files and resources.
3. Add the test code for the sum() function in SampleTest.kt:
   • Define the test testSum() function using the @Test annotation.
   • Check that the sum() function returns the expected value by using the assertEquals() function.
     import org.example.Sample import org.junit.jupiter.api.Assertions.* import kotlin.test.Test class SampleTest { private val testSample: Sample = Sample() @Test fun testSum() { val expected = 42 assertEquals(expected, testSample.sum(40, 2)) } }

Run a test

1. Run the test using the gutter icon:
   
2. Check the result in the Run tool window:
   
   The test function was executed successfully.
3. Make sure that the test works correctly by changing the expected variable value to 43:
   @Test fun testSum() { val expected = 43 assertEquals(expected, classForTesting.sum(40, 2)) }
4. Run the test again and check the result:
   
   The test execution failed.

What's next

Once you've finished your first test, you can:

• Write more tests using other kotlin.test functions. For example, use the assertNotEquals() function.
• Improve your test output with the Kotlin Power-assert compiler plugin. The plugin enriches the test output with contextual information.
• Create your first server-side application with Kotlin and Spring Boot.

Last modified: 24 April 2025