Kotlin Cheatsheet

Testing & Gradle

Use this Kotlin reference while you build software engineering projects, review code for technical interview prep, or polish examples for a software engineer resume.

Gradle Kotlin DSL (build.gradle.kts)

plugins {
    kotlin("jvm") version "2.2.0"        // Kotlin 2.x = K2 compiler (stable since 2024)
    application
}

repositories { mavenCentral() }

dependencies {
    implementation("org.jetbrains.kotlinx:kotlinx-coroutines-core:1.10.2")
    testImplementation(kotlin("test"))                    // kotlin.test + JUnit 5 wiring
    testImplementation("org.jetbrains.kotlinx:kotlinx-coroutines-test:1.10.2")
}

kotlin {
    jvmToolchain(21)                     // pins JDK for compile + test
    compilerOptions {
        allWarningsAsErrors.set(true)
    }
}

tasks.test { useJUnitPlatform() }        // JUnit 5 runner

application { mainClass.set("demo.MainKt") }

./gradlew build          # compile + test
./gradlew test --tests "demo.SolverTest"
./gradlew run
./gradlew dependencies --configuration runtimeClasspath

Version catalogs (gradle/libs.versions.toml) centralize versions: declare kotlin = "2.2.0" under [versions], then alias(libs.plugins.kotlin.jvm) in the plugins block.

kotlin.test basics

import kotlin.test.*

class SolverTest {
    @Test
    fun `adds two numbers`() {                     // backtick names read well in reports
        assertEquals(5, add(2, 3))
    }

    @Test
    fun failures() {
        val e = assertFailsWith<IllegalArgumentException> { parse("") }
        assertTrue("blank" in e.message.orEmpty())
    }

    @BeforeTest fun setUp() { }
    @AfterTest fun tearDown() { }
}

Assertion	Checks
`assertEquals(exp, act)` / `assertNotEquals`	structural equality
`assertTrue { cond }` / `assertFalse`	boolean
`assertNull(x)` / `assertNotNull(x)`	nullability (`assertNotNull` returns non-null)
`assertContains(coll, item)`	membership (also strings)
`assertContentEquals(a, b)`	arrays / iterables element-wise
`assertIs<T>(x)`	type check + smart cast
`assertFailsWith<E> { }`	expected exception, returns it
`fail("msg")`	explicit failure

kotlin.test is a facade — on the JVM it delegates to JUnit 5 (with useJUnitPlatform()). Mixing in raw JUnit 5 (@ParameterizedTest, @Nested) or libraries like Kotest/MockK is normal.

Testing coroutines

import kotlinx.coroutines.test.runTest
import kotlin.test.*

class RepoTest {
    @Test
    fun loadsUser() = runTest {          // virtual time: delay() skips instantly
        val repo = Repo()
        assertEquals("user-1", repo.fetchUser(1))
    }
}

runTest gives a TestScope with virtual time (advanceTimeBy(1_000), runCurrent()); inject Dispatchers into production code so tests can substitute a StandardTestDispatcher.

Parameterized & data-driven

// JUnit 5 params: testImplementation("org.junit.jupiter:junit-jupiter-params")
import org.junit.jupiter.params.ParameterizedTest
import org.junit.jupiter.params.provider.CsvSource

class FizzTest {
    @ParameterizedTest
    @CsvSource("3,Fizz", "5,Buzz", "15,FizzBuzz")
    fun fizz(n: Int, expected: String) = assertEquals(expected, fizzbuzz(n))
}

// or zero-dependency table style:
@Test fun table() {
    listOf(3 to "Fizz", 5 to "Buzz").forEach { (n, want) ->
        assertEquals(want, fizzbuzz(n), "n=$n")
    }
}

Compiler CLI (no Gradle)

kotlinc Main.kt -include-runtime -d main.jar && java -jar main.jar
kotlinc src/ -d out/                 # compile a tree
kotlin -e 'println(1 + 2)'           # one-liner eval