How to Code in Kotlin Using AI

In short: say the Kotlin version, give the formula or rules and one example with the exact answer, ask for kotlin.test tests, and run them.

Why Kotlin with AI

Kotlin is Google's preferred language for Android. Many Android teams write Kotlin every day. AI speeds up the repeated parts, but money and math must be checked with tests.

What you need

• JDK 17 or 21: Kotlin runs on the JVM.
• Kotlin: through Android Studio or IntelliJ (bundled), or the kotlinc command-line compiler.
• kotlin.test: Kotlin's test library; works with JUnit in Gradle projects.
• IDE + assistant: Android Studio or IntelliJ with Copilot, or VS Code with Claude Code.

How it flows

Install and check

1. Install IntelliJ IDEA or Android Studio (Kotlin is included), or the kotlinc compiler from the official Kotlin releases.
2. Check: kotlinc -version (command line) or create a new Kotlin project in the IDE.
3. In Gradle projects, add testImplementation(kotlin("test")) and run ./gradlew test.

Step 1 — the prompt

Step 2 — the code (tested here)

// Monthly loan EMI. Built with an AI assistant, then tested.
import kotlin.math.pow
import kotlin.math.roundToLong

/** EMI = P * r * (1+r)^n / ((1+r)^n - 1), r = monthly rate. Returns rupees, rounded. */
fun emi(principal: Double, annualRatePercent: Double, months: Int): Long {
    require(principal > 0) { "principal must be positive" }
    require(months > 0) { "months must be positive" }
    require(annualRatePercent >= 0) { "rate cannot be negative" }
    if (annualRatePercent == 0.0) return (principal / months).roundToLong()
    val r = annualRatePercent / 12 / 100
    val f = (1 + r).pow(months)
    return (principal * r * f / (f - 1)).roundToLong()
}

fun main() {
    println(emi(500_000.0, 9.0, 60))  // 10379
}

kotlinc Emi.kt -include-runtime -d emi.jar
java -jar emi.jar
# -> 10379

Step 3 — ask for tests, then run them

import kotlin.test.assertEquals
import kotlin.test.assertFailsWith

fun main() {
    assertEquals(10379, emi(500_000.0, 9.0, 60))   // 5 lakh, 9%, 5 years
    assertEquals(10000, emi(120_000.0, 0.0, 12))   // zero interest
    assertFailsWith<IllegalArgumentException> { emi(-1.0, 9.0, 12) }
    assertFailsWith<IllegalArgumentException> { emi(100_000.0, 9.0, 0) }
    println("4 checks passed")
}

# -> 4 checks passed

Settings and config

1. Rules file: “Kotlin 2.x, null-safety, no !!, kotlin.test/JUnit, coroutines for async”.
2. Turn on warnings as errors in CI: kotlin { compilerOptions { allWarningsAsErrors.set(true) } } in Gradle.
3. Use the IDE's “Reformat Code” after accepting AI code.

Watch out (common AI mistakes in Kotlin)

Quick quiz

Save tokens and money

Give the formula in the prompt; the AI does not need to search or guess. Ask for one function plus tests. Paste only the top lines of a Gradle error.

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