Writing Multiple Test Cases Together

In this lesson you will learn how to run several test cases together in Kotlin, and understand what a body of unit tests looks like. This is for anyone searching for "Kotlin unit test how to write".

Testing one function against several input patterns at once is called unit testing. Preparing a spread of cases - normal values, boundary values, special values - helps you find bugs early.

The sample code defines a simple data class TestCase, collects several cases in a list, and feeds each one to add() in a loop to check the result. A data class keeps the boilerplate for grouping values to an absolute minimum.

A common stumbling block for beginners is testing only the happy path and forgetting boundary values such as zero or negative numbers. Covering the cases that are easy to overlook is what matters.

Real Kotlin development uses a test framework such as JUnit, but the underlying idea is exactly what you learn here.

📖 Reference code

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

data class TestCase(val a: Int, val b: Int, val expected: Int)

fun main() {
    val testCases = listOf(
        TestCase(2, 3, 5),
        TestCase(-1, 1, 0),
        TestCase(0, 0, 0)
    )

    var passed = 0
    testCases.forEachIndexed { i, tc ->
        val result = add(tc.a, tc.b)
        val ok = result == tc.expected
        if (ok) passed++
        println("Test ${i + 1}: ${if (ok) "OK" else "FAIL"}")
    }
    println("$passed/${testCases.size} passed")
}

✍️ Your code

Type your code, then press "Run"

🧪 This site can't compile or run Kotlin directly, so it checks on the spot whether what you typed matches the reference code (scoring happens entirely in your browser — nothing is sent anywhere).