67. Testing

The kotlin.test library gives you a small, multiplatform set of assertions that delegate to a real test framework (JUnit on the JVM) at run time. It needs the kotlin-test dependency. Tests are run with the toolchain’s kotlin test command, not with kotlin run.

import kotlin.test.Test
import kotlin.test.assertEquals
import kotlin.test.assertTrue
import kotlin.test.assertFailsWith

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

fun divide(a: Int, b: Int): Int =
    if (b == 0) throw IllegalArgumentException("divide by zero") else a / b

class CalculatorTest {
    @Test
    fun addsNumbers() {
        assertEquals(5, add(2, 3))
    }

    @Test
    fun resultIsPositive() {
        assertTrue(add(2, 3) > 0)
    }

    @Test
    fun rejectsZeroDivisor() {
        assertFailsWith<IllegalArgumentException> {
            divide(1, 0)
        }
    }
}

Each @Test function is one isolated check. assertEquals(expected, actual) compares values, assertTrue checks a boolean, and assertFailsWith<T> { } asserts that the block throws the given exception type — returning the exception so you can inspect its message if needed.

Tests live under a test/ directory (alongside src/). Set product: jvm/lib and add the $kotlin.test dependency in module.yaml, then run the suite with kotlin test:

product: jvm/lib

dependencies:
  - $kotlin.test

Running it:

$ kotlin test
Started CalculatorTest
Started addsNumbers()
Passed addsNumbers()
Started resultIsPositive()
Passed resultIsPositive()
Started rejectsZeroDivisor()
Passed rejectsZeroDivisor()
Completed CalculatorTest

Test run finished after 85 ms
[         3 tests found           ]
[         3 tests started         ]
[         0 tests failed          ]
[         3 tests successful      ]

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