Property tests
- Kotlin course
- Advanced
- Lesson 47/83
- 5 min
Learn property-based testing with kotest-property.
Property testing with kotest-property
kotest-property tests that a property holds for randomly generated inputs. You write a condition with forAll; the library tries hundreds of random values to find a counterexample.
Example: forAll<Int> { n -> (n * 2) % 2 == 0 }. This checks that doubling any integer yields an even number. If a counterexample is found, the test fails and the example is shown. This pattern catches edge cases classic unit tests miss.
forAll and Arb
forAll takes a property function. The type parameter controls random input generation; Arb defines custom generators.
Example: forAll<Int> { n -> n + 0 == n }. Here a random Int is generated. There are built-in generators like Arb.string() and Arb.positiveInt(). When a property fails, the library shows the smallest failing case; this makes understanding the failure easier.
Practice Test the reverse property
In main call forAll<String> { s -> s.reversed().reversed() == s } and print println("ok"). Use the io.kotest.property.forAll import.
Starter code
import io.kotest.property.forAll
// in main test the double-reverse property with forAll<String>
// on success print a success messageSample solution
import io.kotest.property.forAll
fun main() {
forAll<String> { s ->
s.reversed().reversed() == s
}
println("ok")
}Expected output
ok
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 What does kotest-property provide?
- Testing with a single fixed input
- Verifying a property with random inputs Correct answer
- Only UI testing
- Only compile-time checks
Explanation: kotest-property verifies properties with random inputs.
2 Complete the forAll block
____<Int> { n ->
(n * 2) % 2 == 0
}forAll<Int> { n ->
(n * 2) % 2 == 0
}Explanation: forAll tests the property with random inputs.
3 Match the property-test parts
| Term | Meaning |
|---|---|
forAll<T> { } |
Property block |
Arb.string() |
String generator |
Int |
Type parameter |
counterexample |
Failing case |