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  1. DZone
  2. Testing, Deployment, and Maintenance
  3. Testing, Tools, and Frameworks
  4. Kotlintest and Property-Based Testing

Kotlintest and Property-Based Testing

Kotlintest, a port of scalatest written in Kotlin, comes with the support of property-based testing. See how to take advantage of it on a few helpful examples.

By 
Biju Kunjummen user avatar
Biju Kunjummen
·
Aug. 01, 17 · Tutorial
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I was very happy to see that Kotlintest, a port of the excellent scalatest in Kotlin, supports property-based testing.

I was introduced to property-based testing through the excellent "Functional programming in Scala" book.

The idea behind it is simple — the behavior of a program is described as a property, and the testing framework generates random data to validate the property. This is best illustrated with an example using the excellent scalacheck library:

import org.scalacheck.Prop.forAll
import org.scalacheck.Properties

object ListSpecification extends Properties("List") {
    property("reversing a list twice should return the list") = forAll { (a: List[Int]) =>
        a.reverse.reverse == a
    }
}


scalacheck would generate a random list (of integers) of varying sizes and would validate that this property holds for the lists. A similar specification expressed through Kotlintest looks like this:

import io.kotlintest.properties.forAll
import io.kotlintest.specs.StringSpec

class ListSpecification : StringSpec({
    "reversing a list twice should return the list" {
        forAll{ list: List<Int> ->
            list.reversed().reversed().toList() == list
        }
    }
})


If the generators have to be a little more constrained, say if we wanted to test this behavior on lists of integers in the range 1-1000, then an explicit generator can be passed in the following way, again starting with scalacheck:

import org.scalacheck.Prop.forAll
import org.scalacheck.{Gen, Properties}

object ListSpecification extends Properties("List") {
    val intList = Gen.listOf(Gen.choose(1, 1000))
    property("reversing a list twice should return the list") = forAll(intList) { (a: List[Int]) =>
        a.reverse.reverse == a
    }
}


And the equivalent kotlintest code:

import io.kotlintest.properties.Gen
import io.kotlintest.properties.forAll
import io.kotlintest.specs.StringSpec

class BehaviorOfListSpecs : StringSpec({
    "reversing a list twice should return the list" {
        val intList = Gen.list(Gen.choose(1, 1000))

        forAll(intList) { list ->
            list.reversed().reversed().toList() == list
        }
    }
})


Given this, let me now jump onto another example from the scalacheck site, this time to illustrate a failure:

import org.scalacheck.Prop.forAll
import org.scalacheck.Properties

object StringSpecification extends Properties("String") {

    property("startsWith") = forAll { (a: String, b: String) =>
        (a + b).startsWith(a)
    }

    property("concatenate") = forAll { (a: String, b: String) =>
        (a + b).length > a.length && (a + b).length > b.length
    }

    property("substring") = forAll { (a: String, b: String, c: String) =>
        (a + b + c).substring(a.length, a.length + b.length) == b
    }
}


The second property described above is wrong — if two strings are concatenated together, they are ALWAYS larger than each of the parts. This is not true if one of the strings is blank.

If I were to run this test using scalacheck, it correctly catches this wrongly specified behavior:

+ String.startsWith: OK, passed 100 tests.
! String.concatenate: Falsified after 0 passed tests.
> ARG_0: ""
> ARG_1: ""
+ String.substring: OK, passed 100 tests.
Found 1 failing properties.


An equivalent in Kotlintest is the following:

import io.kotlintest.properties.forAll
import io.kotlintest.specs.StringSpec

class StringSpecification : StringSpec({
    "startsWith" {
        forAll { a: String, b: String ->
            (a + b).startsWith(a)
        }
    }

    "concatenate" {
        forAll { a: String, b: String ->
            (a + b).length > a.length && (a + b).length > b.length
        }
    }

    "substring" {
        forAll { a: String, b: String, c: String ->
            (a + b + c).substring(a.length, a.length + b.length) == b
        }
    }
})


On running, it correctly catches the issue with concatenation and produces the following result:

java.lang.AssertionError: Property failed for

Y{_DZ<vGnzLQHf9|3$i|UE,;!%8^SRF;JX%EH+<5d:p`Y7dxAd;I+J5LB/:O)

 at io.kotlintest.properties.PropertyTestingKt.forAll(PropertyTesting.kt:27)


However, there is an issue here — scalacheck found a simpler failure case. It does this with a process called "Test Case minimization," where in the case of a failure, it tries to find the smallest test case that can fail, something that the Kotlintest can learn from.

There are other features where Kotlintest lags in respect to scalacheck, a big one being able to combine generators:

case class Person(name: String, age: Int)

val genPerson = for {
    name <- Gen.alphaStr
    age <- Gen.choose(1, 50)
} yield Person(name, age)

genPerson.sample


However, all in all, I have found the DSL of Kotlintest and its support for property-based testing to be a good start so far, and I look forward to seeing how this library will evolve over time.

If you want to play with these samples a little more, it is available in my GitHub repo here.

Property (programming) Test case Testing Strings Library Blank (solution) Data (computing) Scala (programming language)

Published at DZone with permission of Biju Kunjummen, DZone MVB. See the original article here.

Opinions expressed by DZone contributors are their own.

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