When it comes to testing in Kotlin, we often default to jUnit as the go-to framework.

However, there’s an alternative that offers a variety of powerful features – Kotest.

Kotest provides a variety of spec styles, each with its unique capabilities, that make writing tests a breeze.

Whether you’re looking for a straightforward, linear approach or a more complex, hierarchical structure, there’s a Kotest spec that fits the bill.

This article will walk you through four of these spec styles: StringSpec, FunSpec, FreeSpec, and BehaviorSpec.

By the end, you should have a good understanding of each style and when to use them in your Kotlin testing.

Adding Kotest to your project

To get started with Kotest, you’ll need to add it to your project. With Gradle, this is as simple as adding a few lines to your build.gradle file.

Here’s an example of how you can add the latest version of Kotest to your Gradle project:

dependencies {
    testImplementation 'io.kotest:kotest-runner-junit5:5.6.2'
    testImplementation 'io.kotest:kotest-assertions-core:5.6.2'
}

Way 1

Stepping into StringSpec

import io.kotest.core.spec.style.StringSpec
import io.kotest.matchers.shouldBe

class MyStringSpecTest : StringSpec({
  "length should return size of string" {
    "hello".length shouldBe 5
  }
})

StringSpec is as simple as it gets. It’s ideal for linear test cases where each test is independent of the others. Each test is a string description followed by a lambda containing the test body.

But here’s the thing…

What if we need to group related test cases together or share setup and teardown logic between tests? StringSpec might not cut it.

Way 2

Having Fun with FunSpec

import io.kotest.core.spec.style.FunSpec
import io.kotest.matchers.shouldBe

class MyFunSpecTest : FunSpec({
  context("A string") {
    test("length should return size of string") {
      "hello".length shouldBe 5
    }
  }
})

FunSpec introduces a bit more structure. It allows us to group tests into blocks using the context method. This can be helpful for sharing setup and teardown logic for a group of related tests.

But hang on…

While FunSpec gives us more structure, it doesn’t support nested test groups. That might be a problem for more complex test cases where we need hierarchical organization.

Way 3

Freeing Ourselves with FreeSpec

import io.kotest.core.spec.style.FreeSpec
import io.kotest.matchers.shouldBe

class MyFreeSpecTest : FreeSpec({
  "A string" - {
    "when length is invoked" - {
      "should return the number of characters in the string" {
        "hello".length shouldBe 5
      }
    }
  }
})

FreeSpec allows us to create nested test groups with ease, making it great for complex scenarios. Tests can be organized hierarchically using the - operator.

However…

The syntax can be a bit verbose, and it might be overkill for simpler tests. It’s great for when you need to describe complex behaviors, but for single, standalone tests, other spec styles might be more suitable.

Way 4

Behaving Well with BehaviorSpec

import io.kotest.core.spec.style.BehaviorSpec
import io.kotest.matchers.shouldBe

class MyBehaviorSpecTest : BehaviorSpec({
  Given("a string") {
    When("length is invoked") {
      Then("it should return the number of characters in the string") {
        "hello".length shouldBe 5
      }
    }
  }
})

BehaviorSpec is perfect for Behavior-Driven Development (BDD), allowing us to structure tests with given, when, and then blocks. This format is great for describing the behavior of a system in a language that’s easy for non-programmers to understand.

But remember…

While it’s great for BDD, it might not fit all testing styles. If your team doesn’t use BDD or if you’re writing tests that don’t naturally fit into the given/when/then format, other spec styles might be more suitable.

In summary:

• StringSpec: Best for linear, independent test cases. It lacks support for grouped or nested tests.
• FunSpec: Allows grouping of tests into blocks using the context method, ideal for sharing setup/teardown logic. However, it does not support nested test groups.
• FreeSpec: Enables creation of nested test groups, perfect for complex test scenarios. The syntax can be verbose, and it might be overkill for simpler tests.
• BehaviorSpec: Excellent for Behavior-Driven Development (BDD) with given, when, and then blocks. It may not fit all testing styles, especially if your team doesn’t use BDD or if tests don’t naturally fit into the given/when/then format.

Remember, choosing the right spec style for your tests depends on your specific needs and the nature of the tests you’re writing. There’s no one-size-fits-all answer. Happy testing!