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assertions for kotlin inspired by assertj


repositories {

dependencies {
  testCompile 'com.willowtreeapps.assertk:assertk:0.9'


Simple usage is to wrap the value you are testing in assert() and call assertion methods on the result.

import assertk.assert
import assertk.assertions.*

class PersonTest {
    val person = Person(name = "Bob", age = 18)

    fun testName() {
        // -> expected:<["Alice"]> but was:<["Bob"]>

    fun testAge() {
        assert("age", person.age).isGreaterThan(20)
        // -> expected [age] to be greater than:<20> but was:<10>

You can see all built-in assertions in the docs.


Since null is a first-class concept in kotlin’s type system, you need to be explicit in your assertions.

val nullString: String? = null

will not compile, since hasLength() only makes sense on non-null values. You can use isNotNull() which takes an optional lambda to handle this.

val nullString: String? = null
assert(nullString).isNotNull {
// -> expected to not be null

This will first ensure the string is not null before running any other checks.

Multiple assertions

You can assert multiple things on a single value by providing a lambda as the second argument. All assertions will be run even if the first one fails.

val string = "Test"
assert(string) {
// -> The following 2 assertions failed:
//    - expected to start with:<"L"> but was:<"Test">
//    - expected to have length:<3> but was:<"Test"> (4)

You can wrap multiple assertions in an assertAll to ensure all of them get run, not just the first one.

assertAll {
// -> The following 2 assertions failed:
//    - expected to be true
//    - expected to be false


If you expect an exception to be thrown, you have a couple of options:

The first is to wrap in a catch block to store the result, then assert on that.

val exception = catch { throw Exception("error") }
assert(exception).isNotNull {
// -> expected [message] to be:<["wrong"]> but was:<["error"]>

Your other option is to use an assert with a single lambda arg to capture the error.

assert {
    throw Exception("error")
}.thrownError {
// -> expected [message] to be:<["wrong"]> but was:<["error"]>

This method also allows you to assert on return values.

assert { 1 + 1 }.returnedValue {
// -> expected to be negative but was:<2>

Table Assertions

If you have multiple sets of values you want to test with, you can create a table assertion.

tableOf("a", "b", "result")
    .row(0, 0, 1)
    .row(1, 2, 4)
    .forAll { a, b, result ->
        assert(a + b).isEqualTo(result)
// -> the following 2 assertions failed:
//    on row:(a=<0>,b=<0>,result=<1>)
//    - expected:<[1]> but was:<[0]>
//    on row:(a=<1>,b=<2>,result=<4>)
//    - expected:<[4]> but was:<[3]>

Up to 4 columns are supported.

Custom Assertions

One of the goals of this library is to make custom assertions easy to make. All assertions are just extension methods.

fun Assert<Person>.hasAge(expected: Int) {
    assert("age", actual.age).isEqualTo(expected)

// -> expected [age]:<10> but was:<18>

If you want to customize the message, you usually want to use expected() and show().

fun Assert<Person>.hasAge(expected: Int) {
    if (actual.age == expected) return
    expected("age:${show(expected)} but was age:${show(actual.age)}")

// -> expected age:<10> but was age:<18>

Important Note:

You can’t assume that expected or fail will stop execution in a custom assertion. This is because it might be used in an assertAll block which ensures all assertions are run. Make sure you return early if you want execution to always stop.

fun Assert<...>.myAssertion() {
  while (true) {
    expected("to be something but wasn't")
    break // Need to break out of the loop!

Contributing to assertk

Contributions are more than welcome! Please see the Contributing Guidelines and be mindful of our Code of Conduct.

compile "com.willowtreeapps.assertk:assertk:0.9"

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