Assert-That

Kotlin multiplatform test assertion library inspired by Hamkrest, itself a Kotlinisation of Hamcrest

Usage

Provides a Matcher interface and an assertThat function, and a selection of Matcher implementations that are intended to compare a value to an expectation and manufacture a helpful diagnostic if it does not.

Matching on equality

For example, the matcher produced by equalTo("foo") will match only the string "foo". If you write:

assertThat("bar", equalTo("foo"))

You will get an assertion error like so:

AssertionError: expected: a value that is equal to "foo"
but was "bar"

equalTo is unusual in that it will match any type of object (because all objects can be compared for equality), including nullable ones. Matcher is parameterised, and assertions may be more type-safe.

Matching on features

You can match a property of an object using a has matcher:

assertThat("quux", has(String::length, equalTo(3)))

produces:

AssertionError: expected: a value that has length that is equal to 3
but had length that was 4

You can either define your own extension property, or use an expanded version to extract a feature value:

assertThat("quux", has("length", { it.length }, equalTo(3)))

Combining matchers

Matchers can be combined with the and and or infix functions, and inverted with the negation operator:

Negation

assertThat("mumble", !equalTo("mumble"))

AssertionError: expected: a value that is not equal to "mumble"
but is equal to "mumble"

Combination

assertThat("womble", has(String::length, equalTo(5)) or has(String::length, equalTo(7)))

AssertionError: expected: a value that has length that is equal to 5 or has length that is equal to 7
but had length that was 6

Or more simply:

assertThat("womble", has(String::length, equalTo(5) or equalTo(7)))

AssertionError: expected: a value that has length that is equal to 5 or is equal to 7
but had length that was 6

Get the library

Assert-That is published on Maven Central. You need something like this in build.gradle or build.gradle.kts:

repositories {
    mavenCentral()
}
dependencies {
    testImplementation("org.araqnid.kotlin.assert-that:assert-that:0.1.1")
}