How should I test Kotlin extension functions?

java unit-testing testing kotlin language-interoperability
TheTechWolf picture TheTechWolf · Feb 21, 2017 · Viewed 9.2k times · Source

Can somebody tell me how should I unit-test extension functions in Kotlin? Since they are resolved statically should they be tested as static method calls or as non static ? Also since language is fully interoperable with Java, how Java unit test for Kotlin extension functions should be performed ?

Answer

JB Nizet picture JB Nizet · Feb 21, 2017

Well, to test a method, whether static or not, you call it as real code would do, and you check that it does the right thing.

Assuming this extension method, for example, is defined in the file com/foo/Bar.kt:

fun String.lengthPlus1(): Int {
    return this.length + 1
}

If you write your test in Kotlin (which you would typically do to test Kotlin code), you would write

assertThat("foo".lengthPlus1()).isEqualTo(4);

If you write it in Java (but why would you do that?)

assertThat(BarKt.lengthPlus1("foo")).isEqualTo(4);

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