How to compile Kotlin unit test code that uses hamcrest 'is'

Question

I want to write a unit test for my Kotlin code and use junit/hamcrest matchers, I want to use the is method, but it is a reserved word in Kotlin.

How can I get the following to compile?

class testExample{
  @Test fun example(){
    assertThat(1, is(equalTo(1))
  }
}

Currently my IDE, InteliJ is highlighting that as a compilation error, saying it is expecting a ) after is?

David Soroko · Answer 1 · 2023-07-10T08:51:44.907

46

You can alias is to Is (for example) in the import statement using the as keyword.

For example:

 import org.hamcrest.CoreMatchers.`is` as Is

See https://kotlinlang.org/docs/packages.html#imports

edited Jul 10 '23 at 08:51

answered Oct 14 '16 at 11:34

David Soroko

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You would still need to reference the `is` function when you import it, so you would still need to escape it. But this is a nice trick that would help save you from escaping it all over the place. – thecoshman Oct 14 '16 at 13:07
That is true, just once, something like: import org.hamcrest.CoreMatchers.`is` as Is – David Soroko Oct 14 '16 at 13:28
At which point it's just a style thing. Personally, I'd rather escape the function name personally. – thecoshman Oct 14 '16 at 16:06
Up to you. With 'aliasing' you need to this only once – David Soroko Oct 14 '16 at 16:11

thecoshman · Accepted Answer · 2017-10-26T12:16:49.257

40

In Kotlin, is is a reserved word . To get around this you need to escape the code using backticks, so the following will allow you to compile the code:

class testExample{
  @Test fun example(){
    assertThat(1, `is`(equalTo(1))
  }
}

edited Oct 26 '17 at 12:16

answered Oct 13 '16 at 16:05

thecoshman

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You might be able to make use of infix operator maybe to allow `assertThat(1 is equalTo(1))`... but you would have to wrap a good bit I think. Maybe with the combination of some DSL style work you get to the point one can write `assertThat{ N isEqualTo 1}` but I'm not inclined to put that effort in... yet – thecoshman Oct 09 '17 at 10:47

naXa stands with Ukraine · Answer 3 · 2019-03-20T14:01:50.620

As others pointed out, in Kotlin, is is a reserved word (see Type Checks). But it's not a big problem with Hamcrest since is function is just a decorator. It's used for better code readability, but it's not required for proper functioning.

You are free to use a shorter Kotlin-friendly expression.

equality:

assertThat(cheese, equalTo(smelly))

instead of:

assertThat(cheese, `is`(equalTo(smelly)))

matcher decorator:

assertThat(cheeseBasket, empty())

instead of:

assertThat(cheeseBasket, `is`(empty()))

Another frequently used Hamcrest matcher is a type-check like

assertThat(cheese, `is`(Cheddar.class))

It's deprecated and it's not Kotlin-friendly. Instead, you're advised to use one of the following:

assertThat(cheese, isA(Cheddar.class))
assertThat(cheese, instanceOf(Cheddar.class))

Oh, this is actually quite nice to know. Never knew it was basically a redundant extra call. — thecoshman, Mar 20 '19 at 23:18

How to compile Kotlin unit test code that uses hamcrest 'is'

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