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?

Adenoidectomy

Answer 1

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))
  }
}

Adenoidectomy

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 – Adenoidectomy

Answer 2

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

Mickey

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. – Adenoidectomy

That is true, just once, something like: import org.hamcrest.CoreMatchers.is as Is – Mickey

At which point it's just a style thing. Personally, I'd rather escape the function name personally. – Adenoidectomy

Up to you. With 'aliasing' you need to this only once – Mickey

Answer 3

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.

1. equality:

   assertThat(cheese, equalTo(smelly))
   

   instead of:

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

2. 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))

Incompressible

Oh, this is actually quite nice to know. Never knew it was basically a redundant extra call. – Adenoidectomy