The object keyword in Kotlin creates a singleton in a very convenient way. It can be used, for example, as a state of an operation. Spock is one of the most expressive and readable test frameworks available in the Java ecosystem. Let's see how Kotlin's object can be used in Spock tests.

What Do We Want to Test?

We have a single method, validate, in our Validator interface that returns validation status: Ok or Error.

sealed class ValidationStatus
object Ok : ValidationStatus()
object Error : ValidationStatus()


interface Validator<T> {
    fun validate(value: T): ValidationStatus
}

We also provide a simple implementation of this interface:

class AdultValidator : Validator<Int> {
    override fun validate(value: Int) = if (value >= 18) Ok else Error
}

How to Test it With Spock?

First: The Silly Approach

First, let's write a parameterized test for the validator:

AdultValidator sut = new AdultValidator()

def 'should validate age #age'() {
    expect:
        sut.validate(age) == result
    where:
        age | result
        0   | Error
        17  | Error
        18  | Ok
        19  | Ok
}

We expect it to pass, but it fails... Error and Ok are classes in the code above.

Second: The Naive Approach

We need instances instead, so we modify the test a little:

def 'should validate age #age'() {
    expect:
        sut.validate(age) == result
    where:
        age | result
        0   | new Error()
        17  | new Error()
        18  | new Ok()
        19  | new Ok()
}

And again, this one fails as well. Why? Because the Error and Ok classes do not have overridden equals methods. But why? We expect Kotlin objects (those created with the object keyword, not plain objects) to have it implemented correctly. What is more, it works correctly in Kotlin:

fun isOk(status:ValidationStatus) = status == Ok

Third: The Correct Approach

Let's look into the class file:

$ javap com/github/alien11689/testingkotlinwithspock/Ok.class
Compiled from "Validator.kt"
public final class com.github.alien11689.testingkotlinwithspock.Ok extends com.github.alien11689.testingkotlinwithspock.ValidationStatus {
    public static final com.github.alien11689.testingkotlinwithspock.Ok INSTANCE;
    static {};
}

If we want to access the real object that Kotlin uses in such a comparison, then we should access the class static property INSTANCE:

def 'should validate age #age'() {
    expect:
        sut.validate(age) == result
    where:
        age | result
        0   | Error.INSTANCE
        17  | Error.INSTANCE
        18  | Ok.INSTANCE
        19  | Ok.INSTANCE
}

Now the test passes.

Fourth: An Alternative Approach

We can also check the method result without a specific instance of the object class and use instanceof or Class#isAssignableFrom instead.

def 'should validate age #age'() {
    when:
        ValidationStatus result = sut.validate(age)
    then:
        result.class.isAssignableFrom(expected)
    where:
        age | expected
        0   | Error
        17  | Error
        18  | Ok
        19  | Ok
}

Show me the Code

The code is available here.