Given mocked `Foo` which has `sealed Bar` and given two different exhaustive `when` statements: ```f...

# mockk

[Kevin Worth]

Given mocked Foo which has sealed Bar and given two different exhaustive when statements:

fun Bar.baz() : String {
    when (this) {
        Bam -> "bam"
        Pow -> "pow"
    }
}

// Bar
val baz: String
    get() = when(this) {
        Bam -> "bam"
        Pow -> "pow"
    }
}

and given the mocked Foo.Bar is of type Bar_1_Proxy during the test, why does the extension function result in NoWhenBranchMatchedException but the getter results in an empty string?

[Simon Marquis]

This is expected. You can solve this by using is keyword instead:

fun Bar.baz() : String {
    when (this) {
        is Bam -> "bam"
        is Pow -> "pow"
    }
}

[Simon Marquis]

Or even better, don't mockk data classes 🙂

[Kevin Worth]

Agreed. We decided to use a factory with randomized data in the end. But, are you saying that using is will avoid the exception and both will return empty string? Or both will throw the exception?

[Simon Marquis]

It will prevent the Exception, because when uses reference identity, and mockk can't (or does not support yet) inflating such objects.
For the getter, I'm not sure, but this might reference the actual object and not the mockk proxy object.

[Simon Marquis]

It also depends how you mockk'ed it.

[Kevin Worth]

Can you elaborate on “`when` uses reference identity”? Forgive my ignorance. Am I understanding correctly that the extension will return empty string as soon as we use is?

[Simon Marquis]

in a when statement
• is Bar is similar to if (this is Bar) , it will check the instance has a compatible type
• Bar is similar to if (this === Bar) , it will check for referential equality (pointing to the same object)
https://kotlinlang.org/docs/equality.html

[Simon Marquis]

For the empty string part, I'm not able to answer without more context on how your test looks like

[Kevin Worth]

To me it makes sense that the exception is thrown, whether it checks for compatible type or referential equality, because there’s no way it can know which subtype it should match with. (I haven’t yet tried to confirm what happens with is)

What’s confusing is why the getter doesn’t throw the exception and just says, “Eh, I’ll return empty string”. 🤔

And I don’t believe that has anything to do with how the test is set up, but maybe I’m misunderstanding the question.

[Kevin Worth]

Yeah, even with putting is on all the cases, the exception is still thrown. So, maybe I misunderstood the suggestion/explanation.

[Simon Marquis]

How are the mocks created? Are they relaxed? Are they based on the abstract type?

[Kevin Worth]

Relaxed, yes.

[Simon Marquis]

Then this is probably why it returns an empty string

[Kevin Worth]

Ahh, so relaxed turns the property into empty string rather than calling the getter? 💡

[Simon Marquis]

it returns "default" values

[Simon Marquis]

if the type was String? it would probably have returned null instead