I am staring to get to grips with Kotlin and I have writted about my challenges in starting a new Kotlin project. The project has been progressing well and hopefully will be published this year. I have tripped over an issue in writing unit tests.

In Java classes are only sealed if you use the keyword final. By contrast in Kotlin classes are sealed by default and can only be inherited or overridden if we use the keyword open.

That was perfectly fine while I was writing the code I produced classes like this

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class LocalBinder : Binder() {
	val service: IGpsLoggerService
		get() = this@GpsLoggerService
}

However when I was trying to write a test and provide a mock IGpsLoggerService to the test I ran into problems. I have to say most of the problems revolve around the very poor state of Mockito/Kotlin error messages, I hope this will improve as Kotlin becomes more mainstream in Android development.

In the setup for tests that relied on the GPS service I wrote this

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mockservice = mock(IGpsLoggerService::class.java)
mockbinder = mock(GpsLoggerService.LocalBinder::class.java)
Mockito.`when`(mockbinder.service).thenReturn(mockservice)

However when I tried to run this I got this error

org.mockito.exceptions.base.MockitoException: 
Cannot mock/spy class com.andrewandderek.trailblazer.service.gpslogger.GpsLoggerService$LocalBinder
Mockito cannot mock/spy because :
 - final class

OK, I though that makes sense I need to make the LocalBinder class open. I did that but then I got the following error

org.mockito.exceptions.misusing.CannotStubVoidMethodWithReturnValue: 
'debug' is a *void method* and it *cannot* be stubbed with a *return value*!
Voids are usually stubbed with Throwables:
    doThrow(exception).when(mock).someVoidMethod();
If you need to set the void method to do nothing you can use:
    doNothing().when(mock).someVoidMethod();
For more information, check out the javadocs for Mockito.doNothing().

This made no sense at all, there is no method called debug. I tried to change the property into a method, I tried changing to return type to be a simple Intbut no matter what the method was called or returned I always got the same error. In the end I guessed that the method needed to be open as well. like this

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open class LocalBinder : Binder() {
	open val service: IGpsLoggerService
		get() = this@GpsLoggerService
}

Another approach, which is the one I settled on, was to enable the experimental feature in Mockito to allow mocking of final classes. I know this is experimental but it seems to work for my project, I followed the instructions in this blog.