I looked on the internet and found some examples of how to overcome this. I have used this library on a large project with many thousand of tests and as I updated the version of the library used different tests failed even before we started using the library.example extension github gradle groovy http io jboss kotlin library logging maven. Initially this library depended on a specific version of Kotlin and Mockito, later versions have removed the dependency on Kotlin however I am guessing that testing every combination is not possible. This section of the JUnit 5 Mockito tutorial mainly focuses on Mockito.There is no benefit here as we can overcome this issue with less than 10 lines of code. I prefer not to take a dependency unless there is a significant benefit.One possible solution would be to use a library like mockito-kotlin. If the method being matched is written in Java then I think that it will work as all Java objects are implicitly nullable. This is because it can return void and this is not assignable to a non-nullable parameter. If you have a method written in kotlin that does not take a nullable parameter then we cannot match with it using Mockito.any(). Getting matchers to work with Kotlin can be a problem.

Using kotlin and mockito together throws up some interesting problems, I have already looked at some of these. I have been using Mockito for a while and recently I have started a kotlin project. Mockito-Kotlin extensions are fairly easy and fun to use - they introduce DSL for mocks and stubs, increasing readability and helping avoid long complex mock configurations in test suite.

Mockito with kotlin