kotlin and ArgumentCaptor - IllegalStateException

Question

I have a problem with capturing the Class argument via ArgumentCaptor. My test class looks like this:

@RunWith(RobolectricGradleTestRunner::class)
@Config(sdk = intArrayOf(21), constants = BuildConfig::class)
class MyViewModelTest {
    @Mock
    lateinit var activityHandlerMock: IActivityHandler;

    @Captor
    lateinit var classCaptor: ArgumentCaptor<Class<BaseActivity>>

    @Captor
    lateinit var booleanCaptor: ArgumentCaptor<Boolean>

    private var objectUnderTest: MyViewModel? = null

    @Before
    fun setUp() {
        initMocks(this)
        ...
        objectUnderTest = MyViewModel(...)
    }

    @Test
    fun thatNavigatesToAddListScreenOnAddClicked(){
        //given

        //when
        objectUnderTest?.addNewList()

        //then
        verify(activityHandlerMock).navigateTo(classCaptor.capture(), booleanCaptor.capture())
        var clazz = classCaptor.value
        assertNotNull(clazz);
        assertFalse(booleanCaptor.value);
    }
}

When I run the test, following exception is thrown:
java.lang.IllegalStateException: classCaptor.capture() must not be null
Is it possible to use argument captors in kotlin?

========= UPDATE 1:
Kotlin: 1.0.0-beta-4584
Mockito: 1.10.19
Robolectric: 3.0

========= UPDATE 2:
Stacktrace:

java.lang.IllegalStateException: classCaptor.capture() must not be null

at com.example.view.model.ShoplistsViewModelTest.thatNavigatesToAddListScreenOnAddClicked(ShoplistsViewModelTest.kt:92)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:606)
at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50)
at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47)
at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
at org.junit.internal.runners.statements.RunBefores.evaluate(RunBefores.java:26)
at org.robolectric.RobolectricTestRunner$2.evaluate(RobolectricTestRunner.java:251)
at org.robolectric.RobolectricTestRunner.runChild(RobolectricTestRunner.java:188)
at org.robolectric.RobolectricTestRunner.runChild(RobolectricTestRunner.java:54)
at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
at org.robolectric.RobolectricTestRunner$1.evaluate(RobolectricTestRunner.java:152)
at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
at org.junit.runner.JUnitCore.run(JUnitCore.java:137)
at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:69)
at com.intellij.rt.execution.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:234)
at com.intellij.rt.execution.junit.JUnitStarter.main(JUnitStarter.java:74)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:606)
at com.intellij.rt.execution.application.AppMain.main(AppMain.java:144)

Answer 1

From this blog

"Getting matchers to work with Kotlin can be a problem. If you have a method written in kotlin that does not take a nullable parameter then we cannot match with it using Mockito.any(). This is because it can return void and this is not assignable to a non-nullable parameter. If the method being matched is written in Java then I think that it will work as all Java objects are implicitly nullable."

A wrapper function is needed that returns ArgumentCaptor.capture() as nullable type.

Add the following as a helper method to your test

fun <T> capture(argumentCaptor: ArgumentCaptor<T>): T = argumentCaptor.capture()

Please see, MockitoKotlinHelpers.kt provided by Google in the Android Architecture repo for reference. the capture function provides a convenient way to call ArgumentCaptor.capture(). Call

verify(activityHandlerMock).navigateTo(capture(classCaptor), capture(booleanCaptor))

Update: If the above solution does not work for you, please check Roberto Leinardi's solution in the comments below.