NullPointerException is thrown when stubbing a generic function using the lamda approach

[Nejat]

here are examples to illustrate the issue ...

package bug.stubbing.lambda

import com.nhaarman.mockito_kotlin.mock
import com.nhaarman.mockito_kotlin.whenever
import com.winterbe.expekt.should
import org.junit.jupiter.api.Test

class LamdaStubbingIntialization { 

    // generic interface to illustrate stubbing issue
    interface GenericInterface<out T> {

        fun NonGeneric(): Double

        fun ASecondNonGeneric(): String

        fun Generic(): T
    }

    // stubbing a generic function through the lamda method fails
    // with a NullPointerException
    @Test
    fun FailedStubbingGenerics () {

        val mocked: GenericInterface<Int>

        try {
            mocked = mock {
                on { NonGeneric() }.thenReturn(-1.0)

                on { ASecondNonGeneric() }.thenReturn("multiple stubbing is not a problem")

                on { Generic() }.thenReturn(1)
            }
        } catch (ex: NullPointerException) {
            return@FailedStubbingGenerics
        }

        mocked.NonGeneric().should.be.equal(-1.0)

        mocked.ASecondNonGeneric().should.be.equal("multiple stubbing is not a problem")

        mocked.Generic().should.be.equal(1)

        // just signaling that it worked
        "unexpected pass".should.be.equal("means the mock worked")
    }

    // subbing a non-generic method of a generic interface is fine
    @Test
    fun PassingIncompleteStubbingGenerics () {

        val mocked: GenericInterface<Int> = mock {
            on { NonGeneric() }.thenReturn(-1.0)
        }

        mocked.NonGeneric().should.be.equal(-1.0)

        mocked.Generic().should.be.`null`
    }

    // stubbing a generic function through the non-lamda method works
    // so the problem is not stubbing a generic method
    @Test
    fun PassingStubbingGenerics () {

        val mocked: GenericInterface<Int> = mock()

        whenever(mocked.NonGeneric()).thenReturn(-1.0)
        whenever(mocked.Generic()).thenReturn(1)

        mocked.NonGeneric().should.be.equal(-1.0)

        mocked.Generic().should.be.equal(1)
    }
}

[nhaarman]

What is the stack trace of the exception?

[Nejat]

after removing the try/catch from the FailedStubbingGenerics example, the following stack trace is produced

java.lang.NullPointerException
    at bug.stubbing.lambda.LamdaStubbingIntialization$FailedStubbingGenerics$1$3.invoke(LamdaStubbingIntialization.kt:45)
    at bug.stubbing.lambda.LamdaStubbingIntialization$FailedStubbingGenerics$1$3.invoke(LamdaStubbingIntialization.kt:8)
    at com.nhaarman.mockito_kotlin.KStubbing.on(Mockito.kt:96)
    at bug.stubbing.lambda.LamdaStubbingIntialization.FailedStubbingGenerics(LamdaStubbingIntialization.kt:45)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:498)
    at org.junit.platform.commons.util.ReflectionUtils.invokeMethod(ReflectionUtils.java:252)
    at org.junit.jupiter.engine.execution.ExecutableInvoker.invoke(ExecutableInvoker.java:114)
    at org.junit.jupiter.engine.descriptor.MethodTestDescriptor.lambda$invokeTestMethod$7(MethodTestDescriptor.java:210)
    at org.junit.jupiter.engine.execution.ThrowableCollector.execute(ThrowableCollector.java:40)
    at org.junit.jupiter.engine.descriptor.MethodTestDescriptor.invokeTestMethod(MethodTestDescriptor.java:206)
    at org.junit.jupiter.engine.descriptor.MethodTestDescriptor.execute(MethodTestDescriptor.java:155)
    at org.junit.jupiter.engine.descriptor.MethodTestDescriptor.execute(MethodTestDescriptor.java:63)
    at org.junit.platform.engine.support.hierarchical.HierarchicalTestExecutor.lambda$execute$0(HierarchicalTestExecutor.java:81)
    at org.junit.platform.engine.support.hierarchical.SingleTestExecutor.executeSafely(SingleTestExecutor.java:66)
    at org.junit.platform.engine.support.hierarchical.HierarchicalTestExecutor.execute(HierarchicalTestExecutor.java:77)
    at org.junit.platform.engine.support.hierarchical.HierarchicalTestExecutor.lambda$execute$0(HierarchicalTestExecutor.java:88)
    at org.junit.platform.engine.support.hierarchical.SingleTestExecutor.executeSafely(SingleTestExecutor.java:66)
    at org.junit.platform.engine.support.hierarchical.HierarchicalTestExecutor.execute(HierarchicalTestExecutor.java:77)
    at org.junit.platform.engine.support.hierarchical.HierarchicalTestExecutor.lambda$execute$0(HierarchicalTestExecutor.java:88)
    at org.junit.platform.engine.support.hierarchical.SingleTestExecutor.executeSafely(SingleTestExecutor.java:66)
    at org.junit.platform.engine.support.hierarchical.HierarchicalTestExecutor.execute(HierarchicalTestExecutor.java:77)
    at org.junit.platform.engine.support.hierarchical.HierarchicalTestExecutor.execute(HierarchicalTestExecutor.java:51)
    at org.junit.platform.engine.support.hierarchical.HierarchicalTestEngine.execute(HierarchicalTestEngine.java:43)
    at org.junit.platform.launcher.core.DefaultLauncher.execute(DefaultLauncher.java:124)
    at org.junit.platform.launcher.core.DefaultLauncher.execute(DefaultLauncher.java:84)
    at com.intellij.junit5.JUnit5IdeaTestRunner.startRunnerWithArgs(JUnit5IdeaTestRunner.java:42)
    at com.intellij.rt.execution.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:262)
    at com.intellij.rt.execution.junit.JUnitStarter.main(JUnitStarter.java:84)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:498)
    at com.intellij.rt.execution.application.AppMain.main(AppMain.java:147)