How to mock a Kotlin singleton object?

You can mock Object without any extra library, by using class delegates.

Here is my proposal

val someObjectDelegate : SomeInterface? = null

object SomeObject: by someObjectDelegate ?: SomeObjectImpl

object SomeObjectImpl : SomeInterface {

    fun someFun() {
        println("SomeObjectImpl someFun called")
    }
}

interface SomeInterface {
    fun someFun()
}

In your tests you can set delegate object that will change behaviour, otherwise it will use it's real implementation.

@Beofre
fun setUp() {
  someObjectDelegate = object : SomeInterface {
      fun someFun() {
          println("Mocked function")
      }
  }
  // Will call method from your delegate
  SomeObject.someFun()
}

Of course names above are bad, but for the sake of an example it shows the purpose.

After SomeObject is initialised delegate will handle all the functions.
More you can find in official documentation

There's a very nice mocking library for Kotlin - Mockk, which allows you to mock objects, the exact same way you're desiring.

As of its documentation:

---

Objects can be transformed to mocks following way:

object MockObj {
  fun add(a: Int, b: Int) = a + b
}

mockkObject(MockObj) // aplies mocking to an Object

assertEquals(3, MockObj.add(1, 2))

every { MockObj.add(1, 2) } returns 55

assertEquals(55, MockObj.add(1, 2))

To revert back use unmockkAll or unmockkObject:

@Before
fun beforeTests() {
    mockkObject(MockObj)
    every { MockObj.add(1,2) } returns 55
}

@Test
fun willUseMockBehaviour() {
    assertEquals(55, MockObj.add(1,2))
}

@After
fun afterTests() {
    unmockkAll()
    // or unmockkObject(MockObj)
}

Despite Kotlin language limits you can create new instances of objects if testing logic needs that:

val newObjectMock = mockk<MockObj>()

Just make you object implement an interface, than you can mock you object with any mocking library. Here example of Junit + Mockito + Mockito-Kotlin:

import com.nhaarman.mockito_kotlin.mock
import com.nhaarman.mockito_kotlin.whenever
import org.junit.Assert.assertEquals
import org.junit.Test

object SomeObject : SomeInterface {
    override fun someFun():String {
        return ""
    }
}

interface SomeInterface {
    fun someFun():String
}

class SampleTest {

    @Test
    fun test_with_mock() {
        val mock = mock<SomeInterface>()

        whenever(mock.someFun()).thenReturn("42")

        val answer = mock.someFun()

        assertEquals("42", answer)
    }
}

Or in case if you want mock SomeObject inside callerFun:

import com.nhaarman.mockito_kotlin.mock
import com.nhaarman.mockito_kotlin.whenever
import org.junit.Assert.assertEquals
import org.junit.Test

object SomeObject : SomeInterface {
    override fun someFun():String {
        return ""
    }
}

class Caller(val someInterface: SomeInterface) {
    fun callerFun():String {
        return "Test ${someInterface.someFun()}"
    }
}

// Example of use
val test = Caller(SomeObject).callerFun()

interface SomeInterface {
    fun someFun():String
}

class SampleTest {

    @Test
    fun test_with_mock() {
        val mock = mock<SomeInterface>()
        val caller = Caller(mock)

        whenever(mock.someFun()).thenReturn("42")

        val answer = caller.callerFun()

        assertEquals("Test 42", answer)
    }
}

Besides using Mockk library, which is quite convenient, one could mock an object simply with Mockito and reflection. A Kotlin object is just a regular Java class with a private constructor and an INSTANCE static field, with reflection one can replace the value of INSTANCE with a mocked object. After the test the original should be restored so that the change won't affect other tests

Using Mockito Kotlin (one needs to add an extension configuration as described here to mock final classes):

testCompile "com.nhaarman:mockito-kotlin:1.5.0"

A first fun could replace the value of the static INSTANCE field in the object class and return the previous value

fun <T> replaceObjectInstance(clazz: Class<T>, newInstance: T): T {

    if (!clazz.declaredFields.any {
                it.name == "INSTANCE" && it.type == clazz && Modifier.isStatic(it.modifiers)
            }) {
        throw InstantiationException("clazz ${clazz.canonicalName} does not have a static  " +
                "INSTANCE field, is it really a Kotlin \"object\"?")
    }

    val instanceField = clazz.getDeclaredField("INSTANCE")
    val modifiersField = Field::class.java.getDeclaredField("modifiers")
    modifiersField.isAccessible = true
    modifiersField.setInt(instanceField, instanceField.modifiers and Modifier.FINAL.inv())

    instanceField.isAccessible = true
    val originalInstance = instanceField.get(null) as T
    instanceField.set(null, newInstance)
    return originalInstance
}

Then you could have a fun that will create a mock instance of the object and replace the original value with the mocked one, returning the original so that it can be reset later

fun <T> mockObject(clazz: Class<T>): T {
    val constructor = clazz.declaredConstructors.find { it.parameterCount == 0 }
            ?: throw InstantiationException("class ${clazz.canonicalName} has no empty constructor, " +
                    "is it really a Kotlin \"object\"?")

    constructor.isAccessible = true

    val mockedInstance = spy(constructor.newInstance() as T)

    return replaceObjectInstance(clazz, mockedInstance)
}

Add some Kotlin sugar

class MockedScope<T : Any>(private val clazz: Class<T>) {

    fun test(block: () -> Unit) {
        val originalInstance = mockObject(clazz)
        block.invoke()
        replaceObjectInstance(clazz, originalInstance)
    }
}

fun <T : Any> withMockObject(clazz: Class<T>) = MockedScope(clazz)

And finally, given an object

object Foo {
    fun bar(arg: String) = 0
}

You could test it this way

withMockObject(Foo.javaClass).test {
    doAnswer { 1 }.whenever(Foo).bar(any())

    Assert.assertEquals(1, Foo.bar(""))
}

Assert.assertEquals(0, Foo.bar(""))