Mockito object always giving a NullPointer exception - unit-testing

Question

This is the class that I want to test.
open class Foo(bar :Bar){
fun someMethod() = bar.anotherMethod()
}
And I have a builder class to get Foo mock instance since Mockito cant mock constructor arguments.
open class FooBuilder{
fun makeFoo(bar:Bar) = Foo(bar)
}
Here is my approach to make the mock Objects and verify.
var fooBuilderMock = mock(FooBuilder::class)
var barMock = mock(Bar::class)
var fooMock = mock(Foo::class)
Mockito.`when`(fooBuilderMock.makeFoo(barMock)).thenReturn(fooMock)
fooMock.someMethod()
Mockito.verify(barMock, Mockito.times(1)).anotherMethod()
I am getting a null pointer exception on barMock object.

Answer 1

Don't use the Builder, there is no need for it. Additionally, don't mock the class that you are trying to test. If you do it, your test makes no sense.
class FooTest {
private val barMock = mock(Bar::class)
private lateinit var foo: Foo
#BeforeEach
fun setUp() {
MockitoAnnotations.initMocks(this)
foo = Foo(barMock)
}
#Test
fun `Your test name`() {
// Arrange
Mockito.`when`(barMock.anotherMethod()).thenReturn(// whatever it should return)
// Act
fooMock.someMethod()
// Assert
Mockito.verify(barMock, Mockito.times(1)).anotherMethod()
}
}
As a side note consider taking a look at mockk and using it instead of Mockito. It is implemented in kotlin and thus supports it since day 1. With it, your test would look similar but follow more the kotlin "style":
class FooTest {
#MockK
private lateinit var barMock = mock(Bar::class)
private lateinit var foo: Foo
#BeforeEach
fun setUp() {
MockitoAnnotations.initMocks(this)
foo = Foo(barMock)
}
#Test
fun `Your test name`() {
// Arrange
every { barMock.anotherMethod() } returns //whatever it should return
// Act
foo.someMethod()
// Assert
verify(exactly = 1) { barMock.anotherMethod() }
}
}