Unit testing in Kotlin provides benefits like documenting code, promoting good design, and creating a safety net around code. Tests can be improved by using features like multiline strings, factory functions with named arguments, better mocking with Mockito-Kotlin, and assertions with Kluent. Migrating existing Java unit tests to Kotlin allows leveraging these modern features for more expressive and readable tests.

1. Unit Testing in Kotlin
Egor Andreevici
Software Developer @ 1&1
@EgorAnd

2. Android: In The Java Zone

3. Java Kotlin
+ Official
+ Stable
- Old
- Verbose
+ Modern
+ Expressive
- Stable?
- Support?

4. Starting with Kotlin on Android?
Start small!
Start with unit tests!

5. Unit Tests Are Good

6. Benefits of Unit Tests
Provide a safety net around your code
Document your code… what?
Promote good design practices

7. Bad Documentation
/**
* Get a subview of the root view at a certain position.
*
* @param position Position, 0 <= position < root child count
* @return Subview at given position, or {@code null} if position is out of
* bounds
*/
public View getViewAtPosition(int position) {
if (position >= 0 && position < root.getChildCount()) {
return root.getChildAt(position);
}
return null;
}

8. Bad Documentation
/**
* Get a subview of the root view at a certain position.
*
* @param position Position, 0 <= position < root child count
* @return Subview at given position, or {@code null} if position is out of
* bounds
*/
public View getViewAtPosition(int position) {
if (position >= 0 && position < root.getChildCount()) {
return root.getChildAt(position);
}
throw new IndexOutOfBoundsException("Invalid position " + position);
}

9. A Better Test
@Test public void shouldReturnNullIfCalledWithInvalidPosition() {
doReturn(0).when(mockParent).getChildCount();
assertThat(docs.getViewAtPosition(0)).isNull();
}

10. EvenBetterTest.kt

11. Kotlinize
@Test fun shouldReturnNullIfCalledWithInvalidPosition() {
doReturn(0).`when`<ViewGroup>(mockParent).childCount
assertThat(docs.getViewAtPosition(0)).isNull()
}

12. Better Method Names
@Test fun `Should return null if called with invalid position`() {
doReturn(0).`when`<ViewGroup>(mockParent).childCount
assertThat(docs.getViewAtPosition(0)).isNull()
}

13. Better Mocking with Mockito-Kotlin
● Reified types for mock creation
● Fluent method mocking syntax
● More niceties

14. Better Mocking with Mockito-Kotlin
private var mockParent = mock<ViewGroup>()

15. Better Mocking with Mockito-Kotlin
// private var mockParent = mock<ViewGroup>()
private var mockParent: ViewGroup = mock {
on { childCount } doReturn 0
}

16. Better Mocking with Mockito-Kotlin
@Test fun `Should return null if called with invalid position`() {
whenever(mockParent.childCount) doReturn 0
assertThat(docs.getViewAtPosition(0)).isNull()
}

17. How Does It Work?
inline fun <reified T : Any> mock(): T = Mockito.mock(T::class.java)

18. Better Assertions with Kluent
@Test fun `Should return null if called with invalid position`() {
whenever(mockParent.childCount) doReturn 0
docs.getViewAtPosition(0) `should be` null
}

19. How Does It Work?
import org.junit.Assert.*
infix fun Any?.`should be`(theOther: Any?) = assertSame(theOther, this)
infix fun Any?.shouldBe(theOther: Any?) = this `should be` theOther

20. Other Goodies

21. Multiline Strings
val json =
"""
{
"name": "Bob",
"age": 57
}
"""
@Test fun `Should parse JSON correctly`() {
gson.fromJson(json, Person::class.java) `should equal` Person("Bob", 57)
}

22. “with” for Multiple Assertions
@Test fun `Should parse JSON correctly #2`() {
val person = gson.fromJson(json, Person::class.java)
with(person) {
name `should equal` "Bob"
age `should equal` 57
}
}

23. Factory Functions with Named Args
public class Person {
private String name;
private int age;
private boolean isAKotlinProgrammer;
public Person(String name, int age, boolean isAKotlinProgrammer) {
this.name = name;
this.age = age;
this.isAKotlinProgrammer = isAKotlinProgrammer;
}
…
}

24. Factory Functions with Named Args
fun person(
name: String = "",
age: Int = 0,
isAKotlinProgrammer: Boolean = false) =
Person(name, age, isAKotlinProgrammer)
@Test fun `Should ...`() {
val person = person(name = "Bob", age = 57)
...
}

25. @JvmOverloads
@JvmOverloads
fun person(
name: String = "",
age: Int = 0,
isAKotlinProgrammer: Boolean = false) =
Person(name, age, isAKotlinProgrammer)

26. ...and more magic in store!

27. Thanks!
Egor Andreevici
Software Developer @ 1&1
@EgorAnd