1. Idiomatic Kotlin
(with Android flavouring)
Orion Health Auckland 24th June 2016

2. Idiomatic:
Pronunciation: /ˌɪdɪəˈmatɪk/
ADJECTIVE
1 Using, containing, or denoting expressions that are
natural to a native speaker

3. Or: Learning how to write
Kotlin-y Kotlin instead of
Java-y Kotlin

4. Example:
FragmentPagerAdapter
with 3 static pages

6. Java version
DRY violation!
Meaningful information

8. [
]
Objective (pseudo-code)
• Pass just the meaningful info to a new adapter
• Reduce noise/kill boilerplate
• Don’t repeat ourselves

9. What is this as actual code?
“A list of things that return Fragments”
Java = “A list of objects that return Fragments”
[
]

10. Java attempt
Introduce a SAM (Single Abstract Method) interface:

11. DRY

12. Let’s see the calling code
(e.g. Activity with a
ViewPager)…

14. • Pass just the meaningful info to a new adapter
• Reduce noise/kill boilerplate
• Don’t repeat ourselves
Java version
✔
✔
❌

15. Java version
• Achieved DRY but…
• Far more boilerplate than the original!
• And worse, boilerplate is in calling code rather than
hidden in implementation (anonymous inner
classes)
• Conclusion: not worth it. Stick with adapter coupled
to concrete fragments

16. Kotlin version

17. What is this as actual code?
“A list of things that return Fragments”
Kotlin = “A list of functions that return Fragments”
[
]

18. “A list of functions that return
Fragments”
What does
look like in Kotlin?

19. Function that takes nothing and returns a
Fragment:

20. List of functions that takes nothing and
return a Fragment:

22. Lambda syntax to kill more
boilerplate:

23. Type inference to kill more
boilerplate:

26. Refactor constructor to
accept a list of funcs

28. Replaced .get(n) call with index operator [n]
Replaced explicit .invoke() call with parentheses

29. Curly braces and return statement replaced with =

30. Return types removed, they can be inferred!

31. Let’s see the calling code
(e.g. Activity with a
ViewPager)…

33. • Pass just the meaningful info to a new adapter
• Reduce noise/kill boilerplate
• Don’t repeat ourselves
Kotlin version
✔
✔
✔

34. Recap
• Function types - passing functions as parameters
• Lambda (function literal) syntax
{ OnboardingFragment() }
• Type inference
• Indexing operator .get(n) -> []
• Invoke operator .invoke() -> ()

35. Conclusion
• Reduce your (Java-imposed) tolerance to
boilerplate. It’s even more of a code smell in Kotlin!
• Lots of language tools to kill boilerplate!
• Passing functions around is new territory, be
creative

36. Matthew Clarke
Oblig twitter handle: @kiwiandroiddev
References:
https://kotlinlang.org/docs/reference/operator-
overloading.html
Thank you