Talk about "Nothing"

Talk about "Nothing" in Kotlin愛好会 x GDG Kyushu
https://gdg.connpass.com/event/160628/

Transcript

1. Talk about “Nothing” In Kotlin Kelvin Ko @ko_kelvin
2. What is “Nothing”? • From Kotlin Doc: “ଘࡏ͠ͳ͍஋”
3. What actually it is? • A Class with private construcor • Cannot be instaniated
4. What is “Nothing”? • “A value that never exist”, sounds easy to understand • But • What is the different between Null and Nothing? “ଘࡏ͠ͳ͍஋”
5. Kotlin type system • Nothing is a “Bottom Type” • SubType of any other Types (extends all other types) • In opposite, Any is Super class of any other class (World without Null) (World with Null) Image Reference: https://medium.com/@m.sandovalcalvo/kotlin-type-system-unveiling-the-mystery-50613f0db893
6. So, “Nothing” is • Used as a Type, no instance • Bottom Type (Sub Type of any other Type)
7. Why we need this?
8. Use cases 1. Indicate function never return 2. As a “empty” Type in Generic
9. Example of Function that never return • listenIncomingConnection will run forever (May not be a good way to implement like this, just simple example)
10. Use case • Adding Nothing, IDE will know the println is unreachable This look correct, but actually this line will never run (Video example is uploaded here: https://youtu.be/fjECMd1H-hI)
11. Use case 2: Exception • After throw Exception, program will end • It is more accurate to use “Nothing” as return type to indicate it never return Assume we would Load some data from Database, and throw Exception if it return null (Video example is uploaded here: https://youtu.be/HG210SN4_iI)
12. • So IDE will warn you the code after TODO is unreachable Stdlib’s TODO() • Actually TODO() is implemented like this
13. Why we need a class that is Sub Type of any other classes? Bottom Type Image Reference: https://medium.com/@m.sandovalcalvo/kotlin-type-system-unveiling-the-mystery-50613f0db893
14. 2. Usage in Generic • Example: A Linked List implementation LinkedList Example Reference: https://www.codementor.io/@kotlin_academy/kotlin-s-nothing-its-usefulness-in-generics-j4wcu5taa
15. Linked List implementation • To represent Empty List, add a single instance object • Won’t compile • Kotlin Object cannot have type parameters.
16. Linked List implementation • How about remove Type parameter ? • Won’t compile • The reference to T is unresolved. We must specific a type for T
17. Linked List implementation • Use “Nothing” as Type • It work because Nothing is subtype of any other Type
18. Inside Kotlin Stdlib • emptyList() implementation: •
19. Conclusion • Nothing is • Subtype of any other type (Bottom Type) • Useful in Generic • Indicate function that Never return • Infinite loop • Throw exception
20. I hope you learn Nothing today Thank you for listening