Introduction to Kotlin inline classes

Talk at https://www.meetup.com/pl-PL/GDG-Krakow/events/266069974/

Transcript

1. Introduction to Kotlin inline classes Konrad Szewczuk Android dev @ Miquido @KonradSzewczuk
2. Why inline classes?
11. “Sometimes it is necessary for business logic to create a wrapper around some type. However, it introduces runtime overhead due to additional heap allocations. Moreover, if the wrapped type is primitive, the performance hit is terrible, because primitive types are usually heavily optimized by the runtime, while their wrappers don't get any special treatment. To solve such issues, Kotlin introduces a special kind of class called an inline class” https://kotlinlang.org/docs/reference/inline-classes.html
12. How does inline classes work?
13. When you instantiate an inline class like this: the class isn’t actually instantiated in the compiled code. In fact, as far as the JVM is concerned, you’ve simply written this:
14. Coding time!
15. • At runtime, an inline class can be represented as both, the wrapper type and the underlying type. The compiler prefers using the underlying (wrapped) type of an inline class to optimize the code as much as possible. • In certain situations, however, the compiler needs to use the wrapper itself, so it will be generated during compilation. How does inline classes work
16. Autoboxing
17. • Autoboxing can happen when you use an instance of an inline class where a super type is expected: Autoboxing
18. • Autoboxing also happens when you use inline classes with generics Autoboxing
19. • Autoboxing also can happens when you consider nullability: ◦ nullability of the underlying type ◦ nullability at the usage site • Both cases can happen for underlying reference type or underlying primitive type Autoboxing
20. Autoboxing: Reference type Boxing Underlying type Usage
21. Autoboxing: Primitive type Boxing Underlying type Usage
22. Limitations & restrictions
23. • Inline class must have a public primary constructor with a single value parameter • Inline class must have a single read-only (val) property as an underlying value, which is defined in primary constructor • Underlying value cannot be of the same type that is containing inline class Limitations & restrictions
24. • Inline class cannot have init block • Inline class must be final Inline class can implement only interfaces • Inline class must be a top level class • Inline class cannot have backing fields • Hence, it follows that inline class can have only simple computable properties (no lateinit/delegated properties) Limitations & restrictions
25. Inline classes can be a good choice to use: • To indicate a unit of measure • To use types to protect user from misuse (https://jakewharton.com/inline-classes-make-great-database-ids/) Summary
26. • https://github.com/Kotlin/KEEP/blob/master/proposals/inline-classes.md • https://kotlinlang.org/docs/reference/inline-classes.html • https://kotlinexpertise.com/kotlin-inline-classes/ • https://typealias.com/ References
27. Thank you! Questions? @KonradSzewczuk