[Lightning] Generic's type variance in Kotlin

Transcript

1. Generic Type variance (in Kotlin) The topic that killed my brain
2. Invariance
3. Invariance Why? List<String> strs = new ArrayList<String>();
4. Invariance Why? List<String> strs = new ArrayList<String>(); List<Object> objs = strs; // !!! The cause of the upcoming problem sits here. Java prohibits this!
5. Invariance Why? List<String> strs = new ArrayList<String>(); List<Object> objs = strs; // !!! The cause of the upcoming problem sits here. Java prohibits this! objs.add(1); // Here we put an Integer into a list of Strings
6. Invariance Why? List<String> strs = new ArrayList<String>(); List<Object> objs = strs; // !!! The cause of the upcoming problem sits here. Java prohibits this! objs.add(1); // Here we put an Integer into a list of Strings String s = strs.get(0); // !!! ClassCastException: Cannot cast Integer to String
7. Invariance Why? List<String> strs = new ArrayList<String>(); List<Object> objs = strs; // !!! The cause of the upcoming problem sits here. Java prohibits this! objs.add(1); // Here we put an Integer into a list of Strings String s = strs.get(0); // !!! ClassCastException: Cannot cast Integer to String Note: All generic types in Java are invariant!
8. Invariance A <: B so, for List<T>: List<A> <!: List<B>
9. Covariance interface Producer<out T> { fun produce(): T }
10. Covariance interface Producer<out T> { fun produce(): T } val p = object : Producer<String> { override fun produce(): String = "Hello" }
11. Covariance interface Producer<out T> { fun produce(): T } val p = object : Producer<String> { override fun produce(): String = "Hello" } val gp: Producer<Any> = p
12. Covariance val gp: Producer<Any> = p
13. Covariance val gp: Producer<Any> = p p.produce() // -> String gp.produce() // -> Any
14. Covariance val gp: Producer<Any> = p p.produce() // -> String gp.produce() // -> Any String <: Any Producer<String> <: Producer<Any>
15. Covariance val gp: Producer<Any> = p p.produce() // -> String gp.produce() // -> Any String <: Any Producer<String> <: Producer<Any> -> You can always read Any from a Producer<String>
16. Covariance A <: B so, for Producer<? extends T> // Producer<out T>: Producer<A> <: Producer<B>
17. Contravariance interface Consumer<in T> { fun consume(t: T) }
18. Contravariance interface Consumer<in T> { fun consume(t: T) } val c = object : Consumer<Any> { override fun consume(t: Any): Any = print(t) }
19. Contravariance interface Consumer<in T> { fun consume(t: T) } val c = object : Consumer<Any> { override fun consume(t: Any): Any = print(t) } val gc: Consumer<String> = c
20. Contravariance val gc: Consumer<String> = c
21. Contravariance val gc: Consumer<String> = c c.consume("hello") // <- Any gc.consume("hello") // <- String
22. Contravariance val gc: Consumer<String> = c c.consume("hello") // <- Any gc.consume("hello") // <- String String <: Any Consumer<Any> <: Consumer<String>
23. Contravariance val gc: Consumer<String> = c c.consume("hello") // <- Any gc.consume("hello") // <- String String <: Any Consumer<Any> <: Consumer<String> -> You can always write Strings into a Consumer<Any>
24. Covariance A <: B so, for Consumer<? super T> // Consumer<in T>: Consumer<B> <: Consumer<A>
25. Links https://kotlinlang.org/docs/reference/generics.html#varianc e https://medium.com/@tpolansk/understanding-generics-and- variance-in-kotlin-714c14564c47
26. Thank you Q&A