Practical Kotlin

Introduction talk to some Kotlin features and practices based on my previous experience.

Transcript

1. Practical Kotlin Alex Levin DH Backend Chapter meet-up 17-07-2020, Berlin
2. Disclaimer • Not really an excessive talk • Some things are subjective
3. Agenda • Why Kotlin? • Know your features • Know your library • Avoid overengineering
4. Why Kotlin? • Solved a lot of Java problems • Pretty easy to learn • Not too much compromises
5. Know your features: null • Best part about null in Kotlin - you can choose to not use it • But if you choose to - there are things to help: a?.b a ?: 5 a?.let { … } if (a != null) { … } else { … }
6. Know your features: Extensions • Helping to write more fluent code • Existing “regular” function can be used with `.let`
7. Know your features: Extensions Without extensions: BufferedReader(FileReader(“someFile.txt”))
8. Know your features: Extensions With extensions: File(“someFile.txt”).reader().buffered()
9. Know your features: Reified Generics • Help a lot with boilerplate • Sometimes have more possibilities than providing Class type
10. Know your features: Reified Generics data class Person(val age: Int, val name: String) val mapper = jacksonObjectMapper() val json = """{ "age": 25, "name": "Jason" }""" mapper.readValue(json, Person::class.java)
11. Know your features: Reified Generics data class Person(val age: Int, val name: String) val mapper = jacksonObjectMapper() val json = """{ "age": 25, "name": "Jason" }""" mapper.readValue(json, object: TypeReference<Person>(){})
12. Know your features: Reified Generics inline fun <reified T> ObjectMapper.readValue(s: String) = readValue(s, object: TypeReference<T>() {})
13. Know your features: Reified Generics data class Person(val age: Int, val name: String) val mapper = jacksonObjectMapper() val json = """{ "age": 25, "name": "Jason" }""" mapper.readValue<Person>(json)
14. Know your features: Contracts println(nullableString.capitalize()) // not allowed, no `.capitalize()` for nullable string requireNotNull(nullableString) println(nullableString.capitalize()) // now it’s fine
15. Know your features: Contracts println(nullableString.capitalize()) // not allowed, no `.capitalize()` for nullable string requireNotNull(nullableString) println(nullableString.capitalize()) // now it’s fine
16. Know your features: Overall • A lot of great features • Some of them can be abused
17. Know your library: String Java-way: className.substring(0, className.length - 4) // prone to errors, logic obscured
18. Know your library: String Obvious Kotlin-way: className.dropLast(4) // better, but why 4?
19. Know your library: String Proper Kotlin-way: className.removeSuffix(“Test”) // now I get the idea
20. Know your library: Name Validation I know RegExes: val validCharRegex = """[\s\w\d?\-_'.,/@&()!+:]""".toRegex() return name.all { validCharRegex.matches(it.toString()) } // please gods I hope that’s working
21. Know your library: Name Validation I ACTUALLY know RegExes: val validCharRegex = """[\s\p{gc=L}\d?\-_'.,/@&()!+:]""".toRegex() return name.all { validCharRegex.matches(it.toString()) } // did you notice the change? No? That’s fine
22. Know your library: Name Validation I know Kotlin: val validChars = setOf('-', '\'', '.', ',', '/', '@', '&', '(', ')', '!', '+', ':') return name.all { it.isLetter() || it in validChars } // looks easier, maybe even written by human
23. Know your library: Name Validation I know how to validate person’s name: return true // because you always fail if you try
24. Know your library: Overall • A lot of great stuff in the standard library • Most likely you don’t need Apache/Guava • It’s fine to still use Java standard library
25. Avoid overengineering: null x?.let { … } // Fine idiomatic code x?.let { … } ?: run { … } // Stop it. Get some help
26. Avoid overengineering: null if (x != null) { … } else { … } // More code but less chances to break a leg
27. Avoid overengineering: common algorithms val percent = when { input > 100 -> 100 input < 0 -> 0 else -> input } val percent = input.coerceIn(0, 100)
28. Avoid overengineering: common algorithms val percent = when { input > 100 -> 100 input < 0 -> 0 else -> input } val percent = input.coerceIn(0, 100)
29. Avoid overengineering: DSL for everything • DSLs are cool • But essentially it is just bunch of setters • Sometimes simple constructor is better
30. Avoid overengineering: DSL for everything val r = Rectangle() r.color = 0xFAFAFA r.xSize = 100 r.ySize = 200 // meh
31. Avoid overengineering: DSL for everything val r = Rectangle().apply { color = 0xFAFAFA xSize = 100 ySize = 200 } // better
32. Avoid overengineering: DSL for everything val r = Rectangle( color = 0xFAFAFA, xSize = 100, ySize = 200 ) // best
33. Further reading • “Effective Kotlin” Book • Actual Kotlin doc is pretty good