Safe and sound code with Kotlin

Java достаточно безопасный язык программирования с точки зрения управления памятью и типов. Но с переходом на Kotlin мы можем программировать ещё более безопасно благодаря некоторым фичам языка, недоступных в Java.

Это означает больше времени на бизнес-логику, меньше на велосипеды. И да, с этими фичами некоторые unit-тесты вам больше не понадобятся.

Автор: Руслан Захаров

Multistack developer.

Transcript

1. Safe and sound code with kotlin Code business not code ruX @ GDG Spb “Kotlin HardcoreWhiteNights” 15.06.2017
2. Background • Came from startups #GSD • Worked in very small multifunctional teams • Used different technologies and platforms • Java - 7+ years, kotlin - 4+ • Currently working at Nexmo
3. What is it all about?
4. “Simple” java bean public class User { private String name; private Date birthday; public User(String name) { this.name = name; } public User(String name, Date birthday) { this(name); this.birthday = birthday; } public String getName() { return name; } public void setName(String name) { this.name = name; } public Date getBirthday() { return birthday; } public void setBirthday(Date birthday) { this.birthday = birthday; } @Override public String toString() { return "User{" + "name='" + name + '\'' + ", birthday=" + birthday + '}'; } @Override public boolean equals(Object o) { if (this == o) return true; if (o == null || getClass() != o.getClass()) return false; User user = (User) o; if (name != null ? !name.equals(user.name) : user.name != null) return false; return birthday != null ? birthday.equals(user.birthday) : user.birthday == null; } @Override public int hashCode() { int result = name != null ? name.hashCode() : 0; result = 31 * result + (birthday != null ? birthday.hashCode() : 0); return result; }
5. “Simple” java bean data class User(val name: String, val birthday: Date)
6. NullPointerException
7. Null safety private void display(String lang, Integer age) { requireNonNull(lang, "Language should be non-null"); requireNonNull(age, "Age should be non-null"); System.out.println(lang.toUpperCase() + " is " + age + "years old"); } display(null, 100); display("Java", 22);
8. Null safety fun display(lang: String, age: Int) { println("${lang.toUpperCase()} is $age years old") } display("kotlin", 6) display(null, 6) Kotlin: Null can not be a value of a non-null type String
9. Optionals class JPerson { String name; JAddress address; public JPerson(String name, JAddress address) { this.name = name; this.address = address; } } class JAddress { String street; String zip; Date updatedAt; public JAddress(String street, String zip, Date updatedAt) { this.street = street; this.zip = zip; this.updatedAt = updatedAt; } }
10. Optionals void display(JPerson person) { StringBuilder sb = new StringBuilder(); sb.append(person.name).append(" moved to "); if (person.address != null && person.address.street != null) sb.append(person.address.street); else sb.append("unknown place"); if (person.address != null && person.address.zip != null) sb.append(person.address.zip.toUpperCase()); sb.append(" on "); if (person.address != null && person.address.updatedAt != null) sb.append(person.address.updatedAt.toGMTString()); else sb.append("unknown date"); System.out.println(sb.toString()); }
11. Optionals data class KPerson(val name: String, val address: KAddress? = null) data class KAddress(val street: String?, val zip: String?, val updatedAt: Date) fun display(person: KPerson) { println("${person.name} moved to " + "${person.address?.street ?: "unknown place"} " + "${person.address?.zip.orEmpty()} on " + "${person.address?.updatedAt?.toGMTString() ?: "unknown date"} ") }
12. Numbers int n = 2_000_000_000; long b = n + n; System.out.println(b); -294967296
13. Numbers val n: Int = 2_000_000_000 val b: Long = n + n println(b) Error:(9, 19) Kotlin: Type mismatch: inferred type is Int but Long was expected
14. Numbers val n: Int = 2_000_000_000 val b: Long = n.toLong() + n println(b) 4000000000
15. Collections void mutatorMethod(List<Integer> list) { list.add(5); } void run() { List<Integer> list1 = genList1(); mutatorMethod(list1); List<Integer> list2 = genList2(); mutatorMethod(list2); } List<Integer> genList1() { return new ArrayList<>( asList(1,2,3,4) ); } List<Integer> genList2() { return unmodifiableList( asList(1,2,3,4) ); } Exception in thread "main" java.lang.UnsupportedOperationException at java.util.Collections$UnmodifiableCollection.add(Collections.java:1055) at collections.JavaImmutableCollection.mutatorMethod(JavaImmutableCollection.java:31)
16. Collections fun mutatorMethod(list: MutableList<Int>) { list.add(5) } fun run() { val list1 = genList1() mutatorMethod(list1) val list2 = genList2() mutatorMethod(list2) } fun genList1() : MutableList<Int> { return ArrayList( asList( 1, 2, 3, 4) ) } fun genList2() : List<Int> { return asList(1, 2, 3, 4) } Error:(27, 19) Kotlin: Type mismatch: inferred type is List<Int> but MutableList<Int> was expected
17. Named arguments void run() { checkParcelSize(439394234, 23, 21, 10, 3.5, 7.8, 200); } boolean checkParcelSize(long id, double width, double height, double length, double weight, double price, double value) { return true; // pretends we checked something }
18. Named arguments fun run() { checkParcelSize(id = 439394234, weight = 3.5 width = 23.0, height = 21.0, length = 10.0, price = 7.8, value = 200.0 ) } fun checkParcelSize(id: Long, width: Double, height: Double, length: Double, weight: Double, price: Double, value: Double ): Boolean { return true }
19. Named constructor arguments class MyService( val config: Map<String, String>, val input: File, val output: File, val offsetX: Int, val offsetY: Int, val items: List<String> ) val service = MyService( config = loadConfig(), items = items, input = File("input.txt"), output = File("output.txt"), offsetX = 42, offsetY = 64 ) (for DI haters)
20. Keys name refactoring class JUser { private final String login; private final String password; public JUser(String login, String password) { this.login = login; this.password = password; } public JSONObject toJson() { return new JSONObject() .put( KEY_LOGIN, login) .put( KEY_PASSWORD, password); } public static final String KEY_LOGIN = "login"; public static final String KEY_PASSWORD = "password"; }
21. Keys name refactoring data class KUser(val login: String, val password: String) { fun toJson() = JSONObject() .put(KUser::login.name, login) .put(KUser::password.name, password) } fun main(args: Array<String>) { val user = KUser("Bob", "123456") println(user.toJson()) }
22. Sealed classes abstract class JEventBase { } class JMouseMove extends JEventBase { public int x, y; public JMouseMove(int x, int y) { this.x = x; this.y = y; } } class JMouseClick extends JEventBase { public int x, y, btn; public JMouseClick(int x, int y, int btn) { this.x = x; this.y = y; this.btn = btn; } } class JKeyPressed extends JEventBase { char key; public JKeyPressed(char key) { this.key = key; } }
23. Sealed classes List<JEventBase> events = Arrays. asList( new JMouseClick(42, 200, 1), new JMouseMove(46, 203), new JMouseMove(50, 210), new JMouseClick(40, 210, 3), new JKeyPressed('j') ); events.forEach( this::process);
24. Sealed classes private void process(JEventBase event) { String log = ""; if (event instanceof JMouseClick) { log = "Mouse click button #" + ((JMouseClick) event).btn + " at " + ((JMouseClick) event).x + ", " + ((JMouseClick) event).y; } else if (event instanceof JMouseMove) { log = "Mouse move to " + ((JMouseMove) event).x + ", " + ((JMouseMove) event).y; } else if (event instanceof JKeyPressed) { log = "Key '" + ((JKeyPressed) event).key + "' pressed"; } else throw new IllegalArgumentException("Got unknown event, should never happen!"); System.out.println(event.getClass().getSimpleName() + "> " + log); }
25. Sealed classes sealed class KEventBase data class KMouseMove(val x: Int, val y: Int) : KEventBase() data class KMouseClick(val x: Int, val y: Int, val btn: Int) : KEventBase() data class KKeyPressed(val key: Char) : KEventBase() val events = listOf( KMouseClick( 42, 200, 1), KMouseMove( 46, 203), KMouseMove( 50, 210), KMouseClick( 40, 210, 3), KKeyPressed( 'j') ) events.forEach(::process)
26. Sealed classes fun process(event: KEventBase) { val log = when(event) { is KMouseClick -> "Mouse clicked button #${event.btn} at ${event.x}, ${event.y}" is KMouseMove -> "Mouse moved to ${event.x}, ${event.y}" is KKeyPressed -> "Key '${event.key}' pressed" } println("${event::class.java.simpleName}> $log") }
27. Type safe units math open class DistanceUnit(val multiplier: Double, val unitAbbr: String) object Meters : DistanceUnit( 1.0, "m") object Kilometers : DistanceUnit( 1e3, "km") object Millimeters : DistanceUnit( 1e-3, "mm") object Centimetres : DistanceUnit( 1e-2, "cm") object Miles : DistanceUnit( 1609.34, "mi") object Yards : DistanceUnit( 0.9144, "yd") part #1
28. Type safe units math data class Distance(val distance: Double, val unit: DistanceUnit) { override fun toString() = "${distance}${unit.unitAbbr}" fun to(otherUnit: DistanceUnit) = Distance( distance * unit.multiplier / otherUnit.multiplier, otherUnit) operator fun plus(that: Distance) = Distance( this.distance + that.to(unit).distance, unit) operator fun minus(that: Distance) = Distance( this.distance - that.to(unit).distance, unit) } part #1
29. Type safe units math val twoKm = Distance(2.0, Kilometers) val `5m` = Distance(5.0, Meters) val `42mm` = Distance(42.0, Millimeters) println(twoKm) // 2km println("$`42mm` = ${`42mm`.to(Centimetres)}") // 42.0mm = 4.2cm println("$twoKm = ${twoKm.to(Meters)} = ${twoKm.to(Miles)}") // 2.0km = 2000.0m = 1.2427454732996135mi println(twoKm - `5m`) // 1.995km println(Distance(10.0, Meters) - `5m` + `42mm`) // 5.042m part #1
30. Type safe units math data class Distance(val distance: Double, val unit: DistanceUnit) { fun to(otherUnit: DistanceUnit) = Distance( distance * unit.multiplier / otherUnit.multiplier, otherUnit) val asMeters get() = to(Meters) val asKilometers get() = to(Kilometers) val asMillimeters get() = to(Millimeters) val asCentimetres get() = to(Centimetres) val asMiles get() = to(Miles) val asYards get() = to(Yards) override fun toString() = "${distance}${unit.unitAbbr}" } part #2
31. Type safe units math fun Number.to(unit: DistanceUnit) = Distance( this.toDouble(), unit) val Number.asMeters get() = to(Meters) val Number.asKilometers get() = to(Kilometers) val Number.asMillimeters get() = to(Millimeters) val Number.asCentimetres get() = to(Centimetres) val Number.asMiles get() = to(Miles) val Number.asYards get() = to(Yards) part #2 val tenMeters = 10.asMeters val nineYards = 9.asYards
32. Type safe units math val twoKm = 2.asKilometers val `5m` = 5.asMeters val `42mm` = 42.asMillimeters println("$twoKm = ${twoKm.asMeters} = ${twoKm.asMiles}") // 2.0km = 2000.0m = 1.2427454732996135mi println(twoKm - `5m`) // 1.995km println(10.asMeters - `5m` + `42mm`) // 5.042m val snake = (10.asCentimetres + 1.asMeters).asMeters println("snake is $snake(${snake.asYards}) long") // snake is 1.1m(1.2029746281714786yd) long part #2
33. Type safe units math data class SpeedUnit(val distanceUnit: DistanceUnit, val timeUnit: TimeUnit) open class DistanceUnit(val multiplier: Double, val unitAbbr: String) { operator fun div(timeUnit: TimeUnit) = SpeedUnit( this, timeUnit) } data class Speed(val speed: Double, val unit: SpeedUnit) { fun to(speedUnit: SpeedUnit): Speed { val dd = unit.distanceUnit.multiplier / speedUnit.distanceUnit.multiplier val dt = unit.timeUnit.multiplier / speedUnit.timeUnit.multiplier val s = speed * (dd / dt) return Speed(s, speedUnit) } override fun toString() = "$speed${unit.distanceUnit.unitAbbr}/${unit.timeUnit.unitAbbr}" } part #3
34. Type safe units math println(10.asSeconds + 5.asMinutes) // 310.0s val speed = 10.asKilometers / 5.asHours val speedInMPerSecond = speed.to(Meters / Seconds) println("$speed = $speedInMPerSecond") // 2.0km/h = 0.556m/s val speedInMph = speed.to(Miles / Hours) println("$speed = $speedInMph") // 2.0km/h = 1.2427454732996135mi/h val `50mph` = 50.asMiles / 1.asHours println("$`50mph` = ${`50mph`.to(Kilometers / Hours)}") // 50.0mi/h = 80.467km/h part #3
35. https://github.com/ruXlab/safe-kotlin-features-showcase http://ruX.vc @ruxeg