Shining a Light into Kotlin's Dark Corners (Kotlin London, May 2016)

Our experience going from Java to Kotlin

Transcript

1. Shining a Light into Kotlin's Dark Corners Nat Pryce Duncan McGregor
2. Kotlin at Springer Nature
4. We use null much more than in Java data class AuthorDetails( val name: String, val email: String?, val averageImpactFactor: Double ) data class Pagination(val count: Int, val offset: Int = 0) { fun next() = copy(offset = offset + count) } interface AuthorSearch { fun search(impactFactor: Double?, keywords: List<String> = emptyList(), title: String? = null, coauthors: List<String> = emptyList(), abstract: String? = null, pagination: Pagination? = null): List<AuthorDetails> }
5. “Nullable” is a special case val maybeName: String? = "alice"; val nameList = listOf("bob", "carol", "dave") Mapping maybeName?.let { it.toUpperCase() } nameList.map { it.toUpperCase() } Filtering maybeName?.let { if ("a" in it) it else null } nameList.filter { "a" in it }
6. Kotlin has limited polymorphism fun <T : AutoCloseable, U> T.use(block: (T) -> U) = try { block(this); } finally { close() } fun <T : Image, U> T.use(block: (T) -> U) = try { block(this); } finally { flush() } fun <U> SplashScreen.use(block: (SplashScreen)->U) = try { block(this); } finally { close() } fun <U> MembershipKey.use(block: (MembershipKey)->U) = try { block(this); } finally { drop() }
7. No destructuring pattern match… but flow-sensitive typing makes up for it sealed class Shape { class Rect(val p1: Point, val p2: Point) : Shape() class Circle(val centre: Point, val radius: Double) : Shape() } Shape s = getTheShape() val area = when(s) { is Shape.Rect -> abs(s.p2.x - s.p1.x) * abs(s.p2.y - s.p1.y) is Shape.Circle -> PI * s.radius * s.radius } fun Shape.area(): Double = when (this) { is Shape.Rect -> abs(p2.x - p1.x) * abs(p2.y - p1.y) is Shape.Circle -> PI * radius * radius }
8. Default public and final methods
9. Subtyping, Nullability & Generics assertThat(optionalText, isNotNullAnd(equalTo("hello"))) fun <T> isNotNullAnd(valueMatcher: Matcher<T>): Matcher<T?> = object:Matcher<T?> { override fun invoke(actual: T?): MatchResult { return when (actual) { null -> MatchResult.Mismatch("was null") else -> valueMatcher(actual) } } override val description: String get() { return "is not null & " + valueMatcher.description } }
10. Touching the Void @Test public void callable_returning_nothing_is_a_bit_of_a_pain() throws Exception { Callable<Void> callable = new Callable<Void>() { @Override public Void call() throws Exception { System.out.println("Doing a thing"); return null; } }; assertNull(callable.call()); } @Test public void even_with_lambdas() throws Exception { Callable<Void> callable = () -> { System.out.println("Doing a thing"); return null; }; assertNull(callable.call()); }
11. Touching the Void fun `implicitly returns Unit`() : Unit {} fun `explicitly returns Unit`() : Unit { return Unit }
12. Touching the Void @Test fun `Callable of Unit is OK`() { val callable = object: Callable<Unit> { override fun call() { println("Doing a thing") } } assertEquals(Unit, callable.call()) } @Test fun `and even better as a lambda`() { val callable = Callable<Unit> { println("Doing a thing") } assertEquals(Unit, callable.call()) }
13. Touching the Void @Test fun `can extend Unit to give fluent API`() { returnAThing().shouldEqual(3) doAThing().shouldEqual(Unit) null.shouldEqual(null) } fun returnAThing() = 3 fun doAThing() { println("Doing a thing") } fun <T:Any?> T.shouldEqual(x: T) = assertEquals(x, this)
14. Nothing – for the point of no return
15. Kotlin works well for old-fashioned Java… But what about modern Java?
16. Language support for the Singleton pattern object alice { val name = "Alice" } val bob = object { val name = "Bob" } fun example() { val carol = object { val name = "Carol" } assert( alice.name == "Alice" ) assert( bob.name == "Bob" ) assert( carol.name == "Carol" ) } Error: Unresolved reference: name
17. Spaces in Identifiers private val wordBoundaryRegex = Regex("(\\p{Ll})(\\p{Lu})") private fun String.`space Out Camel Case`() = this.replace(wordBoundaryRegex, "$1 $2")
18. Spaces in Identifiers class SmokeTests : AcceptanceTest( asA = "user named James", want = "to know things are running") { val james = actorNamed("James") @Test fun `The server is running`() { given (james) { loadsTheHomePage() } then (james) { shouldSee(::`the page title`, equalTo("Journal")) } } @Test fun `Search returns some results`() { given (james) { loadsTheHomePage() } then (james) { shouldSee(::`the results table`, isntThere) } wheN (james) { searchesForTitle("fred") } then (james) { shouldSee(::`the results table`, hasSomeContent) } } }
19. Infixation val config = ConfigurationProperties.systemProperties() overriding EnvironmentVariables() overriding ConfigurationProperties.fromResource("default.conf") infix fun Configuration.overriding(defaults: Configuration) = Override(this, defaults) (n) An unhealthy obsession with infix functions.
20. fun operationFor(query: (String) -> QueryBuilder) = fun(search: String, client: HttpElasticClient) = client.query(this, query(search)) fun with = fun currying
21. Return is a bit weird
22. Wish List Checked exceptions • sadly will never happen Sealed data classes • scheduled for v1.1 Persistent data structures • planned for the stdlib Maybe one day • Type classes • Higher-kinded types