IDCamp Sharing Session - Kotlin 101

Keynote raw can be downloaded here
https://drive.google.com/open?id=1_3fgofTAQq2rtmJDBI32VgHHkc3IhbPw

Transcript

1. Kotlin 101 @echohikaru @ekosuhariyadi
2. What is Kotlin • Open source and free to use Programming language by JetBrains • Targeting Java Platform: server-side, Android, anywhere Java runs • Statically typed Language • Object Oriented and Functional Language • First-class functions • Immutability • No side-effects • Pragmatic, Concise, Safe And Focus on Interoperability
3. Kotlin History • Internal project in JetBrains unveiled in July 2011 after 1 year development • Seeking language for future development • Scala is great, but compile time and interoperability is obvious deficiency • Open sourced on Feb 2012 • First v1.0 released on 15 Feb 2016 
 (takes 6 years for first stable release) • Named after Kotlin Island, near St. Petersburg, Russia
 Where most JetBrains’ Kotlin development team work • “Some part” of language design is based on “Effective Java” book by 
 Joshua Bloch
4. Fun Facts • C takes 1 year to first stable release • C++ takes 4 years • Java takes 4 years • Python takes 3 years • Javascript only takes 10 days :D
 #DuaMingguJadi
5. Why Kotlin? • (Android) Stuck on Java 6 • No streams • No lambda, method references • No try-with-resources
6. Why Kotlin? • (Android) Stuck on Java 6 • No streams Use RxJava(2) • No lambda, method references • No try-with-resources
7. Why Kotlin? • (Android) Stuck on Java 6 • No streams Use RxJava(2) • No lambda, method references Recent Partial Java 8 Desugar Support • No try-with-resources
8. Why Kotlin? • (Android) Stuck on Java 6 • No streams Use RxJava(2) • No lambda, method references Recent Partial Java 8 Desugar Support • No try-with-resources minSdkVersion = 19 or Use Retrolambda
9. Why Kotlin? • (Android) Stuck on Java 6 • Inability to add methods to platform types in Java • Achieved by Util class • Nullability problems (*NullPointerException) in Java • General verbosity / boilerplate code in Java • Too much code to achieve minimal jobs
10. Taste of Kotlin data class Person( val name: String, val age: Int? = null ) fun main() { val persons = listOf( Person("Alice"), Person("Bob", age = 33) ) val oldest = persons.maxBy { it.age ?: 0 } println("The oldest is: $oldest") } // The oldest is: Person(name=Bob, age=33) Data class Nullable type (Int?); default value for argument Top-level function Named argument Lambda expression; Elvis operator String template Autogenerated toString
11. Taste of Kotlin • Property declaration • val (*value) for immutable property, once initialised can’t be changed • var (*variable) for mutable property, can be reassigned with new “data” • fun keyword to declare method/function • So literally we are having fun when code in Kotlin
12. Taste of Kotlin fun someFunction(): Unit { // function always have return value var say: String = "Kotlin is pragmatic language" say = "also safe and concise" val hello: String = "Domo arigatou Mr. Roboto" hello = "Konnichiwa" // compile time error return Unit } fun sum(x: Int, y: Int): Int { return x + y }
13. Taste of Kotlin fun someFunction() { // function always have return value var say = "Kotlin is pragmatic language" say = "also safe and concise" val hello = "Domo arigatou Mr. Roboto" hello = "Konnichiwa" // compile time error } fun sum(x: Int, y: Int): Int { return x + y }
14. Taste of Kotlin fun someFunction() { // function always have return value var say = "Kotlin is pragmatic language" say = "also safe and concise" val hello = "Domo arigatou Mr. Roboto" hello = "Konnichiwa" // compile time error } fun sum(x: Int, y: Int) = x + y // one-liner
15. Working with Null data class Contact( var name: String, var email: String? = null ) fun main() { val eko = Contact( name = "Eko Suhariyadi", email = "eko.suhariyadi@gmail.com" ) eko.name = null // compile error, because name is non null String eko.email = null // ok, since email is nullable String }
16. Working with Null val eko = Contact("Eko Suhariyadi") // return other value when email is null; using elvis operator val email = eko.email ?: "email unavailable" // cascading null safety val emailStrSize = eko.email?.length // safe scoping eko.email?.let { // this is only executed when email is not null // send email }
17. Conciseness // Java view.setOnClickListener(new View.OnClickListener() { @Override public void onClick(View v) { // do something } })
18. Conciseness // Kotlin view.setOnClickListener { v: View -> // do something }
19. Conciseness // Kotlin view.setOnClickListener { v -> // do something }
20. Conciseness // Kotlin view.setOnClickListener { // do something }
21. Data Class • Write POJO class in Java is quite verbose • Overloading constructor • Getter & setter • toString • hashCode & equals • IDE could help for code generation • Or by using libraries such as AutoValue or Immutable
22. Data Class public class Todo { private String description; private Long createdAt; private boolean done; public Todo(String description, Long createdAt, boolean done) { this.description = description; this.createdAt = createdAt; this.done = done; } // ... }
23. Data Class public class Todo { private String description; private Long createdAt; private boolean done; public Todo(String description, Long createdAt, boolean done) { this.description = description; this.createdAt = createdAt; this.done = done; } public Todo(String description, Long createdAt) { this.description = description; this.createdAt = createdAt; this.done = true; } // … }
24. Data Class public class Todo { private String description; private Long createdAt; private boolean done; public Todo(String description, Long createdAt, boolean done) { this.description = description; this.createdAt = createdAt; this.done = done; } public String getDescription() { return description; } public void setDescription(String description) { this.description = description; } public Long getCreatedAt() { return createdAt; } public void setCreatedAt(Long createdAt) { this.createdAt = createdAt; } public boolean isDone() { return done; } public void setDone(boolean done) { this.done = done; } // ... }
25. Data Class import java.util.Objects; public class Todo { private String description; private Long createdAt; private boolean done; public Todo(String description, Long createdAt, boolean done) { this.description = description; this.createdAt = createdAt; this.done = done; } public String getDescription() { return description; } public void setDescription(String description) { this.description = description; } public Long getCreatedAt() { return createdAt; } public void setCreatedAt(Long createdAt) { this.createdAt = createdAt; } public boolean isDone() { return done; } public void setDone(boolean done) { this.done = done; } @Override public boolean equals(Object o) { if (this == o) return true; if (!(o instanceof Todo)) return false; Todo todo = (Todo) o; return done == todo.done && Objects.equals(description, todo.description) && Objects.equals(createdAt, todo.createdAt); } @Override public int hashCode() { return Objects.hash(description, createdAt, done); } @Override public String toString() { return "Todo{" + "description='" + description + '\'' + ", createdAt=" + createdAt + ", done=" + done + '}'; } }
26. Data Class data class TodoKotlin( var description: String, var createdAt: Long, var done: Boolean = true )
27. Data Class data class TodoKotlin @JvmOverloads constructor( var description: String, var createdAt: Long, var done: Boolean = true )
28. Data Class \> javap TodoKotlin Compiled from "TodoKotlin.kt" public final class TodoKotlin { public final java.lang.String getDescription(); public final void setDescription(java.lang.String); public final long getCreatedAt(); public final void setCreatedAt(long); public final boolean getDone(); public final void setDone(boolean); public TodoKotlin(java.lang.String, long, boolean); public TodoKotlin(java.lang.String, long, boolean, int, kotlin.jvm.internal.DefaultConstructorMarker); public TodoKotlin(java.lang.String, long); public final java.lang.String component1(); public final long component2(); public final boolean component3(); public final TodoKotlin copy(java.lang.String, long, boolean); public static TodoKotlin copy$default(TodoKotlin, java.lang.String, long, boolean, int, java.lang.Object); public java.lang.String toString(); public int hashCode(); public boolean equals(java.lang.Object); }
29. Data Class • Kotlin data class generate compiled code almost same with Java POJO • But with steroid (extra method) • component(N) for destructure • copy for prototype pattern
30. Data Class val todo = TodoKotlin("Sample Todo", System.currentTimeMillis(), false) val (description, createAt, finished) = todo // destructure val (description1, _, finished1) = todo // destructure specific field(s) // create new object by cloning from other val cloneTodo = todo.copy(description = "Clone Sample Todo", done = true)
31. Extension Functions • Ability to add functions or properties to existing class without modify the class itself • In Java, we usually using static Utility classes
32. Extension Functions // ViewGroupUtils.java public static View inflate(ViewGroup parent, int layoutResId) { return LayoutInflater.from(parent.getContext()) .inflate(layoutResId, parent, false); } @Override ViewHolder onCreateViewHolder(ViewGroup parent, int viewType) { final View rootView = ViewGroupUtils.inflate(parent, viewType); return new ViewHolder(rootView); }
33. Extension Functions // Kotlin fun ViewGroup.inflate(layoutResId: Int): View = LayoutInflater.from(this.context) .inflate(layoutResId, this, false) override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): ViewHolder { val rootView = parent.inflate(viewType) return ViewHolder(rootView) }
34. Lambda Expression • “Anonymous function/code block” • Java • Retrolambda • Java 8 Desugar Support (Partial)
35. Lambda Expression • Implementation • Event listener, Callbacks • Higher-order functions • With Kotlin’s perks • Can be inlined for performant custom control structure • Closures
36. Lambda Expression // Java Observable.just("Hello Rx") .subscribe( new Consumer<String>() { @Override public void accept(String s) throws Exception { System.out.println(s); } }, new Consumer<Throwable>() { @Override public void accept(Throwable throwable) throws Exception { throwable.printStackTrace(); } }, new Action() { @Override public void run() throws Exception { System.out.println("Completed"); } });
37. Lambda Expression // Kotlin Observable.just("Hello Rx") .subscribe( { s -> println(s) }, { throwable -> throwable.printStackTrace() }, { println("Completed") })
38. Lambda Expression // Kotlin Observable.just("Hello Rx") .subscribe( { println(it) }, { it.printStackTrace() }, { println("Completed") })
39. Lambda Expression // Kotlin Method Reference Observable.just("Hello Rx") .subscribe( ::println, Throwable::printStackTrace, { println("Completed") })
40. Higher-order Functions • Function that takes functions as parameter and or returns a function
41. Higher-order Functions fun listCalculator(numbers: List<Int>, calculator: (List<Int>) -> Int): Int = calculator(numbers) fun main() { val numbers = (1..10).toList() val sum = listCalculator(numbers, { var accumulator = 0 it.forEach { accumulator += it } accumulator }) val avg = listCalculator(numbers, { var accumulator = 0 it.forEach { accumulator += 1 } accumulator / numbers.size }) println("sum=$sum, avg=$avg")
42. Higher-order Functions fun listCalculator(numbers: List<Int>, calculator: (List<Int>) -> Int): Int = calculator(numbers) fun main() { val numbers = (1..10).toList() val sum = listCalculator(numbers) { var accumulator = 0 it.forEach { accumulator += it } accumulator } val avg = listCalculator(numbers) { var accumulator = 0 it.forEach { accumulator += 1 } accumulator / numbers.size } println("sum=$sum, avg=$avg") } When function is last parameter We can do this
43. Extension Function Expressions • Recap • Extension Functions -> Functions added to a Type/Class without modifying the original • Function Expressions -> Undeclared function bodies used as an expression (ie. data/ value) • Higher-order Functions -> Function which takes functions as parameter and or returns a function
44. Extension Function Expressions db.beginTransaction() try { db.delete("movie", "title = ?", arrayOf("Rurouni Kenshin")) db.markTransactionSuccessful() } finally { db.endTransaction() }
45. Extension Function Expressions fun SQLiteDatabase.useTransaction(func: () -> Unit) { beginTransaction() try { func() markTransactionSuccessful() } finally { endTransaction() } }
46. Extension Function Expressions db.beginTransaction() try { db.delete("movie", "title = ?", arrayOf("Rurouni Kenshin")) db.markTransactionSuccessful() } finally { db.endTransaction() }
47. Extension Function Expressions db.useTransaction { db.delete("movie", "title = ?", arrayOf("Rurouni Kenshin")) }
48. Extension Function Expressions fun SQLiteDatabase.useTransaction(func: () -> Unit) { beginTransaction() try { func() markTransactionSuccessful() } finally { endTransaction() } }
49. Extension Function Expressions fun SQLiteDatabase.useTransaction(func: (SQLiteDatabase) -> Unit) { beginTransaction() try { func(this) markTransactionSuccessful() } finally { endTransaction() } }
50. Extension Function Expressions db.useTransaction { db.delete("movie", "title = ?", arrayOf("Rurouni Kenshin")) }
51. Extension Function Expressions db.useTransaction { it.delete("movie", "title = ?", arrayOf("Rurouni Kenshin")) }
52. Extension Function Expressions fun SQLiteDatabase.useTransaction(func: (SQLiteDatabase) -> Unit) { beginTransaction() try { func(this) markTransactionSuccessful() } finally { endTransaction() } }
53. Extension Function Expressions fun SQLiteDatabase.useTransaction(func: SQLiteDatabase.() -> Unit) { beginTransaction() try { this.func() markTransactionSuccessful() } finally { endTransaction() } }
54. Extension Function Expressions db.useTransaction { it.delete("movie", "title = ?", arrayOf("Rurouni Kenshin")) }
55. Extension Function Expressions db.useTransaction { delete("movie", "title = ?", arrayOf("Rurouni Kenshin")) }
56. Extension Function Expressions fun SQLiteDatabase.useTransaction(func: SQLiteDatabase.() -> Unit) { beginTransaction() try { this.func() markTransactionSuccessful() } finally { endTransaction() } }
57. Extension Function Expressions fun SQLiteDatabase.useTransaction(func: SQLiteDatabase.() -> Unit) { beginTransaction() try { func() markTransactionSuccessful() } finally { endTransaction() } }
58. Extension Function Expressions inline fun SQLiteDatabase.useTransaction(func: SQLiteDatabase.() -> Unit) { beginTransaction() try { func() markTransactionSuccessful() } finally { endTransaction() } } // with inline keyword, code block will be inlined with caller code // so, there in no overhead for creating lambda
59. Extension Function Expressions • Higher-order functions is very convenience to create our own DSL • For Android developer • There is anko library, but sadly no longer being maintain • Jetpack Kotlin extension
60. Other Kotlin Cool • Collection API • Standar lib functions • apply, with, let, run • If Expressions • When Expression • Sealed class
61. Thank you and Have a nice Kotlin!