KOTLIN A QUICK INTRODUCTION Vinh Nguyen @ntvinh11586 Saigon, May 2018 

AGENDA • Overview • Basic features • Kotlin in Android • In production: TripNow • Resources • Q&A 

Overview 

What is Kotlin? Kotlin island, Russia (59.9360229, 29.5408354,9) 

What is Kotlin? 

What is actual Kotlin? • Programming language • Created by JetBrains • Statically-typed language • JVM Based, compiles to Java bytecode, Javascript, or machine code (Native) • Object-oriented language, support functional programming • Simple, lightweight, interoperate • Is open source 

History Time Version 2010 Kotlin project started 2016 Version 1.0 2017 Google I/O, Official support 2018 Version 1.2.41 

Market share 

Why Kotlin? “Concise, Safe, Interoperable, Tool-friendly” – kotlinlang 

Basic Features 

Null safety Artist artist = null;
 artist.print(); Java 

Null safety var artist: Artist? = null
 artist.print() var artist: Artist? = null
 artist?.print() if (artist != null) { artist.print()
 } var artist: Artist? = null
 artist!!.print() Kotlin Won’t compile Will do nothing Smart cast Will crash var artist: Artist = null
 artist.print() Won’t compile 

Mutability/Immutability var hello = “Hello world, var!” hello = “Hello world, again, var!!” Mutability Immutability val hello: String = “Hello world, val!” hello = “Hello world, again, val!!” 

Properties Java (POJO) public class Artist { private String field = ""; public String getField() { return field;
 }
 public void setField(String field) { this.field = field;
 } } 

Properties Kotlin class Artist(var property: String = "") { } 

Data classes data class Artist( val id: Long, val name: String, var url: String, var address: String = "", var description: String = "None" ) 

Control flow: If var max = a if (a < b) max = b var max: Int if (a > b) { max = a } else { max = b } val max = if (a > b) a else b 

Control flow: When when (x) { 1 -> print("x == 1") 2 -> print("x == 2") else -> { print("x is neither 1 nor 2") } } when { x.isOdd() -> print("x is odd") x.isEven() -> print("x is even") else -> print("x is funny") } value = when { x.isOdd() -> "x is odd" x.isEven() -> "x is even" else -> “x is funny" } 

Control flow: For for (i in 1..3) { println(i) } for (i in 6 downTo 0 step 2) { println(i) } for (i in array.indices) { println(array[i]) } for ((index, value) in array.withIndex()) { println("the element at $index is $value") } 

Functional Programming Lambda Function - (T) -> R Java public interface Callback {
 void invoke(T result);
 }
 public void asyncOperation(int value, Callback callback) { ... callback.invoke(true);
 } asyncOperation(5, new Callback() { @Override public void invoke(Boolean result) { System.out.println("Result: " + result);
 } }); 

Functional Programming Lambda Function - (T) -> R Kotin fun asyncOperation(value: Int, callback: (Boolean) -> Unit) { ... callback(true) } asyncOperation(5) { result -> println("result: $result") } asyncOperation(5) { println("result: $it") } 

Functional Programming Collections • Array, List • Set • Map • Iterable • … • filter { } • sort { } • map { } • zip { } • first { } • firstOrNull { } • flatmap { } • … 

Extension Function fun Context.toast(message: CharSequence) { Toast.makeText(this, message, duration).show() } context.toast("I'm a context") activity.toast("I'm an activity") service.toast("I'm a service”) 

Default arguments fun displayGreeting(message: String, name: String = "Guest") { println("Hello $name, $message") } displayGreeting("Welcome", "John") // Hello John, Welcome displayGreeting("Welcome") // Hello Guest, Welcome 

Interoperability // Java Palette.generateAsync(bitmap, new Palette.PaletteAsyncListener() { @Override public void onGenerated(Palette palette) { ... } }); // Kotlin Palette.generateAsync(bitmap) { palette -> ... } 

Interoperability // Kotlin, ContextUtils.kt fun Context.toast(message: CharSequence) { Toast.makeText(this, message, duration).show() } // Java ContextUtilsKt.toast(context, “message”) 

Kotlin in Android 

Project configuration •~/build.gradle •ext.kotlin_version = '1.2.41' •org.jetbrains.kotlin:kotlin-gradle-plugin: $kotlin_version •~/app/build.gradle •apply plugin: 'kotlin-android' •apply plugin: 'kotlin-android-extensions' •org.jetbrains.kotlin:kotlin-stdlib- jdk7:$kotlin_version 

Java compile 

Kotlin compile 

Gradle build times (no gradle daemon) 

Gradle build times (daemon running) 

Gradle build times (incremental, since 1.0.2) No file changed 

Gradle build times (incremental, since 1.0.2) One independent file changed 

Gradle build times (incremental, since 1.0.2) One core file changed 

Demo 

In production TripNow 

Introduction • Travel community • Search places, tours, blogs • Reviews & ratings • Create own travel trip (Web) • Future: Book tours, hotels,… • … 

Project target • Research new technologies • Apply the good architecture, clean code • Easy to maintain, following requirements changed faster • Make product stable, less errors, good performance • Cover by unit test • … 

Kotlin usage • Make object immutability and safe from NULL • Reduce boilerplate code • Apply functional & lambda practice • Implement extended methods, toolkits, utils by using extension function • … 

Example Immutability and null safety data class TourSchedule( val id: Int = 0, val startDate: String = "", val endDate: String = "", val price: Double = 0.0 ) 

Example Reduce boilerplate code data class TourSchedule( val id: Int = 0, val startDate: String = "", val endDate: String = "", val price: Double = 0.0 ) 

Example Reduce boilerplate code public final class TourSchedule { private final int id; @NotNull private final String startDate; @NotNull private final String endDate; private final double price; public final int getId() { return this.id; } @NotNull public final String getStartDate() { return this.startDate; } @NotNull public final String getEndDate() { return this.endDate; } public final double getPrice() { return this.price; } public TourSchedule(int id, @NotNull String startDate, @NotNull String endDate, double price) { Intrinsics.checkParameterIsNotNull(startDate, "startDate"); Intrinsics.checkParameterIsNotNull(endDate, "endDate"); super(); this.id = id; this.startDate = startDate; this.endDate = endDate; this.price = price; } 

Example Reduce boilerplate code public TourSchedule(int var1, String var2, String var3, double var4, int var6, DefaultConstructorMarker var7) { if ((var6 & 1) != 0) { var1 = 0; } if ((var6 & 2) != 0) { var2 = ""; } if ((var6 & 4) != 0) { var3 = ""; } if ((var6 & 8) != 0) { var4 = 0.0D; } this(var1, var2, var3, var4); } public TourSchedule() { this(0, (String)null, (String)null, 0.0D, 15, (DefaultConstructorMarker)null); } public final int component1() { return this.id; } @NotNull public final String component2() { return this.startDate; } @NotNull public final String component3() { return this.endDate; } 

Example Reduce boilerplate code @NotNull public final TourSchedule copy(int id, @NotNull String startDate, @NotNull String endDate, double price) { Intrinsics.checkParameterIsNotNull(startDate, "startDate"); Intrinsics.checkParameterIsNotNull(endDate, "endDate"); return new TourSchedule(id, startDate, endDate, price); } // $FF: synthetic method // $FF: bridge method @NotNull public static TourSchedule copy$default(TourSchedule var0, int var1, String var2, String var3, double var4, int var6, Object var7) { if ((var6 & 1) != 0) { var1 = var0.id; } if ((var6 & 2) != 0) { var2 = var0.startDate; } if ((var6 & 4) != 0) { var3 = var0.endDate; } if ((var6 & 8) != 0) { var4 = var0.price; } return var0.copy(var1, var2, var3, var4); } 

Example Reduce boilerplate code public String toString() { return "TourSchedule(id=" + this.id + ", startDate=" + this.startDate + ", endDate=" + this.endDate + ", price=" + this.price + ")"; } public int hashCode() { int var10000 = ((this.id * 31 + (this.startDate != null ? this.startDate.hashCode() : 0)) * 31 + (this.endDate != null ? this.endDate.hashCode() : 0)) * 31; long var10001 = Double.doubleToLongBits(this.price); return var10000 + (int)(var10001 ^ var10001 >>> 32); } public boolean equals(Object var1) { if (this != var1) { if (var1 instanceof TourSchedule) { TourSchedule var2 = (TourSchedule)var1; if (this.id == var2.id && Intrinsics.areEqual(this.startDate, var2.startDate) && Intrinsics.areEqual(this.endDate, var2.endDate) && Double.compare(this.price, var2.price) == 0) { return true; } } return false; } else { return true; } } } 

Example FP & Lambda compositeDisposable.add( selectedCityRepository.onRxSelectedCityUpdated() .observeOn(schedulerProvider.ui()) .doOnNext { view?.finishLoadingPages() } .doOnNext { loadHomeData() } .doOnNext { view?.scrollListToTop() } .doOnNext { view?.showSearchView() } .subscribe({}, { Log.e("log", Log.getStackTraceString(it)) }, {}) ) 

Example Extended method fun TourScheduleEntity.toTourSchedule() = TourSchedule( id = this.id, startDate = this.startDate ?: "", endDate = this.endDate ?: "", price = this.price ) 

Example Toolkits, Utils fun View.hideKeyboard() { context.hideKeyboard(this) } fun Context.hideKeyboard(view: View) { val inputMethodManager = getSystemService(Activity.INPUT_METHOD_SERVICE) as InputMethodManager inputMethodManager.hideSoftInputFromWindow(view.windowToken, 0) } fun View.showKeyboard() { context.showKeyboard(this) } fun Context.showKeyboard(view: View) { val inputMethodManager = getSystemService(Activity.INPUT_METHOD_SERVICE) as InputMethodManager inputMethodManager.showSoftInputFromInputMethod(view.windowToken, 0) } 

Learning resources • https://kotlinlang.org/ • https://try.kotlinlang.org/#/Examples/ • http://www.kotlinweekly.net/ • https://antonioleiva.com/kotlin/ • Book: Kotlin in Action • Book: The Joy of Kotlin (MEAP) 

Q&A 

Thank you!