Kotlin for Android 101

By Brice

Transcript

1. /39 Kotlin for Android 101 2018.05.09 Brice Buzzvil 1
2. /39 Wait, wait, wait. Brice, YOUR QUIZ!! You ruined my weekends! 2
3. /39 Quiz • In an array, the most of the numbers are twice, but two different numbers are only once. ex) {1, 5, 6, 6, 1, 2} Find the algorithm that can find the two numbers which has time complexity O(n) and space complexity O(1). No sort No radix sort Not sorted XOR 3
4. /39 Solution {1, 5, 6, 6, 1, 2} 1. XOR all the numbers = 1^5^6^6^1^2 = 2^5 = 010^101 = 111 2. Choose any bit which is 1 • Iterate all the bits or try this: The smallest 1 = x & -x 3. Find all the numbers the bit is 0, and XOR to find one answer • G0 = {6, 6, 2}, XOR(G0) = 6^6^2 = 2 4. Find all the numbers the bit is 1, and XOR to find the other answer • G1 = {1, 5, 1}, XOR(G1) = 1^5^1 = 5 4
5. /39 Solution (python3 code except lambda keyword) from functools import reduce def solution(array): xor = reduce(x, y: x ^ y, array, 0) bit = xor & -xor a = reduce(x, y: x ^ y, filter(x: not x & bit, array), 0) b = reduce(x, y: x ^ y, filter(x: x & bit, array), 0) return a, b print(solution([1, 5, 6, 6, 1, 2])) # (2, 5) 5
6. /39 Kotlin for Android 101 2018.05.09 Brice Buzzvil 6
7. /39 References • Home Page • Source code • Kotlin Koans – tutorial • Official References • 깡샘의 코틀린 프로그래밍 7
8. /39 The Billion Dollar Mistake I call it my billion-dollar mistake. It was the invention of the null reference in 1965. At that time, I was designing the first comprehensive type system for references in an object oriented language (ALGOL W). My goal was to ensure that all use of references should be absolutely safe, with checking performed automatically by the compiler. But I couldn't resist the temptation to put in a null reference, simply because it was so easy to implement. This has led to innumerable errors, vulnerabilities, and system crashes, which have probably caused a billion dollars of pain and damage in the last forty years. - Tony Hoare, 2009 Kotlin's type system is aimed at eliminating the danger of null references from code, also known as the The Billion Dollar Mistake. 8
9. /39 • Statically typed programming language • Can be compiled to • Java virtual machine bytecode • JavaScript source code • Designed by JetBrains • First appeared in 2011 • Fully supported by Google on the Android OS (Android Studio 3.0) 9
10. /39 Hello, world! package hello fun main(args: Array<String>) { println("Hello World!") } 10
11. /39 Characteristics • *.kt • Decoupling file and class • Decoupling package and file location • No semicolon (;) at the end of lines • No ‘static’ keyword • Null-aware types • Functional programming • Simplified and modern features added version of Java 11
12. /39 Kotlin for Android • Compatibility: Fully compatible with JDK 6 • Performance: Runs as fast as equivalent java one • Interoperability: 100% interoperable with Java • Footprint: Compact runtime library (proguarded, <=100KB) • Compilation Time: Supports efficient incremental compilation • Learning Curve: Automated Java to Kotlin converter, guides • http://kotlinlang.org/docs/reference/android-overview.html 12
13. /39 Normal File / Class File • Normal File: not a class-only file • Class File: a class-only file 13
14. /39 Variable val value: Int val value2 = 1 var value3: String var value4 = "test" var value5: Boolean = false var value6: Boolean? = null 14
15. /39 Val / Var • Value (val): assign-once variable • Variable (var): mutable variable 15
16. /39 Types • Numbers • Characters • Booleans • Arrays • Strings • Any • Unit • Nothing 16
17. /39 Numbers • Types • Double • Float • Long • Int • Short • Byte • Char • Literals • 123 • 1_000_000 • 123L • 0x0F • 0xFF_AB_CD • 0b001 17
18. /39 Characters • Characters are not a number 18
19. /39 Booleans • true / false • ||, &&, ! 19
20. /39 Arrays • class Array<T> private constructor() { val size: Int operator fun get(index: Int): T operator fun set(index: Int, value: T): Unit operator fun iterator(): Iterator<T> // ... } • val x: IntArray = intArrayOf(1, 2, 3) 20
21. /39 Strings • for (c in str) { println(c) } • val text = """ |Tell me and I forget. |Teach me and I remember. |Involve me and I learn. |(Benjamin Franklin) """.trimMargin() • val s = "abc" println("$s.length is ${s.length}") // prints "abc.length is 3" 21
22. /39 Any, Unit, Nothing • Any • The topmost class of Kotlin • Unit • There is no return value (similar to void, but a type) • val unit: Unit = kotlin.Unit • Nothing • There is no return value / Never returns • fun myFun(): Nothing { while (true) {...} } fun myFun2(): Nothing { throw Exception() } 22
23. /39 ? • type: @NotNull • type?: @Nullable 23
24. /39 Control Flow • If • when • for • while 24
25. /39 If • If is an expression à it returns a value • val result = if (a > b) a else b • val max = if (a > b) { print("Choose a") a } else { print("Choose b") b } 25
26. /39 When • Improved switch operator • var x: Any when (x) { 0 -> print("x == 0") 1, 2 -> print("x == 1 or x == 2") parseInt(s) -> print("x == s") is String -> print("x length: ${x.length}") // Smart cast else -> print("otherwise") } • when { x.isOdd() -> print("x is odd") x.isEven() -> print("x is even") else -> print("x is funny") } 26
27. /39 For • for (i in 1..3) print(i) for ((i, v) in (1..3).withIndex()) { print(i) print(v) } for (i in 6 downTo 0 step 2) { print(i) } 27
28. /39 Class • open class Demo constructor(name: String) { val firstProperty = "First property: $name".also(::println) init { println("First initializer block that prints ${name}") } val secondProperty = "Second property: ${name.length}".also(::println) init { println("Second initializer block that prints ${name.length}") } constructor(name: String, age: Int): this(name) { } } val demo = Demo("brice") 28
29. /39 Properties and Fields • class Demo constructor() { var name: String = "" } • class Demo constructor() { var name: String = "" get() = field set(value) { field = value } } • BuzzScreen.getInstance().getUserProfile().setUserId("testId"); • BuzzScreen.getInstance().userProfile.userId = "testId" 29
30. /39 Data Type Object (DTO, POJO, POCO) • data class Customer(val name: String, val email: String) • getters and setters for all properties • name = c.name vs name = c.getName() • c.name = “name” vs c.setName(“name”) • equals() • hashCode() • toString() • copy() • component1(), … for all properties 30
31. /39 Destructuring Declarations • data class Customer(val name: String, val email: String) • val (name, email) = Customer("name", "name@gmail.com") • val name = customer.component1() val email = customer.component2() 31
32. /39 Functions • fun read(b: Array<Byte>, off: Int = 0, len: Int = b.size): Int { ... } • read(byteArrayOf(1, 2, 3).toTypedArray(), off = 0) 32
33. /39 High Order Functions and Lambdas • High Order Function: A function that takes functions as params or return functions • Lambda Expression • max(strings, { a, b -> a.length < b.length }) • LINQ Style Code • students.filter { it.avgGrade > 4.0 } .sortedBy { it.avgGrade } .take(10) .sortedWith(compareBy( { it.surname }, { it.name })) • Anonymous Function • fun(x: Int, y: Int): Int = x + y • Closures • var sum = 0 ints.filter { it > 0 }.forEach { sum += it } print(sum) 33
34. /39 Equality • Structural Equality • a == b • a?.equals(b) ?: (b === null) • Referential Equality • a === b 34
35. /39 Calling Java code from Kotlin • getter and setter / isBoolean : access it as property • fun calendarDemo() { val calendar = Calendar.getInstance() if (calendar.firstDayOfWeek == Calendar.SUNDAY) { // call getFirstDayOfWeek() calendar.firstDayOfWeek = Calendar.MONDAY // call setFirstDayOfWeek() } if (!calendar.isLenient) { // call isLenient() calendar.isLenient = true // call setLenient() } } • https://kotlinlang.org/docs/reference/java-interop.html 35
36. /39 Calling Kotlin code from Java • Properties -> getter / setter • Package Level Functions • https://kotlinlang.org/docs/reference/java-to-kotlin-interop.html // example.kt package demo class Foo fun bar() { } new demo.Foo(); demo.ExampleKt.bar(); 36
37. /39 Tools – Kotlin Android Extensions • // Using R.layout.activity_main from the 'main' source set import kotlinx.android.synthetic.main.activity_main.* class MyActivity : Activity() { override fun onCreate(savedInstanceState: Bundle?) { super.onCreate(savedInstanceState) setContentView(R.layout.activity_main) // Instead of findViewById<TextView>(R.id.textView) textView.setText("Hello, world!") } } 37
38. /39 Apps built with Kotlin 38
39. /39 Kotlin Koans https://try.kotlinlang.org/ 39
40. /39 Questions 40