這份 Kotlin 快速指南會概略說明 Android Kotlin 基本概念課程所涵蓋的主題，並以程式碼片段提供輔助。
瞭解詳情
- 如需完整參考資料，請參閱 Kotlin 語言說明文件。
- 如需更多可用於練習的程式碼片段，請參閱 Kotlin Koans。注意：系統會將您導向由 JetBrains 開發的外部 kotlinlang.org 網站。
單元 1
Kotlin 程式
用於顯示文字的最小型 main() 程式
fun main() {
println("Hello, world!")
}
顯示一行文字
println("This is the text to print!")
內嵌註解
// This is a comment line.
// This is another comment.
變數
// Assign once, cannot change.
val age = "5"
val name = "Rover"
// Assign and change as needed.
var roll = 6
var rolledValue: Int = 4
使用字串範本顯示變數
println("You are already ${age}!")
println("You are already ${age} days old, ${name}!")
資料類型
Int // Whole positive and negative numbers and zero
String // Piece of text between quotes
IntRange // Setting a range such as 1..6
Boolean // Can only be true or false
不含引數的函式
// Define the function.
fun printHello () {
println ("Hello Kotlin")
}
// Call the function.
printHello()
含引數的函式
fun printBorder(border: String, timesToRepeat: Int) {
repeat(timesToRepeat) {
print(border)
}
println()
}
傳回值的函式
fun roll(): Int {
val randomNumber = (1..6).random()
return randomNumber
}
基本運算子
* // multiplication
/ // division
+ // addition
- // subtraction
= // assignment
邏輯運算子
> // greater than
< // less than
== // equal
>= // greater or equal
<= // less or equal
!= // not equal
建立隨機數字
val randomNumber = diceRange.random()
fun roll() {
val anotherRandomNumber = (1..6).random()
println(randomNumber)
}
控管程式流程
使用 repeat() 重複動作
fun printBorder() {
repeat(23) {
print("=")
}
}
建立 repeat() 迴圈的巢狀結構
fun printCakeBottom(age: Int, layers: Int) {
repeat(layers) {
repeat(age + 2) {
print("@")
}
println()
}
}
使用 if/else 的條件陳述式
fun main() {
val num = 4
if (num > 4) {
println("The variable is greater than 4")
} else if (num == 4) {
println("The variable is equal to 4")
} else {
println("The variable is less than 4")
}
}
使用 when 的條件陳述式
when (rollResult) {
luckyNumber -> println("You won!")
1 -> println("So sorry! You rolled a 1. Try again!")
2 -> println("Sadly, you rolled a 2. Try again!")
3 -> println("Unfortunately, you rolled a 3. Try again!")
4 -> println("No luck! You rolled a 4. Try again!")
5 -> println("Don't cry! You rolled a 5. Try again!")
6 -> println("Apologies! you rolled a 6. Try again!")
}
將 when 陳述式的結果指派給變數
// Determine which drawable resource ID to use based on the dice roll.
val drawableResource = when (diceRoll) {
1 -> R.drawable.dice_1
2 -> R.drawable.dice_2
3 -> R.drawable.dice_3
4 -> R.drawable.dice_4
5 -> R.drawable.dice_5
else -> R.drawable.dice_6
}
類別
含屬性和方法的簡易類別
class Dice {
var sides = 6
fun roll() {
val randomNumber = (1..6).random()
println(randomNumber)
}
}
含參數的類別
class Dice (val numSides: Int) {
fun roll(): Int {
val randomNumber = (1..numSides).random()
return randomNumber
}
}
建立執行個體
val myFirstDice = Dice(6)
單元 2
類別
抽象類別
abstract class Dwelling() {
}
含抽象屬性的抽象類別
abstract class Dwelling() {
abstract val buildingMaterial: String
}
含抽象方法的抽象類別
abstract class Dwelling() {
abstract fun floorArea(): Double
}
將類別設為 open，即可做為子類別使用
open class RoundHut(residents: Int) {
}
以擴充父類別的方式來建立子類別
class SquareCabin : Dwelling()
覆寫父類別的函式
override fun floorArea(): Double {
}
呼叫函式的父類別實作
override fun floorArea(): Double {
return super.floorArea() * floors
}
清單
定義唯讀清單
val numbers = listOf(1, 2, 3, 4, 5, 6)
取得清單大小
numbers.size
取得清單的第一個項目
numbers[0]
取得反向排序的清單副本
listOf("red", "blue", "green").reversed()
定義可變動的字串清單
val entrees = mutableListOf<String>()
將項目加入可變動的清單
entrees.add("spaghetti")
修改可變動清單中的項目
entrees[0] = "lasagna"
從可變動的清單中移除項目
entrees.remove("lasagna")
迴圈
使用 for 迴圈疊代清單中的項目
for (element in myList) {
// Perform an operation with each item
println(element)
}
使用 while 迴圈疊代清單中的項目
var index = 0
while (index < myList.size) {
println(myList[index])
index++
}
字串
字串中的字元數
val name = "Android"
println(name.length)
在字串範本中使用變數
val number = 10
println("$number people")
在字串範本中使用運算式
val number = 10
val groups = 5
println("${number * groups} people")
其他
擴增指派
a += b // a = a + b
a -= b // a = a - b
a *= b // a = a * b
a /= b // a = a / b
使用 with 簡化物件存取方法
with(squareCabin) {
println("Capacity: ${capacity}")
println("Material: ${buildingMaterial}")
println("Has room? ${hasRoom()}")
}
從 Kotlin 數學資料庫匯入
import kotlin.math.PI
在 Kotlin 數學資料庫中使用常數的完整名稱 (無需匯入陳述式)
kotlin.math.PI * radius * radius
同時執行鏈結呼叫 (用於存取屬性和呼叫方法)
val stringInTextField = binding.costOfService.text.toString()
函式引數的變數數量
fun addToppings(vararg val toppings: String)
套件宣告
package com.example.affirmations.model
單元 3
集合
從清單建立集合
val numbers = listOf(0, 3, 8, 4, 0, 5, 5, 8, 9, 2)
val setOfNumbers = numbers.toSet()
定義集合
val set1 = setOf(1,2,3)
val set2 = mutableSetOf(3, 4, 5)
集合運算
set1.intersect(set2) // 3
set1.union(set2) // 1, 2, 3, 4, 5
映射
定義可變動的映射
val peopleAges = mutableMapOf<String, Int>(
"Fred" to 30,
"Ann" to 23
)
在可變動的映射中設定值
peopleAges.put("Barbara", 42)
peopleAges["Joe"] = 51
集合族運算
對集合族進行疊代
peopleAges.forEach { print("${it.key} is ${it.value}, ") }
// Fred is 31, Ann is 23, Barbara is 42, Joe is 51,
轉換集合族中的每個項目
println(peopleAges.map { "${it.key} is ${it.value}" }.joinToString(", ") )
// Fred is 31, Ann is 23, Barbara is 42, Joe is 51
篩選集合族中的項目
val filteredNames = peopleAges.filter { it.key.length < 4 }
println(filteredNames)
// {Ann=23, Joe=51}
其他集合族運算
val words = listOf("about", "acute", "balloon", "best", "brief", "class")
val filteredWords = words.filter { it.startsWith("b", ignoreCase = true) }
.shuffled() // [brief, balloon, best]
.take(2) // [brief, balloon]
.sorted() // [balloon, brief]
範圍函式
let
arguments?.let {
letterId = it.getString(LETTER).toString()
}
apply
binding?.apply {
...
flavorFragment = this@FlavorFragment
}
其他
幕後屬性
private var _currentScrambledWord = "test"
val currentScrambledWord: String
get() = _currentScrambledWord
安全呼叫
val letterId = intent?.extras?.getString("letter").toString()
Lambda 函式
val triple: (Int) -> Int = { a: Int -> a * 3 }
println(triple(5))
伴生物件
class DetailActivity: AppCompatActivity() {
...
companion object {
const val LETTER = "letter"
}
...
}
// accessible outside DetailActivity
DetailActivity.LETTER
屬性委任
// syntax
var <property-name> : <property-type> by <delegate-class>()
// example
private val viewModel: GameViewModel by viewModels()
延遲初始化
private var wordsList: MutableList<String> = mutableListOf() // has a value at initialization
private lateinit var currentWord: String // needs to be assigned after initialization
Elvis 運算子
var quantity : Int? = null
quantity ?: 0 // 0
quantity = 4
quantity ?: 0 // 4
單元 4
協同程式
宣告懸置函式
suspend fun getValue(): Double {
// long running work or calls to other suspend functions
}
在 GlobalScope 中執行懸置函式
GlobalScope.launch {
val output = getValue()
}
透過其他懸置函式呼叫懸置函式
suspend fun processValue() {
val value = getValue()
// modify the value
}
存取協同程式 Job
val job: Job = GlobalScope.launch {
val output = getValue()
}
取消協同程式 Job
job.cancel()
執行懸置函式，並封鎖目前的執行緒，直到函式完成為止
runBlocking {
val output = getValue()
}
以非同步方式延後執行懸置函式
runBlocking {
val output = await { getValue() }
println("Output is ${output.await()}")
}
其他
宣告物件
object DataProviderManager {
}
攔截例外狀況
try {
// code that may throw an error
} catch (exception: Exception) {
// handle the thrown exception
}
建立列舉類別
enum class Direction {
NORTH, SOUTH, WEST, EAST
}
存取列舉類別值
val direction = Direction.NORTH
檢查列舉值
when (direction) {
Directon.NORTH -> {
}
Direction.SOUTH -> {
}
Direction.WEST -> {
}
Direction.EAST -> {
}
}