Kotlin 开发速查手册

Kotlin 开发速查手册是 Kotlin 编程语言的单页参考表

Kotlin 简介main() 函数打印声明注释执行顺序数据类型和变量可变变量不可变变量类型推断字符串连接字符串模板内置属性和函数字符转义序列算术运算符操作顺序增强赋值运算符递增和递减运算符数学库条件表达式if 表达式else 表达式Else-If 表达式比较运算符逻辑运算符AND 运算符： &&或运算符：||NOT 运算符评估顺序嵌套条件当表达式范围运算符等式运算符Collections不可变列表可变列表访问列表元素大小属性列表操作不可变集可变集访问集合元素不可变映射可变映射检索映射键和值添加和删除地图条目函数函数函数参数默认参数命名参数返回声明单表达式函数函数字面量类类示例类实例主构造函数初始化块成员函数另见

Kotlin 简介

main() 函数

fun main() {
  // Code goes here
}

main() 函数是每个 Kotlin 程序的起点，在执行之前必须包含在代码中

打印声明

println("Greetings, earthling!")
print("Take me to ")
print("your leader.")

/* 
打印:
Greetings, earthling!
Take me to your leader.
*/

注释

// 这是单行注释

/*
这个
注释
用于
多
行
*/

执行顺序

fun main() {
  println("I will be printed first.")
  println("I will be printed second.")
  println("I will be printed third.")
}

数据类型和变量

可变变量

var age = 25
age = 26

不可变变量

val goldenRatio = 1.618

类型推断

// 以下变量在双引号内分配了一个文本值
// 因此推断的类型是 String

var color = "Purple"

字符串连接

var streetAddress = "123 Main St."
var cityState = "Brooklyn, NY" 

println(streetAddress + " " + cityState) 
// 打印: 123 Main St. Brooklyn, NY

字符串模板

var address = "123 Main St."
println("The address is $address") 
// 打印: The address is 123 Main St.

内置属性和函数

var monument = "the Statue of Liberty"

println(monument.capitalize())
// 打印: The Statue of Liberty
println(monument.length)
// 打印: 21

字符转义序列

print("\"Excellent!\" I cried. \"Elementary,\" said he.") 

// 打印: "Excellent!" I cried. "Elementary," said he.
  • \n 插入新行
  • \t 插入标签
  • \r 插入回车
  • \' 插入单引号
  • \" 插入双引号
  • \\ 插入反斜杠
  • \$ 插入美元符号

算术运算符

5 + 7  // 12
9 - 2  // 7
8 * 4  // 32
25 / 5 // 5 
31 % 2 // 1

+ 加法、- 减法、* 乘法、/ 除法和 % 模数

操作顺序

5 + 8 * 2 / 4 - 3 // 6 
3 + (4 + 4) / 2   // 7
4 * 2 + 1 * 7     // 15
3 + 18 / 2 * 1    // 12  
6 - 3 % 2 + 2     // 7

增强赋值运算符

var batteryPercentage = 80

// 长语法
batteryPercentage = batteryPercantage + 10 

// 带有增广赋值运算符的短句法
batteryPercentage += 10

递增和递减运算符

var year = 2019 
year++ // 2020
year-- // 2019

数学库

Math.pow(2.0, 3.0)  // 8.0
Math.min(6, 9)      // 6 
Math.max(10, 12)    // 12
Math.round(13.7)    // 14

条件表达式

if 表达式

var morning = true

if (morning) {
  println("Rise and shine!")
}
// 打印: Rise and shine!

else 表达式

var rained = false

if (rained) {
  println("今天不需要给植物浇水。")
} else {
  println("植物需要浇水！")
}
// 打印: 植物需要浇水！

Else-If 表达式

var age = 65

if (age < 18 ) {
  println("您被视为未成年人")
} else if (age < 60) {
  println("您被视为成年人")
} else {
  println("你被认为是高级")
}

// 打印: 你被认为是高级

比较运算符

var myAge = 19
var sisterAge = 11
var cousinAge = 11

myAge > sisterAge  // true
myAge < cousinAge  // false
myAge >= cousinAge // true
myAge <= sisterAge // false

逻辑运算符

var humid = true
var raining = true
var jacket = false

println(!humid)
// 打印: false
println(jacket && raining)
// 打印: true
println(humid || raining)
// 打印: true

AND 运算符： &&

var humid = true
var raining = true
var shorts = false
var sunny = false

// true AND true
println(humid && raining) //  true
// true AND false
println(humid && shorts)  //  false
// false AND true
println(sunny && raining) //  false
// false AND false
println(shorts && sunny)  // false

或运算符：||

var late = true
var skipBreakfast = true
var underslept = false
var checkEmails = false

// true OR true
println(skipBreakfast || late) //  true
// true OR false
println(late || checkEmails) //  true
// false OR true
println(underslept || late) //  true
// false OR false
println(checkEmails || underslept) // false

NOT 运算符

var hungry = true
var full = false

println(!hungry) //  false
println(!full) //  true

评估顺序

!true && (false || true) // false
/*
(false || true) 被评估首先返回 true。
然后，评估 !true && true 并返回最终结果 false
*/

!false && true || false // true
/*
!false 被评估首先返回 true。 
然后评估 true && true，返回 true。
那么，真|| 评估 false 最终返回 true
*/

嵌套条件

var studied = true
var wellRested = true

if (wellRested) {
  println("祝你今天好运！")
  if (studied) {
    println("你应该为考试做好准备！")
  } else {
    println("考试前花几个小时学习！")
  }
}

// 打印: 祝你今天好运！
// 打印: 你应该为考试做好准备！

当表达式

var grade = "A"

when(grade) {
  "A" -> println("很棒的工作！")
  "B" -> println("做得太好了！")
  "C" -> println("你通过了！")
  else -> println("关！下次一定要多准备！")
}
// 打印: 很棒的工作！

范围运算符

var height = 46 // inches

if (height in 1..53) {
  println("抱歉，您必须至少 54 英寸才能乘坐过山车"）
}
// Prints: 抱歉，您必须至少 54 英寸才能乘坐过山车

等式运算符

var myAge = 22
var sisterAge = 21

myAge == sisterAge // false
myAge !== sisterAge // true

Collections

不可变列表

var programmingLanguages = listOf("C#", "Java", "Kotlin", "Ruby")

可变列表

var fruits = mutableListOf("Orange", "Apple", "Banana", "Mango")

访问列表元素

var cars = listOf("BMW", "Ferrari", "Volvo", "Tesla")

println(cars[2]) // Prints: Volvo

大小属性

var worldContinents = listOf("Asia", "Africa", "North America", "South America", "Antarctica", "Europe", "Australia")

println(worldContinents.size) // Prints: 7

列表操作

var seas = listOf("Black Sea", "Caribbean Sea", "North Sea") 
println(seas.contains("North Sea")) // Prints: true

// contains() 函数对任何列表执行读取操作并确定元素是否存在
seas.add("Baltic Sea") // 错误：无法对不可变列表执行写操作
// add() 函数只能在可变列表上调用，因此上面的代码会引发错误

不可变集

var primaryColors = setOf("Red", "Blue", "Yellow")

可变集

var womenInTech = mutableSetOf("Ada Lovelace",  "Grace Hopper",  "Radia Perlman",  "Sister Mary Kenneth Keller")

访问集合元素

var companies = setOf("Facebook", "Apple", "Netflix", "Google")

println(companies.elementAt(3))
// Prints: Google
println(companies.elementAt(4))
// Returns and Error
println(companies.elementAtOrNull(4))
// Prints: null

不可变映射

var averageTemp = mapOf("winter" to 35,  "spring" to 60,  "summer" to 85, "fall" to 55)

可变映射

var europeanDomains = mutableMapOf("Germany" to "de", "Slovakia" to "sk", "Hungary" to "hu", "Norway" to "no")

检索映射键和值

var oscarWinners = mutableMapOf("Parasite" to "Bong Joon-ho", "Green Book" to "Jim Burke", "The Shape Of Water" to "Guillermo del Toro")

println(oscarWinners.keys)
// Prints: [Parasite, Green Book, The Shape Of Water]

println(oscarWinners.values)
// Prints: [Bong Joon-ho, Jim Burke, Guillermo del Toro]

println(oscarWinners["Parasite"])
// Prints: Bong Joon-ho

添加和删除地图条目

var worldCapitals = mutableMapOf("United States" to "Washington D.C.", "Germany" to "Berlin", "Mexico" to "Mexico City", "France" to "Paris")

worldCapitals.put("Brazil", "Brasilia")
println(worldCapitals)
// Prints: {United States=Washington D.C., Germany=Berlin, Mexico=Mexico City, France=Paris, Brazil=Brasilia}

worldCapitals.remove("Germany")
println(worldCapitals)
// Prints: {United States=Washington D.C., Mexico=Mexico City, France=Paris, Brazil=Brasilia}

函数

函数

fun greet() {
  println("Hey there!")
}

fun main() {
  // Function call
  greet() // Prints: Hey there!
}

函数参数

fun birthday(name: String, age: Int) {
   println("Happy birthday $name! You turn $age today!")
}

fun main() {
  birthday("Oscar", 26) 
  // Prints: Happy birthday Oscar! You turn 25 today!
  birthday("Amarah", 30) 
  // Prints: Happy birthday Amarah! You turn 30 today!
}

默认参数

fun favoriteLanguage(name, language = "Kotlin") {
  println("Hello, $name. Your favorite programming language is $language")  
}

fun main() {
  favoriteLanguage("Manon") 
  // Prints: Hello, Manon. Your favorite programming language is Kotlin
  
  favoriteLanguage("Lee", "Java") 
  // Prints: Hello, Lee. Your favorite programming language is Java
}

命名参数

fun findMyAge(currentYear: Int, birthYear: Int) {
   var myAge = currentYear - birthYear
   println("I am $myAge years old.")
}

fun main() {
  findMyAge(currentYear = 2020, birthYear = 1995)
  // Prints: I am 25 years old.
  findMyAge(birthYear = 1920, currentYear = 2020)
  // Prints: I am 100 years old.
}

返回声明

// Return type is declared outside the parentheses
fun getArea(length: Int, width: Int): Int {
  var area = length * width

  // return statement
  return area
}

fun main() {
  var myArea = getArea(10, 8)
  println("The area is $myArea.")
  // Prints: The area is 80.
}

单表达式函数

fun fullName(firstName: String, lastName: String) = "$firstName $lastName"

fun main() {
  println(fullName("Ariana", "Ortega")) 
  // Prints: Ariana Ortega
  println(fullName("Kai", "Gittens")) 
  // Prints: Kai Gittens
}

函数字面量

fun main() {
  // Anonymous Function:
  var getProduct = fun(num1: Int, num2: Int): Int {
     return num1 * num2
  }
  println(getProduct(8, 3)) 
  // Prints: 24

  // Lambda Expression
  var getDifference = { num1: Int, num2: Int -> num1 - num2 }
  println(getDifference(10, 3))
  // Prints: 7
}

类

类示例

// 具有包含默认值的属性的类
class Student {
  var name = "Lucia"
  var semester = "Fall"
  var gpa = 3.95
}

// 没有类体的简写语法
class Student

类实例

// Class
class Student {
  var name = "Lucia"
  var semester = "Fall"
  var gpa = 3.95
}

fun main() {
  var student = Student()   
  // Instance
  println(student.name)     
  // Prints: Lucia
  println(student.semester) 
  // Prints: Fall
  println(student.gpa)      
  // Prints: 3.95  
}

主构造函数

class Student(val name: String, val gpa: Double, val semester: String, val estimatedGraduationYear: Int) 

fun main() {
  var student = Student("Lucia", 3.95, "Fall", 2022) 
  println(student.name)     
  // Prints: Lucia
  println(student.gpa)      
  // Prints: 3.95
  println(student.semester) 
  // Prints: Fall
  println(student.estimatedGraduationYear) 
  // Prints: 2022
}

初始化块

class Student(val name: String, val gpa: Double, val semester: String, val estimatedGraduationYear: Int) {
  init {
    println("$name has ${estimatedGraduationYear - 2020} years left in college.")
  }
}

fun main() {
  var student = Student("Lucia", 3.95, "Fall", 2022)
  // Prints: Lucia has 2 years left in college. 
}

成员函数

class Student(val name: String, val gpa: Double, val semester: String, val estimatedGraduationYear: Int) {

  init {
    println("$name has ${estimatedGraduationYear - 2020} years left in college.")
  }

  // 成员函数
  fun calculateLetterGrade(): String {
    return when {
      gpa >= 3.0 -> "A"
      gpa >= 2.7 -> "B"
      gpa >= 1.7 -> "C"
      gpa >= 1.0 -> "D"
      else -> "E"
    }
  }
}

// 创建实例并调用函数时，将执行 when 表达式并返回字母等级
fun main() {
  var student = Student("Lucia", 3.95, "Fall", 2022) 
  // Prints: Lucia has 2 years left in college. 
  println("${student.name}'s letter grade is ${student.calculateLetterGrade()}.") 
  // Prints: Lucia's letter grade is A. 
}

另见