Kotlin

Basic Concepts

intro

used to develop Android App

Hello World

fun main(args: Array<String>) {
    println("Hello World!")
}

Data Types

Int
Double
Boolean
Char

Variables

变量

TIP

变量: var

var num: Int = 12
var course: String = "Kotlin"

常量

TIP

常量: val

val num = 12

TIP

Kotlin支持类型自动推断

val num = 12
val course = "Kotlin"

Operators

Arithmetic Operators

+ - * / %

println("Hello " + "Kotlin")  // "Hello Kotlin"

Assignment Operators

+= -= *= /= %=

Comparison Operators

> < >= <= == !=

Logocal Operators

&& || !

Increment & Decrement Operators

i++
i--
++i
--i

Comments

// this is a single comment

/*
this is a multi-line comment
*/

Input

readLine()              // String
readLine()!!.toInt()    // Int
readLine()!!.toDouble() // Double

not null assertion operator

WARNING

!! 表示非空

var age = readLine()
parintln("You entered " + age)

Control

if Statement

TIP

三元运算写法与python相同多一个when，与switch有类似之处

if (condition) {
    // statement
} else if (condition) {
    // statement
} else if (condition) {
    // statement
} else {
    // statement
}

三元运算

var res = if (condition) ... else ...

Example

var x = 2
var y = if (x > 1) x + 2 else x -2

when

var res = when {
    condition1 -> ...
    condition2 -> ...
    else -> ...
}

Example

val age = 19
val group = when {
    age >= 18 -> "Adult"
    else -> "Child"
}

For loops

for(x in nums) {...}

TIP

for (x in nums) {...}

var nums = arrayOf(2, 4, 7, 9)
for (num in nums) {
    println(num)
}

arr.forEach

TIP

arr.forEach kotlin porvides an it keyword for name elements in forEach.

fun main(args: Array<String>) {
    var arr = arrayOf(1, 2, 3)
    arr.forEach {
        // v -> println(v * 4)
        println(it * 4)
    }
}

TIP

2..5 => [2, 3, 4] 'a'..'e' => ['a', 'b', 'c', 'd', 'e']

var nums = arrayOf(2, 4, 7, 9)
for (num in 2..5) {
    println(num)
}

while Loops

var i = 0
while (condition) {
    // statement
    i++
}

break & continue

break
continue

Arrays

var array = arrayOf("Tom", "Bob", "Amy")
var nums = 2..6

TIP

2..5 => [2, 3, 4, 5] 'a'..'e' => ['a', 'b', 'c', 'd', 'e']

var nums = arrayOf(1, 2, 3, 8)
println(nums[0])   // 1

var s = "Kotlin"
println(s[2])     // 't'

var nums = arrayOf(1, 2, 3, 8, 10)
val res = nums.filter({it > 5})
println(res)   // [8, 10]

Function

Define Function

fun isEven(num: Int): Boolean {
    return num%2 == 0
}
fun main(args: Array<String>) {
    println(isEven(6)) // true
}

Function Arguments

fun welcome(name: String) {
    println("hello, " + name)
}

fun plus(x: Int, y: Int) {
    println(x + y)
}

fun main(args: Array<String>) {
    welcome("Amy")    // "hello, Amy"
    plus(3, 4)         // 7
}

Return

fun sum(x: Int, y: Int): Int {
    return (x + y)
}

fun main(args: Array<String>) {
    println(sum(3, 4)) // 7
}

Anonymous Function 匿名函数

TIP

kotlin automatically understands the return type from the Arguments

// val f: (Int, Int) -> Int = (a, b -> a + b)
val f = { a: Int, b: Int -> a + b }

println(f(2, 8)) // 10

{String, String} -> Boolean

fun main(args: Array<String>) {
    var arr = arrayOf(1, 2, 3)
    arr.forEach(
        // v -> println( v * 4)
        println(it * 4)
    )
}

Higher-order Function 自定义方法

fun apply(x: Int, action: (Int) -> Int): Int {
    return action(x)
}
fun main(args: Array<String>) {
    println(apply(4, {x -> x*2})) // 8
}

Classes

TIP

OOP: Object-Oriented Programming

Defined Class

class User {
    var name = ""
    var age = 0
}

Create object of the class

var u1 = User()
u1.name = "James"
ui.age = 12

Constructors

class User(var name: String, var age: Int) {
}

fun main(args: Array<String>) {
    val u1 = User("James", 12)
    println(u1.name) // "James"
}

multple constructors

class User {
    var name = ""
    var age = 0
    constructor(name: String) {
        name = name
    }
    constructor(name: String, age: Int) {
        name = name
        age = age
    }
}

Getter & Setter

class User {
    var name = ""
    var age = 0

    get() = filed
    set(value) {
        if(value < 0) {
            filed = 18
        } else {
            filed = value
        }

        // filed = if (value < 0) 18 else value
    }
}

Inheritance

TIP

open 定义父类

open class User (var name: String, var age: Int) {
}

class Admin (name: String, age: Int, var country: String): User(name, age) {}

fun main(args: Array<String>) {
    val a = Admin("Amy", 23, "USA")
    println(a.country)    // "USA"
}

class Cat: Animal{}

Visiblity Modifiers

public
protected
private

privete

class User(var name: String, private age: Int) {}

fun main(args: Array<String>) {
    val u1 = User("Amy", 23)
    println(u1.age)     // error
}

protected

open class Account() {
    protected var balance = 0
}

class Saving(): Account() {
    // some code
}

Abstract Classes

TIP

Abstract classes are always open, so don't need to yse the open keyword

abstract class User(var name: String, var age: Int) {}

class Admin(name: String, age: Int): User(name, age){}

class Moderator(name: String, age: Int, var country: String): User(name, age) {}

TIP

抽象类可以包含抽象方法--方法没有定义具体实现，需要implement abstractoverride

abstract class User(var name: String, var age: Int) {
    abstract fun display()
}

class Admin(name: String, age: Int): User(name, age){
    override fun display() {
        println(name + " is " + age + " years old")
    }
}

class Moderator(name: String, age: Int, var country: String): User(name, age) {
    override fun display(
        println(name + " is from " + country)
    )
}

Code Projects

1. Water Consumption

fun main(args: Array<String>) {
    var years = readLine()!!.toInt()
    val res = years * 365 * 15
    println(res)
}

2. Parking Fee

fun main(args: Array<String>) {
    var hours = readLine()!!.toInt()
    var total: Double = 0.0

    total = when {
        hours >= 24 -> (hours / 24) * 15.0 + (hours % 24) * 0.5
        hours <= 5 -> hours * 1.0
        else 5.0 + ((hours - 5) * 0.5)
    }

    /*
    if (hours >= 24) {
        total = ((hours / 24) * 15.0) + ((hours % 24) * 0.5)
    } else if (hours <= 5) {
        total = hours * 1.0
    } else {
        total = 5.0 + ((hours - 5) * 0.5)
    }
    */
    println(total)
}

3. Shipping Calculator

fun shippingCost(amount: Double, international: Boolean): Double {
    return (
        if(international) (if(amount * 0.15 < 50) amount * 0.15 else 50.0)
        else (if(amount > 75) 0.0 else amount * 0.1)
    )
}
fun main(args: Array<String>) {
    val total = readLine()!!.toDouble()
    val international = readLine()!!.toBoolean()
    println(shippingCost(total, international))
}

3. Music Player

class MusicPlayer {
    private var songs: Array<String> = arrayOf()
    //your code goes here
    fun add(song: String) {
        songs += song
    }

    fun show() {
        songs.forEach {
            println(it)
        }
    }

    fun play() {
        println("Playing " + songs[0])
    }
}
fun main(args: Array<String>) {
    val m = MusicPlayer()

    while(true) {
        var input = readLine()!!
        if(input == "stop") {
            break
        }
        m.add(input)
    }
    m.show()
    m.play()
}

# Kotlin

# Basic Concepts

# intro

# Hello World

# Data Types

# Variables

# Operators

# Comments

# Input

# Control

# if Statement

# For loops

# while Loops

# break & continue

# Arrays

# Function

# Define Function

# Function Arguments

# Return

# Anonymous Function 匿名函数

# Higher-order Function 自定义方法

# Classes

# Defined Class

# Create object of the class

# Constructors

# Getter & Setter

# Inheritance

# Visiblity Modifiers

# Abstract Classes

# Code Projects