我和科特林的第一次经历
我和科特林的第一次经历
原文：https://medium.com/hackernoon/my-first-experiences-with-kotlin-b3fcf5e111f7
一些可能对初学者有用的快速笔记
Kotlin 是一种新语言，至少大多数 Android 开发者听说过它，或者正在使用它来代替 T2 Java T3。
出于我的好奇心，我开始学习它，想知道它有什么吸引人的地方，让一些开发者为之疯狂！所以在我的学习过程中，我做了一些笔记，这些笔记可能看起来太小了，但包含了初学者和你以后搜索的关键词。
这是我喜欢在学习过程开始时做的笔记:
变量:
val immutableName: String = "Mohsen"var mutableName: String = "Mohsen"
var mutableName = "Mohsen"var nullableName: String? = nullval numbers = arrayListOf(1, 2, 3, 4, 5)
print("Even Numbers are ${numbers.filter { i -> i % 2 == 0 }}")val countries = listOf(Pair("Iran", "Tehran"), Pair("France", "Paris"), Pair("Germany", "Berlin"))for ((country, city) in countries) {
print("Country: $country - City: $city")
}功能:
// fun stands for function!fun addNumbers(first: Int, second: Int): Int {
return first + second
}
addNumbers(10, 20)
addNumbers(first = 10, second = 20)
addNumbers(second = 1, first = 2) // !!!// you can have multiple params
fun addNumbers(vararg numbers: Int): Int {
return numbers.sum()
}
addNumbers(1, 2, 3, 4, 5, 6) // default value
fun addTwoNumbers(first: Int, secound: Int = 0): Int {
return first + secound
}
addTwoNumbers(2)班级:
// no fields in classes! only properties and functions
class Customer(val id: Int = 0, var name: String) { // secondary constructor
constructor(id: Int) : this(id, "")
// initialize method
init() {
}
// public by default property
val test = 0
// visible inside Customer.kt
private fun foo() {}
// property is visible everywhere
// setter is visible only in Customer.kt
public var bar: Int = 5
private set
// visible inside the same module
internal val baz = 6
}数据类别:
// mutable data classes similar to AutoValue library which provide equals, toString, copy, etc.
data class Customer(val id: Int = 0, var name: String)扩展功能:
fun String.convertSpacesToUnderscores(): String {
return this.replace(" ", "_")
}print("This is my sample String.".convertSpacesToUnderscores())空:
val x: String = null //error!
val x: String? = null// if x is not null range to 10
x?.rangeTo(10)让:
var customer: Customer? = null
customer?.let {
// it = user and not null and only read once!
println(it.name)
}someMethod().let { result ->
//method only run once!
}任何:
fun getStringLen(obj: Any): Int? = if (obj is String) obj.length else null同伴:
// the same as Static methods and you have to access the
// method like this: Customer.Companion.what()class Customer {
companion object {
fun what(customer: Customer): String {
return customer.id.toString() + customer.name
}
}
}// If you want the function looks like java static methods
// like this: Customer.what() have to add JvmStatic
// It is useful for calling static methods from NDK libraries
class Customer {
companion object {
@JvmStatic
fun what(customer: Customer): String {
return customer.id.toString() + customer.name
}
}
}包装功能:
// functions with no classes, just the same as java import static
package com.google.testfun packageFunction() {
println("I am a package function!!")
}列表，数组列表，映射:
val list = listOf("a", "b")
val mutableList = mutableListOf("a", "b")for (s in list) {
// ...
}val map = mapOf("key" to "value")
val mutableMap = mutableMapOf("key" to "value")for ((key, value) in map) {
// ...
}接口:
interface TestInterface {
fun run(t:Int): String
fun test(){
// default body!
}
}通用:
class Box<T>(t: T) {
// ...
}
Val box: Box(1) // box: Box<Int>枚举:
enum class Direction(val degree: Int) {
NORTH(0), EAST(90), SOUTH(180), WEST(270)
}铸造类型:
val y = x as Int
val y = x as? Int // nullable// smart cast
If (x is String)
x.toUpperCase()运算符重载:
operator fun plus(other: Point): Point = Point(x + other.x, y + other.y)继承:
// all classes are final by default and for inheritance should be open
open class Shape {
}class Circle: Shape() {
}中缀:
infix fun String.hello(name:String): String{
return "Hello, $name. $this"
}"Test" hello "test" // == "Test".hello("test")代表们:
interface Repository<T : Record> {
fun getById(id: Int): T
fun getAll(): List<T>
}class Controller(repository: Repository<Customer>) : Repository<Customer> by repository {
fun customerList(): List<Customer> {
return getAll()
}
fun customerById(id: Int): Customer {
return getById(id)
}
}var test: String by Delegates.observable("") {
prop, old, new ->
*println*("$old -> $new")
}密封:
// sealed means there is no PageResult child more than ones that are here in this file
sealed class PageResultclass Success(val url: String) : PageResult()
class Error(val code: Int, val message: String) : PageResult()fun getPage(url: String): PageResult {
val someOperationIsSuccessfull = false
if (someOperationIsSuccessfull)
return Success(url)
else
return Error(404, "Not found")
}fun callingPage() {
val result = getPage("http://google.com")
when (result) {
is Success -> println(result.url)
is Error -> println(result.message)
}
}懒惰:
class User(firstName: String, lastName: String) {
val fullName: String by lazy { "$firstName $lastName" } fun printFullName() {
println("User full name is: $fullName")
}
}本地回报:
**fun** myFunction() {
**val** numbers = 1..100
numbers.*forEach* **{
if**(**it**%5==0)
**return**@forEach
**}** *println*(**"Hello!!"**) //Hello will be printed!
}Tailrec:
//tailrec convert a recursive function call to a goto loop **tailrec fun** factorial(number: Int, accumulator: Int = 1): Int {
**when** (number) {
0 -> **return** accumulator
**else** -> **return** *factorial*(number - 1, accumulator * number)
}
}Java 开发人员的一些不寻常的语法！！：
// multiline string literals
print("""1\. test
2\. unit test
3\. what a test""")print("""1\. test
2\. unit test
3\. what a test""".trimIndent())print("""|1\. test
|2\. unit test
|3\. what a test""".trimMargin())fun max(a: Int, b: Int) = if (a > b) a else b// no for (int i=0;i<10;I++) loop!!!
val myArray = *arrayOf*("A","B","C","D")
for (index in 0 *until* myArray.size){
*println*(myArray.get(index))
}for ( i in 1..10 )
*println*(myArray.get(index))
}for ( i in 100 downTo 1 step 5)
*println*(myArray.get(index))
}var i = 0
while (i < 10)
println (i++)// no switch case default
when (i) {
0 -> println("zero")
1..5 -> println("number")
is String -> println("$i is string")
!is String -> println("is not string")
else -> println("unknown!!!")
}val isString = when ("Hello") {
is String -> true
else -> false
}val isInRange = i in 5..9
val isMember = "hello" in array// set alias while importing!
Import com.google.Test as MyTest// loop operators
while (true){
if(false) break
if(false) continue
if(false) return
}// labels
outer@ for (l in list) {
for (m in map) {
if (l == m.key) break@outer
}
}// Pair and Triple data types
val myPair = Pair(1, "Mohse")
val myTriple = Triple(1, "A", "Mohsen")
val (id, name) = myPair// measure runtime in ms!!
val took = measureTimeMillis {
// ...
}// deprecate a method
@Deprecated ("Use another method!", level = WARNING/ERROR/HIDDEN, replaceWith = ReplaceWith("newPrintFullName()", imports = "com.google.test"))
fun printFullName() {
// …
}// rename method for Java
@JvmName("methodNameInJava")
fun printFullName() {
// …
}// inject using qualifier annotation
@field:[Inject ApplicationContext]
lateinit var context: Context// exclude field from serialization and gson
@Transient
var id: String? = nullKotlin Android 扩展:
// add plugin to build.gradle
apply plugin: 'kotlin-android-extensions'import kotlinx.android.synthetic.main.<layout>.*// access views without findViewById!!
myText.text = "Hello world!"我期待你们的评论，Kotlin 的经验，以及任何我作为初学者可能会错过的不寻常的语法。如果你认为这篇文章有用，请点击分享💚按钮，关注我获取更多文章 Mohsen Mirhoseini 。
[## 穆赫森·米尔霍西尼
高级 Android 开发人员
mohsenoid.com](http://mohsenoid.com) 
