Kotlin惯用语法(优先推荐阅读)

作者：陈桐 2018.9.28

####惯用语法

函数定义

//有返回值
fun sum(a: Int, b: Int): Int {
    return a + b
}

//无返回值
fun printSum(a: Int, b: Int) {
println("sum of $a and $b is ${a + b}")
}

定义变量

//只读
val a: Int = 1

//变量
var x = 5

字符串模板 $

var a = 1 
val s1 = "a is $a"

条件表达式统一写法

if (a > b) {
        return a
} else {
        return b 
}

空值校验 null

当某个变量的值可以为 null 的时候，必须在声明处的类型后添加 ? 来标识该引用可为空

val str: String ?= null

数据类型检测

fun getStringLength(obj: Any): Int?{
    if(obj is String){
        return obj.length
    }
    return null
}

使用for循环

1、类似java增强for循环

//推荐这种写法
val items = listOf("apple", "banana", "kiwifruit") 
for (item in items) {
    println(item)
}

或

val items = listOf("apple", "banana", "kiwifruit") 
for (index in items.indices) 
{
    println("item at $index is ${items[index]}") 
}

while循环方法优点：对index进行精确控制

val items = listOf("apple", "banana", "kiwifruit") 
var index = 0
while (index < items.size) {
    println("item at $index is ${items[index]}")
    index++ 
}

区间方法

//升序
val n = 11
for (x in 1..n step 2) 
{ 
    print(x)
}

或

//降序
for (x in 9 downTo 0 step 3) 
{
    print(x) 
}

区间检测

1、是否在某范围内

val x = 10
val y = 9
if (x in 1..y+1) {
    println("fits in range") 
}

when 表达式 (取代switch)

when (x) {
    1 -> print("x == 1") 
    2 -> print("x == 2") 
    else -> { // default
        print("x is neither 1 nor 2") 
    }
}

集合遍历

//推荐用法
for (item in items) {
    println(item)
}

或

//推荐用法
for ((key, value) in map) {
    println("$key -> $value")
}

或判断集合是否包含

when {
    "orange" in items -> println("juicy")
    "apple" in items -> println("apple is fine too")
}

或使用 lambda 表达式来过滤(filter)与映射(map)集合:

val fruits = listOf("banana", "avocado", "apple", "kiwifruit")

fruits
    .filter{ it.startsWith("a") }
    .sortedBy { it }
    .map { it.toUpperCase() }
    .forEach { println(it) }

创建对象不需要“new”关键字

 val rectangle = Rectangle(5.0, 2.0)

####习惯用法

创建实体类

//推荐实体类要有默认参数
data class Customer(val name: String = "", var email: String = "")

var customer: Customer = Customer("chentong","7045xxx")
println(customer.toString())

var customer01: Customer = Customer(name="kk")
customer01.email = "1986xxx"    println(customer01.toString())

//强烈推荐这种写法 语义清晰
var customer02: Customer = Customer(name="tong",email="890xxx")
println(customer02.toString())

var customer03: Customer = Customer("03")
customer03.email = "1986xxx"
println(customer03.toString())

var customer04: Customer = Customer()
println(customer04.toString())

1）自带getter、setter、toString、hashcode方法 2）val(只读) 没有setter方法 3）如果生成的类需要含有一个无参的构造函数，则所有的属性必须指定默认值

函数默认参数

fun foo(a: Int = 0, b: String = "") { ...... }

过滤 list(filter 内是判断条件)

val listdata = listOf(1, 2, 3, -1)
println(listdata.toString())
    
var newdata = listdata.filter{ it > 0 } 
println(newdata.toString())

map创建

//打印map
fun printMap(map: Map<String,Any?>){
    for ((key, value) in map) {
        println("$key -> $value")
    }
}

//强烈推荐第一种写法 语义清晰
var map = HashMap<String,Any?>()
map.put("chen","tong")
map.put("yang","yue")
printMap(map)

var map01 = mutableMapOf<String, Any?>()
map01["chen01"] = "tong"
map01["yang01"] = "yue"
printMap(map01)

var map02: HashMap<String,String> = HashMap()
map02.put("chen02","tong")
map02.put("yang02","yue")
printMap(map02)

var chen03 = "tong"
var yang03 ="yue"
var yuan03 = "dong"
//左key,右value    
var map03 = mutableMapOf("chen03" to chen03, "yang03" to yang03,"yuan03" to yuan03)
printMap(map03)

val map04: MutableMap<String, Any?> = mutableMapOf()
map04.put("chen04","tong")
map04.put("yang04","yue")
map04["yuan04"] = "dong"
printMap(map04)

强烈推荐第一种写法 原因： 1、简单少，语法清晰， 2、与java写法一致，上手成本低，易于理解 3、分辨map是到底哪种实现方式很重要

list创建

//list打印
fun printList(list: List<Any?>){
    for (item in list) {
        println("$item")
    }
    println("===end===")
}

val lists = listOf("lists", "123", "456") 
//没有add方法
printList(lists)

//强烈推荐这种写法
var lists01 = ArrayList<String>()
lists01.add("lists01")
lists01.add("banana")    
printList(lists01)

val lists02: MutableList<String> = mutableListOf("lists02","02121")
lists02.add("testadd")
printList(lists02)

//推荐这种写法
var lists03: MutableList<String> = mutableListOf()
lists03.add("lists03")
lists03.add("676767")
printList(lists03)

推荐第2、4种写法，清晰，简洁

只读list map

val list = listOf("a", "b", "c")

val map = mapOf("a" to 1, "b" to 2, "c" to 3)

访问map

var map = HashMap<String,String>()
map.put("chen","tong")
map["zhang"] = "san"     

println(map.get("zhang"))    
println(map.get("chen"))

println(map["zhang"]) 
println(map["chen"])

快速访问推荐下面👇

println(map["key"]) 
map["key"] = value

延迟属性

val p: String by lazy { 
// 计算该字符串
}

创建单例

object Resource {
    val name = "Name"
}

#####安全判断

if not null缩写

val lists = listOf("123")
println(lists?.size)

if not null{} else{} 缩写

val lists = listOf("123")
println(lists?.size ?: "empty")
println("内容" ?: "empty")
println(null ?: "empty")

if not null

val lists = listOf("123")
lists?.let {
	// 代码会执行到此处, 假如data不为null
    println(lists.size)
}

返回 when 表达式

fun transform(color: String): Int { 
    return when (color) {
        "Red" -> 0
        "Green" -> 1
        "Blue" -> 2
        else -> throw IllegalArgumentException("Invalid color param value")
	} 
}

println(transform("Red"))

println(transform("1"))

单表达式用法

fun theAnswer() = 42
//等于
fun theAnswer(): Int {
    return 42
}

一个对象调用多种方法

class Turtle {
    fun penDown(){
    	println("penDown()")
    }
	
    fun penUp(){
    	println("penUp()")
	}
    
	fun turn(degrees: Double){
    	println("turn($degrees)" )
	}
    
	fun forward(pixels: Double){
    	println("forward($pixels)")
	}
}

val myTurtle = Turtle()
with(myTurtle) { // 画一个 100 像素的正方形
    penDown()
	for(i in 1..4) 
    {
        forward(100.0)
        turn(90.0)
    }
    penUp() 
}

优先使用try 、if 与 when表达形式

return if (x) foo() else bar()

return when(x) {
    0 -> "zero"
    else -> "nonzero"
}

下面代码不建议使用

if (x)
    return foo()
else
    return bar()

when(x) {
    0 -> return "zero"
    else -> return "nonzero"
}

if 适用两个条件 when 适用多个条件