作者：陳桐 2018.9.28
####kotlin模擬執行器 try.kotlinlang.org/
####慣用語法
- 函式定義
//有返回值
fun sum(a: Int, b: Int): Int {
return a + b
}
//無返回值
fun printSum(a: Int, b: Int) {
println("sum of $a and $b is ${a + b}")
}
複製程式碼- 定義變數
//只讀
val a: Int = 1
//變數
var x = 5
複製程式碼- 字串模板 $
var a = 1
val s1 = "a is $a"
複製程式碼- 條件表示式統一寫法
if (a > b) {
return a
} else {
return b
}
複製程式碼- 空值校驗 null
當某個變數的值可以為 null 的時候，必須在宣告處的型別後新增 ? 來標識該引用可為空
val str: String ?= null
複製程式碼- 資料型別檢測
fun getStringLength(obj: Any): Int?{
if(obj is String){
return obj.length
}
return null
}
複製程式碼- 使用for迴圈
1、類似java增強for迴圈
//推薦這種寫法
val items = listOf("apple", "banana", "kiwifruit")
for (item in items) {
println(item)
}
複製程式碼或
val items = listOf("apple", "banana", "kiwifruit")
for (index in items.indices)
{
println("item at $index is ${items[index]}")
}
複製程式碼while迴圈方法 優點：對index進行精確控制
val items = listOf("apple", "banana", "kiwifruit")
var index = 0
while (index < items.size) {
println("item at $index is ${items[index]}")
index++
}
複製程式碼區間方法
//升序
val n = 11
for (x in 1..n step 2)
{
print(x)
}
複製程式碼或
//降序
for (x in 9 downTo 0 step 3)
{
print(x)
}
複製程式碼- 區間檢測
1、是否在某範圍內
val x = 10
val y = 9
if (x in 1..y+1) {
println("fits in range")
}
複製程式碼- when 表示式 (取代switch)
when (x) {
1 -> print("x == 1")
2 -> print("x == 2")
else -> { // default
print("x is neither 1 nor 2")
}
}
複製程式碼- 集合遍歷
//推薦用法
for (item in items) {
println(item)
}
複製程式碼或
//推薦用法
for ((key, value) in map) {
println("$key -> $value")
}
複製程式碼或 判斷集合是否包含
when {
"orange" in items -> println("juicy")
"apple" in items -> println("apple is fine too")
}
複製程式碼或 使用 lambda 表示式來過濾(filter)與對映(map)集合:
val fruits = listOf("banana", "avocado", "apple", "kiwifruit")
fruits
.filter{ it.startsWith("a") }
.sortedBy { it }
.map { it.toUpperCase() }
.forEach { println(it) }
複製程式碼- 建立物件 不需要“new”關鍵字
val rectangle = Rectangle(5.0, 2.0)
複製程式碼####習慣用法
- 建立實體類
//推薦實體類要有預設引數
data class Customer(val name: String = "", var email: String = "")
var customer: Customer = Customer("chentong","7045xxx")
println(customer.toString())
var customer01: Customer = Customer(name="kk")
customer01.email = "1986xxx" println(customer01.toString())
//強烈推薦這種寫法 語義清晰
var customer02: Customer = Customer(name="tong",email="890xxx")
println(customer02.toString())
var customer03: Customer = Customer("03")
customer03.email = "1986xxx"
println(customer03.toString())
var customer04: Customer = Customer()
println(customer04.toString())
複製程式碼1）自帶getter、setter、toString、hashcode方法 2）val(只讀) 沒有setter方法 3）如果生成的類需要含有一個無參的建構函式，則所有的屬性必須指定預設值
- 函式預設引數
fun foo(a: Int = 0, b: String = "") { ...... }
複製程式碼- 過濾 list(filter 內是判斷條件)
val listdata = listOf(1, 2, 3, -1)
println(listdata.toString())
var newdata = listdata.filter{ it > 0 }
println(newdata.toString())
複製程式碼- map建立
//列印map
fun printMap(map: Map<String,Any?>){
for ((key, value) in map) {
println("$key -> $value")
}
}
//強烈推薦第一種寫法 語義清晰
var map = HashMap<String,Any?>()
map.put("chen","tong")
map.put("yang","yue")
printMap(map)
var map01 = mutableMapOf<String, Any?>()
map01["chen01"] = "tong"
map01["yang01"] = "yue"
printMap(map01)
var map02: HashMap<String,String> = HashMap()
map02.put("chen02","tong")
map02.put("yang02","yue")
printMap(map02)
var chen03 = "tong"
var yang03 ="yue"
var yuan03 = "dong"
//左key,右value
var map03 = mutableMapOf("chen03" to chen03, "yang03" to yang03,"yuan03" to yuan03)
printMap(map03)
val map04: MutableMap<String, Any?> = mutableMapOf()
map04.put("chen04","tong")
map04.put("yang04","yue")
map04["yuan04"] = "dong"
printMap(map04)
複製程式碼強烈推薦第一種寫法 原因： 1、簡單少，語法清晰， 2、與java寫法一致，上手成本低，易於理解 3、分辨map是到底哪種實現方式很重要
- list建立
//list列印
fun printList(list: List<Any?>){
for (item in list) {
println("$item")
}
println("===end===")
}
val lists = listOf("lists", "123", "456")
//沒有add方法
printList(lists)
//強烈推薦這種寫法
var lists01 = ArrayList<String>()
lists01.add("lists01")
lists01.add("banana")
printList(lists01)
val lists02: MutableList<String> = mutableListOf("lists02","02121")
lists02.add("testadd")
printList(lists02)
//推薦這種寫法
var lists03: MutableList<String> = mutableListOf()
lists03.add("lists03")
lists03.add("676767")
printList(lists03)
複製程式碼推薦第2、4種寫法，清晰，簡潔
- 只讀list map
val list = listOf("a", "b", "c")
val map = mapOf("a" to 1, "b" to 2, "c" to 3)
複製程式碼- 訪問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"])
複製程式碼快速訪問推薦下面?
println(map["key"])
map["key"] = value
複製程式碼- 延遲屬性
val p: String by lazy {
// 計算該字串
}
複製程式碼- 建立單例
object Resource {
val name = "Name"
}
複製程式碼#####安全判斷
- if not null縮寫
val lists = listOf("123")
println(lists?.size)
複製程式碼- if not null{} else{} 縮寫
val lists = listOf("123")
println(lists?.size ?: "empty")
println("內容" ?: "empty")
println(null ?: "empty")
複製程式碼- if not null
val lists = listOf("123")
lists?.let {
// 程式碼會執行到此處, 假如data不為null
println(lists.size)
}
複製程式碼- 返回 when 表示式
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"))
複製程式碼- 單表示式用法
fun theAnswer() = 42
//等於
fun theAnswer(): Int {
return 42
}
複製程式碼- 一個物件呼叫多種方法
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) { // 畫一個 100 畫素的正方形
penDown()
for(i in 1..4)
{
forward(100.0)
turn(90.0)
}
penUp()
}
複製程式碼- 優先使用try 、if 與 when表達形式
return if (x) foo() else bar()
return when(x) {
0 -> "zero"
else -> "nonzero"
}
複製程式碼下面程式碼不建議使用
if (x)
return foo()
else
return bar()
when(x) {
0 -> return "zero"
else -> return "nonzero"
}
複製程式碼if 適用兩個條件 when 適用多個條件