kotlin模拟运行器
https://try.kotlinlang.org/
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]}")
}
或
for (i in array.indices) {
println(array[i])
}
for ((index, value) in array.withIndex()) {
println("the element at $index is $value")
}
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 -> $value")
}
或 判断集合是否包含
when {
"orange" in items -> println("juicy")
"apple" in items -> println("apple is fine too")
}
或 使用 lambda 表达式来过滤(filter)与映射(map)集合:
val fruits = listOf("banana", "avocado", "apple", "kiwifruit")
fruits
.filter{ it.startsWith("a") }
.sortedBy { it }
.map { it.toUpperCase() }
.forEach { println(it) }
- 创建对象 不需要“new”关键字
val rectangle = Rectangle(5.0, 2.0)
习惯用法
- 创建实体类
//推荐实体类要有默认参数
data class Customer(val name: String = "", var email: String = "")
var customer: Customer = Customer("chentong","7045xxx")
println(customer.toString())
var customer01: Customer = Customer(name="kk")
customer01.email = "1986xxx" println(customer01.toString())
//强烈推荐这种写法 语义清晰
var customer02: Customer = Customer(name="tong",email="890xxx")
println(customer02.toString())
var customer03: Customer = Customer("03")
customer03.email = "1986xxx"
println(customer03.toString())
var customer04: Customer = Customer()
println(customer04.toString())
1）自带getter、setter、toString、hashcode方法
2）val(只读) 没有setter方法
3）如果生成的类需要含有一个无参的构造函数，则所有的属性必须指定默认值
- 函数默认参数
fun foo(a: Int = 0, b: String = "") { ...... }
- 过滤 list(filter 内是判断条件)
val listdata = listOf(1, 2, 3, -1)
println(listdata.toString())
var newdata = listdata.filter{ it > 0 }
println(newdata.toString())
- map创建
//打印map
fun printMap(map: Map){
for ((key, value) in map) {
println("$key -> $value")
}
}
//强烈推荐第一种写法 语义清晰
var map = HashMap()
map.put("chen","tong")
map.put("yang","yue")
printMap(map)
var map01 = mutableMapOf()
map01["chen01"] = "tong"
map01["yang01"] = "yue"
printMap(map01)
var map02: HashMap = HashMap()
map02.put("chen02","tong")
map02.put("yang02","yue")
printMap(map02)
var chen03 = "tong"
var yang03 ="yue"
var yuan03 = "dong"
//左key,右value
var map03 = mutableMapOf("chen03" to chen03, "yang03" to yang03,"yuan03" to yuan03)
printMap(map03)
val map04: MutableMap = mutableMapOf()
map04.put("chen04","tong")
map04.put("yang04","yue")
map04["yuan04"] = "dong"
printMap(map04)
强烈推荐第一种写法
原因：
1、简单少，语法清晰，
2、与java写法一致，上手成本低，易于理解
3、分辨map是到底哪种实现方式很重要
- list创建
//list打印
fun printList(list: List){
for (item in list) {
println("$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()
lists01.add("lists01")
lists01.add("banana")
printList(lists01)
val lists02: MutableList = mutableListOf("lists02","02121")
lists02.add("testadd")
printList(lists02)
//推荐这种写法
var lists03: MutableList = 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()
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
基础知识
数字面值中间加下划线，易于读
val oneMillion = 1_000_000
kotlin 不使用位运算
数组Array
- if 表达式 会返回值，或代码块中值
// 传统用法
var max = a
if (a < b) max = b
// 作为表达式
val max = if (a > b) a else b
*for循环 补充
for (i in array.indices) {
println(array[i])
}
for ((index, value) in array.withIndex()) {
println("the element at $index is $value")
}
- 标签 @ 即跳转地址 不建议采用
合理使用 return 、break、continue取代
fun foo() {
listOf(1, 2, 3, 4, 5).forEach lit@{
if (it == 3) return@lit // 局部返回到该 lambda 表达式的调用者，即 forEach 循环
print(it)
}
print(" done with explicit label")
}
类与对象
- 类的写法
class Person{
var name: String = "chentong"
//主构造函数
constructor()
//主构造函数 初始化代码
init{
name = name + " from Init"
}
constructor(name: String){
this.name = name+" from name"
}
constructor(name: String, from: String){
this.name = name + " from " + from
}
}
var person = Person()
println(person.name)
var person01 = Person("yan")
println(person01.name)
var person02 = Person("kun"," China ")
println(person02.name)
- 类继承
open class Base(p: Int)
class Derived(p: Int) : Base(p)
- 方法覆盖
open class Base {
open fun v() { ... }
fun nv() { ... }
}
class Derived() : Base() {
override fun v() { ... }
}
- 属性覆盖
open class Foo {
open val x: Int get() { ...... }
}
class Bar1 : Foo() {
override val x: Int = ......
}
子类不覆盖基类open成员
抽象类
open class Base {
open fun f() {}
}
abstract class Derived : Base() { override abstract fun f()
}
接口
接口不建议写方法体，也不建议写属性字段
原则就是清晰，易懂无歧义
interface MyInterface {
fun bar()
fun foo() {// 可选的方法体}
}
class Child : MyInterface {
override fun bar() {// 方法体 }
}
可见性修饰
private :文件内可见
protected :只在本类与子类可见，外界引用不可见
internal:能⻅到类声明的本模块内的任何客戶端都可⻅其 internal 成员（相同模块）
public ：默认是public
扩展
class C {
fun foo(){println("member") }
}
fun C.foo(i: Int) {println("extension")}
泛型
class Box(t: T) {
var value = t
}
val box = Box()
形变
in/out
解决通配符问题
类型擦除
延时属性lazy 线程安全的
val lazyValue: String by lazy {
println("computed!") "Hello"
}
fun main(args: Array) {
println(lazyValue)
println(lazyValue)
}