习惯用法

函数的默认参数

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

过滤list

val  result  = args.filter{it > 0}

模板字符串 ，String内插

val name = "vincent"
println("my name is  $name")

类型判断

when(x){
is  Number -> ...
in 1..100 -> ...
in 1 until 100  -> ...
else -> ...
}

遍历map/pair型list

for((key ,value)  in map){
println("$key->$value")
}

只读list

val  list = listOf(1,2,3)

只读map

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

访问map

println(map["key"])

延迟属性

val  list  by lazy {
    listOf("1" ,"2","3")
}

扩展函数

fun  String.spaceTocamelcase(){...}

创建单例

object Resource{
  val name = "vincnet"
}

if not null 缩写

val  files = File("Test").listFIles()
println(files?.size)

if not null and else

val  files = File("Test").listFIles()
println(files?.size?:"empity")

if null 执行语句

val  list = ...
val element = list[3]?:  throw IllegalStateException(" is missing!")

在可能为空的集合中取第一个元素

val emails = ... // 可能会是空集合
val mainEmail = emails.firstOrNull() ?: ""

if not null 执行代码

list?.let{

}