//类型后面加?表示可为空var agrs_t: String? = null
//抛出空指针异常val v1 = agrs_t!!.toInt()
//不做处理返回 null   //the safe call operator, written ?.val v2 = agrs_t?.toLong()
//age为空时返回-1,   非空正常执行val v3 = agrs_t?.toInt() ?: -1

//显示判断处理val l: Int = if (b != null) b.length else -1val b: String? = "Kotlin"if (b != null && b.length > 0) {print("String of length ${b.length}")} else {print("Empty string")}//安全处理符处理
//the safe call operator, written ?.val lt = b?.length ?: -1//为空为-1val files = File("Test").listFiles()println(files?.size ?:"empty")

class A { // implicit label @Ainner class B { // implicit label @Bfun Int.foo() { // implicit label @fooval a = this@A // A's thisval b = this@B // B's thisval c = this // foo()'s receiver, an Intval c1 = this@foo // foo()'s receiver, an Intval funLit = lambda@ fun String.() {val d = this // funLit's receiver}val funLit2 = { s: String ->// foo()'s receiver, since enclosing lambda expression// doesn't have any receiverval d1 = this}}}
}

 fun testHandleMethodParams(){val strSample = "handle filter"val tvalue = transformResult (strSample){//函数作为参数getParams(strSample)}}fun getParams(str1: String): Int = str1.length/*** arg0 为实参* p1 为形参，内部无法引用*  ->函数返回值*/fun <T, R> transformResult(args:T,transform: (T) -> R): R {return transform(args)}

 /*** 此方法接收一个无参数的函数并且无返回,Unit表示无返回*/private fun getResults(method: () -> Unit) {method()}

fun listOperate(){val items = listOf(1, 2, 3, 4)items.first() //== 1items.last() //== 4val subItems = items.filter { it % 2 == 0 }   // returns [2, 4]var sub2 = items.slice(1..3)var sub3 =items.filterNot { it>100 }//高阶函数//遍历listitems.forEach{
//        it= it*2+1print(it)}items.forEachIndexed { index, value -> if (value > 8) println("value of index $index is $value") }//T->R 集合中的元素通过某个方法转换后的结果存到一个集合val list11 = items.map { (it - 2+1).toLong() }val list111 = items.mapIndexed { index, it -> if(index>5) 10 * it }//合并两个集合
//    items.flatMap { listOf(it, it + 1) }//    map：遍历每一个元素
//    flatMap：遍历每一个元素，并铺平(展开内部嵌套结构)元素var list_map = listOf(listOf(10,20),listOf(30,40),listOf(50,60))var mapList = list_map.map{ element -> element.toString() }var flatMapList = list_map.flatMap{ element -> element.asIterable() }
//    map [[10, 20], [30, 40], [50, 60]]
//   flatmap [10, 20, 30, 40, 50, 60]// filter all 符合条件elementval list12 = items.filter { mypre(it) }//filter 到第一个不符合条件元素返回，后面不再过滤val list13 = items.takeWhile { (it%2!=0) }//提取前3个元素val list14 = items.take(3)//提取后3个val list15 = items.takeLast(3)//去除重复元素var list16 = items.distinct()//指定key过滤条件var list17 = items.distinctBy { it*101+100 }
//    list1.fold(5,operation = )//匹配交集元素var list18 = items.intersect(listOf<Int>(2,3,4))//匹配两个集合内所以不同元素，差集合var list19 = items.union(listOf<Int>(2,3,4))//返回匹配条件元素var list21 = items.any { (it in 10..20) }var list22 = items.count { it >20 }//初始值，累计计算处理var list23 = items.fold(10) { total, next -> total + next }var list24 =items.reduce { total, next -> total - next }var sumFilterResult : Int = 0items.filter { it < 7  }.forEach { sumFilterResult += it }val rwList = mutableListOf(1, 2, 3)// 去除null元素rwList.requireNoNulls()  //returns [1, 2, 3]//检测list内有符合条件元素if (rwList.none { it > 6 }) println("No items above 6")  // prints "No items above 6"//第一个元素，或为空对象val item = rwList.firstOrNull()}fun mypre(t:Int): Boolean{    return t%2==0}