1，定义函数
比如
fun sum(a: Int, b: Int): Int { return a + b }如果不需要返回值，则可以
fun printSum(a: Int, b: Int) { println("sum of $a and $b is ${a + b}") }2，定义变量
如果是只读变量，用val声明，如果是可修改的变量，用var声明
val a: Int = 1 val b = 2 // 自动推断类型`Int` val c: Int // 如果没有初始值，则需要提供类型 c = 3 // 稍后赋值 var x = 5 // 自动推断类型`Int` x += 13，字符串模板
var a = 1 // simple name in template: val s1 = "a is $a" a = 2 // arbitrary expression in template: val s2 = "${s1.replace("is", "was")}, but now is $a"4，if表达式
fun maxOf(a: Int, b: Int) = if (a > b) a else b5，对于可能为null的值，必须判断
fun parseInt(str: String): Int? { // 如果不是int，就返回null } fun printProduct(arg1: String, arg2: String) { val x = parseInt(arg1) val y = parseInt(arg2) // Using `x * y` yields error because they may hold nulls. if (x != null && y != null) { // x and y are automatically cast to non-nullable after null check println(x * y) } else { println("either '$arg1' or '$arg2' is not a number") } }6, 用is 关键字判断对象类型，相当于java的instanceOf
fun getStringLength(obj: Any): Int? { if (obj !is String) return null // `obj` is automatically cast to `String` in this branch return obj.length }7, list遍历
val items = listOf("apple", "banana", "kiwifruit") for (item in items) { println(item) } val items = listOf("apple", "banana", "kiwifruit") var index = 0 while (index < items.size) { println("item at $index is ${items[index]}") index++ }fun describe(obj: Any): String = when (obj) { 1 -> "One" "Hello" -> "Greeting" is Long -> "Long" !is String -> "Not a string" else -> "Unknown" }8, 范围
val x = 10 val y = 9 if (x in 1..y+1) { println("fits in range") } val list = listOf("a", "b", "c") if (-1 !in 0..list.lastIndex) { println("-1 is out of range") } if (list.size !in list.indices) { println("list size is out of valid list indices range, too") } //遍历 for (x in 1..5) { print(x) } //步长 for (x in 1..10 step 2) { print(x) } println() for (x in 9 downTo 0 step 3) { print(x) }9，集合
for (item in items) { println(item) } when { "orange" in items -> println("juicy") "apple" in items -> println("apple is fine too") } //lambda表达式 val fruits = listOf("banana", "avocado", "apple", "kiwifruit") fruits .filter { it.startsWith("a") } .sortedBy { it } .map { it.toUpperCase() } .forEach { println(it) }10,创建对象
val rectangle = Rectangle(5.0, 2.0) //不需要'new' val triangle = Triangle(3.0, 4.0, 5.0)参考文献： https://kotlinlang.org/docs/reference/coding-conventions.html
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