Kotlin入门基础1--一句话教程

1，定义函数

fun 函数名(参数名:类型,参数名：类型,...):返回类型{    ......}

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 += 1

3，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 b

5，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 = 0while (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 = 10val y = 9if (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