定义包
定义包的规范应位于源文件的顶部：
| package com.web3
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| import java.util.*
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| // ... |
不需要匹配目录和包：源文件可以任意放在文件系统中。
定义函数
具有返回Int类型的两个Int参数的函数：
| fun sum(a: Int, b: Int): Int {
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| return a + b
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| }
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| fun main(args: Array<String>) {
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| print("sum of 3 and 5 is ")
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| println(sum(3, 5))
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| } |
函数与表达主体和推断返回值的类型：
| fun sum(a: Int, b: Int) = a + b
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| fun main(args: Array<String>) {
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| println("sum of 19 and 23 is ${sum(19, 23)}")
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| }
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函数返回无意义值：
| fun printSum(a: Int, b: Int): Unit {
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| println("sum of $a and $b is ${a + b}")
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| }
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| fun main(args: Array<String>) {
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| printSum(-1, 8)
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| } |
Unit返回类型可以省略：
| fun printSum(a: Int, b: Int) {
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| println("sum of $a and $b is ${a + b}")
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| }
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| fun main(args: Array<String>) {
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| printSum(-1, 8)
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| }
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定义局部变量
分配一次(只读)局部变量：
| fun main(args: Array<String>) {
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| val a: Int = 1 // immediate assignment
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| val b = 2 // `Int` type is inferred
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| val c: Int // Type required when no initializer is provided
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| c = 3 // deferred assignment
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| println("a = $a, b = $b, c = $c")
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| } |
可变变量：
| fun main(args: Array<String>) {
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| var x = 5 // `Int` type is inferred
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| x += 1
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| println("x = $x")
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| } |
Kotlin的注释
Kotlin中的注释类似于Java和JavaScript，Kotlin支持行尾和块注释。
| // This is an end-of-line comment
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| /* This is a block comment
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| on multiple lines. */ |
与Java不同的是，Kotlin中的块注释可以嵌套。
使用字符串模板
| fun main(args: Array<String>) {
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| var a = 1
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| // simple name in template:
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| val s1 = "a is $a"
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| a = 2
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| // arbitrary expression in template:
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| val s2 = "${s1.replace("is", "was")}, but now is $a"
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| println(s2)
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| } |
使用条件表达式
| fun maxOf(a: Int, b: Int): Int {
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| if (a > b) {
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| return a
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| } else {
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| return b
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| }
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| }
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| fun main(args: Array<String>) {
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| println("max of 0 and 42 is ${maxOf(0, 42)}")
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| } |
使用
if表达式：
| fun maxOf(a: Int, b: Int) = if (a > b) a else b
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| fun main(args: Array<String>) {
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| println("max of 0 and 42 is ${maxOf(0, 42)}")
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| } |
使用可空值来检查null值
当可能为null值时，引用必须被明确地标记为可空(null)。
如果str不包含整数，则返回null：
| fun parseInt(str: String): Int? {
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| // ...
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| } |
使用可返回
null值的函数：
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| fun parseInt(str: String): Int? {
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| return str.toIntOrNull()
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| }
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| fun printProduct(arg1: String, arg2: String) {
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| val x = parseInt(arg1)
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| val y = parseInt(arg2)
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| // Using `x * y` yields error because they may hold nulls.
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| if (x != null && y != null) {
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| // x and y are automatically cast to non-nullable after null check
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| println(x * y)
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| }
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| else {
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| println("either '$arg1' or '$arg2' is not a number")
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| }
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| }
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| fun main(args: Array<String>) {
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| printProduct("6", "7")
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| printProduct("a", "7")
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| printProduct("a", "b")
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| } |
上面代码，也可以编写成下面 -
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| fun parseInt(str: String): Int? {
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| return str.toIntOrNull()
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| }
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| fun printProduct(arg1: String, arg2: String) {
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| val x = parseInt(arg1)
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| val y = parseInt(arg2)
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| // ...
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| if (x == null) {
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| println("Wrong number format in arg1: '${arg1}'")
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| return
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| }
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| if (y == null) {
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| println("Wrong number format in arg2: '${arg2}'")
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| return
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| }
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| // x and y are automatically cast to non-nullable after null check
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| println(x * y)
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| }
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| fun main(args: Array<String>) {
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| printProduct("6", "7")
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| printProduct("a", "7")
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| printProduct("99", "b")
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| } |
使用类型检查和自动转换
is运算符检查表达式是否是类型的实例。 如果一个不可变的局部变量或属性是指定类型，则不需要显式转换：
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| fun getStringLength(obj: Any): Int? {
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| if (obj is String) {
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| // `obj` is automatically cast to `String` in this branch
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| return obj.length
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| }
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| // `obj` is still of type `Any` outside of the type-checked branch
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| return null
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| }
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| fun main(args: Array<String>) {
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| fun printLength(obj: Any) {
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| println("'$obj' string length is ${getStringLength(obj) ?: "... err, not a string"} ")
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| }
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| printLength("Incomprehensibilities")
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| printLength(1000)
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| printLength(listOf(Any()))
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| } |
上面代码，也可以编写成下面 -
| fun getStringLength(obj: Any): Int? {
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| if (obj !is String) return null
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| // `obj` is automatically cast to `String` in this branch
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| return obj.length
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| }
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| fun main(args: Array<String>) {
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| fun printLength(obj: Any) {
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| println("'$obj' string length is ${getStringLength(obj) ?: "... err, not a string"} ")
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| }
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| printLength("Incomprehensibilities")
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| printLength(1000)
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| printLength(listOf(Any()))
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| }
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上面代码，还可以编写成下面 -
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| fun getStringLength(obj: Any): Int? {
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| // `obj` is automatically cast to `String` on the right-hand side of `&&`
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| if (obj is String && obj.length > 0) {
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| return obj.length
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| }
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| return null
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| }
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| fun main(args: Array<String>) {
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| fun printLength(obj: Any) {
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| println("'$obj' string length is ${getStringLength(obj) ?: "... err, is empty or not a string at all"} ")
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| }
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| printLength("Incomprehensibilities")
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| printLength("")
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| printLength(1000)
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| } |
使用for循环
| fun main(args: Array<String>) {
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| val items = listOf("apple", "banana", "kiwi")
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| for (item in items) {
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| println(item)
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| }
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| } |
上面代码，可以编写成如下 -
| fun main(args: Array<String>) {
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| val items = listOf("apple", "banana", "kiwi")
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| for (index in items.indices) {
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| println("item at $index is ${items[index]}")
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| }
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| } |
使用 while 循环
| fun main(args: Array<String>) {
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| val items = listOf("apple", "banana", "kiwi")
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| var index = 0
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| while (index < items.size) {
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| println("item at $index is ${items[index]}")
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| index++
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| }
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| } |
使用 when 表达式
| fun describe(obj: Any): String =
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| when (obj) {
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| 1 -> "One"
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| "Hello" -> "Greeting"
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| is Long -> "Long"
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| !is String -> "Not a string"
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| else -> "Unknown"
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| }
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| fun main(args: Array<String>) {
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| println(describe(1))
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| println(describe("Hello"))
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| println(describe(1000L))
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| println(describe(2))
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| println(describe("other"))
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| } |
使用范围
使用in操作符检查数字是否在指定范围内：
| fun main(args: Array<String>) {
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| val x = 10
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| val y = 9
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| if (x in 1..y+1) {
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| println("fits in range")
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| }
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| } |
检查一个数字是否超出指定范围：
| fun main(args: Array<String>) {
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| val list = listOf("a", "b", "c")
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| if (-1 !in 0..list.lastIndex) {
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| println("-1 is out of range")
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| }
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| if (list.size !in list.indices) {
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| println("list size is out of valid list indices range too")
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| }
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| }
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上面代码，执行结果如下 -
| -1 is out of range
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| list size is out of valid list indices range too |
迭代范围：
| fun main(args: Array<String>) {
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| for (x in 1..5) {
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| print(x)
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| }
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| }
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或过程：
| fun main(args: Array<String>) {
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| for (x in 1..10 step 2) {
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| println(x)
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| }
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| println("===============================")
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| for (x in 9 downTo 0 step 3) {
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| println(x)
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| }
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| } |
上面代码，执行结果如下 -
| 1
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| 3
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| 5
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| 7
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| 9
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| ===============================
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| 9
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| 6
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| 3
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| 0 |
使用集合
迭代集合：
| fun main(args: Array<String>) {
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| val items = listOf("apple", "banana", "kiwi")
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| for (item in items) {
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| println(item)
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| }
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| } |
检查集合是否包含一个对象，使用
in运算符：
| fun main(args: Array<String>) {
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| val items = setOf("apple", "banana", "kiwi")
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| when {
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| "orange" in items -> println("juicy")
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| "apple" in items -> println("apple is fine too")
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| }
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| } |
使用
lambda表达式过滤映射集合：
| fun main(args: Array<String>) {
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| val fruits = listOf("banana", "avocado", "apple", "kiwi")
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| fruits
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| .filter { it.startsWith("a") }
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| .sortedBy { it }
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| .map { it.toUpperCase() }
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| .forEach { println(it) }
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| } |
