Kotlin基础语法

package定义与import

package my.demo

import kotlin.text.*

// ...

程序入口

fun main() {
    println("Hello world!")
}

另一种接受可变数量String参数的main

fun main(args: Array<String>) {
    println(args.contentToString())
}

打印标准输出

print("Hello ")
print("world!")

println("Hello world!")
println(42)

函数

fun sum(a: Int, b: Int): Int {
    return a + b
}

函数体可以是表达式

fun sum(a: Int, b: Int) = a + b

无意义返回值

fun printSum(a: Int, b: Int): Unit {
    println("sum of $a and $b is ${a + b}")
}

Unit返回类型可以省略

变量

val：定义常量

val a: Int = 1  // immediate assignment
val b = 2   // `Int` type is inferred
val c: Int  // Type required when no initializer is provided
c = 3       // deferred assignment

var：定义变量

var x = 5 // `Int` type is inferred
x += 1

可以在顶层定义变量

val PI = 3.14
var x = 0

fun incrementX() { 
    x += 1 
}

创建类Class和实例instances

class Shape

class Rectangle(var height: Double, var length: Double) {
    var perimeter = (height + length) * 2
}

类构造函数里面的变量默认可用

val rectangle = Rectangle(5.0, 2.0)
println("The perimeter is ${rectangle.perimeter}")

类之间继承使用:，一个类可继承，标记为open

open class Shape

class Rectangle(var height: Double, var length: Double): Shape {
    var perimeter = (height + length) * 2
}

注释

// This is an end-of-line comment

/* This is a block comment
   on multiple lines. */

块注释可以内嵌

/* The comment starts here
/* contains a nested comment */
and ends here. */

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"

条件表达式

fun maxOf(a: Int, b: Int): Int {
    if (a > b) {
        return a
    } else {
        return b
    }
}

在kotlin，if还可以当成表达式使用。

fun maxOf(a: Int, b: Int) = if (a > b) a else b

for循环

val items = listOf("apple", "banana", "kiwifruit")
for (item in items) {
    println(item)
}

val items = listOf("apple", "banana", "kiwifruit")
for (index in items.indices) {
    println("item at $index is ${items[index]}")
}

while循环

val items = listOf("apple", "banana", "kiwifruit")
var index = 0
while (index < items.size) {
    println("item at $index is ${items[index]}")
    index++
}

when表达式

fun describe(obj: Any): String =
    when (obj) {
        1          -> "One"
        "Hello"    -> "Greeting"
        is Long    -> "Long"
        !is String -> "Not a string"
        else       -> "Unknown"
    }

Ranges

使用in判断数值是否位于一个区间范围

val x = 10
val y = 9
if (x in 1..y+1) {
    println("fits in range")
}

判断数值是否超出区间

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")
}

区间遍历

for (x in 1..5) {
    print(x)
}

阶梯式遍历区间

for (x in 1..10 step 2) {
    print(x)
}
println()
for (x in 9 downTo 0 step 3) {
    print(x)
}

集合

遍历集合

for (item in items) {
    println(item)
}

检查集合中是否包含某个元素使用in

when {
    "orange" in items -> println("juicy")
    "apple" in items -> println("apple is fine too")
}

使用lambda表达式对集合进行filter和map操作

val fruits = listOf("banana", "avocado", "apple", "kiwifruit")
fruits
    .filter { it.startsWith("a") }
    .sortedBy { it }
    .map { it.toUpperCase() }
    .forEach { println(it) }

空值和空值判断

当为null时，必须显示标记为nullable。nullable类型的名称后面带有?。

如果str无法转换成数字，返回null

fun parseInt(str: String): Int? {
    // ...
}

null的函数返回值的范例

fun parseInt(str: String): Int? {
    return str.toIntOrNull()
}

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("'$arg1' or '$arg2' is not a number")
    }    
}

fun main() {
    printProduct("6", "7")
    printProduct("a", "7")
    printProduct("a", "b")
}

类型检查和自动转换

is操作符用来检查表达式是否是某个类型实例。如果变量进行过is类型判断，无需再显示进行转换。

fun getStringLength(obj: Any): Int? {
    if (obj is String) {
        // `obj` is automatically cast to `String` in this branch
        return obj.length
    }

    // `obj` is still of type `Any` outside of the type-checked branch
    return null
}

fun getStringLength(obj: Any): Int? {
    if (obj !is String) return null

    // `obj` is automatically cast to `String` in this branch
    return obj.length
}

fun getStringLength(obj: Any): Int? {
    // `obj` is automatically cast to `String` on the right-hand side of `&&`
    if (obj is String && obj.length > 0) {
        return obj.length
    }

    return null
}

参考资料：
kotlinlang.org basic-syntax