Try Kotlin

整理来自try kotlin网站的一些示例代码。

fun main(args: Array) {
    for (name in args)
        println("Hello, $name!")
}

fun main(args: Array) {
    val language = if (args.size == 0) "EN" else args[0]
    println(when (language) {
        "EN" -> "Hello!"
        "FR" -> "Salut!"
        "IT" -> "Ciao!"
        else -> "Sorry, I can't greet you in $language yet"
    })
}

/**
 * Here we have a class with a primary constructor and a member function.
 * Note that there's no `new` keyword used to create an object.
 * See http://kotlinlang.org/docs/reference/classes.html#classes
 */

class Greeter(val name: String) {
    fun greet() {
        println("Hello, ${name}");
    }
}

fun main(args: Array) {
    Greeter(args[0]).greet()
}

可能返回null时，返回值要加上?标识

fun parseInt(str: String): Int? {
    try {
        return str.toInt()
    } catch (e: NumberFormatException) {
        println("One of the arguments isn't Int")
    }
    return null
}

is操作符和智能转换

fun getStringLength(obj: Any): Int? {
    if (obj is String)
        return obj.length // no cast to String is needed
    return null
}

for循环

fun main(args: Array) {
    for (arg in args)
        println(arg)

    // or
    println()
    for (i in args.indices)
        println(args[i])
}

ranges & in

fun main(args: Array) {
    val x = args[0].toInt()
    //Check if a number lies within a range:
    val y = 10
    if (x in 1..y - 1)
        println("OK")

    //Iterate over a range:
    for (a in 1..5)
        print("${a} ")

    //Check if a number is out of range:
    println()
    val array = arrayListOf()
    array.add("aaa")
    array.add("bbb")
    array.add("ccc")

    if (x !in 0..array.size - 1)
        println("Out: array has only ${array.size} elements. x = ${x}")

    //Check if a collection contains an object:
    if ("aaa" in array) // collection.contains(obj) is called
        println("Yes: array contains aaa")

    if ("ddd" in array) // collection.contains(obj) is called
        println("Yes: array contains ddd")
    else
        println("No: array doesn't contains ddd")
}

destructuring declarations 解构声明

Kotlin可以将一个对象解构为多个变量，这种语法被称为解构声明。解构声明一次创建多个变量。

fun main(args: Array) {
    val pair = Pair(1, "one")

    val (num, name) = pair

    println("num = $num, name = $name")
}

class Pair(val first: K, val second: V) {
    operator fun component1(): K {
        return first
    }

    operator fun component2(): V {
        return second
    }
}

数据类 Data classes

我们经常创建那种什么都不做只持有数据的Bean类，这种情况下许多函数都是从数据对象类里自动产生的。在Kotlin类里，也可以用data标记成数据类。

/**
 *  Data class gets component functions, one for each property declared
 *  in the primary constructor, generated automatically, same for all the
 *  other goodies common for data: toString(), equals(), hashCode() and copy().
 *  See http://kotlinlang.org/docs/reference/data-classes.html#data-classes
 */

data class User(val name: String, val id: Int)

fun getUser(): User {
    return User("Alex", 1)
}

fun main(args: Array) {
    val user = getUser()
    println("name = ${user.name}, id = ${user.id}")

    // or

    val (name, id) = getUser()
    println("name = $name, id = $id")

    // or

    println("name = ${getUser().component1()}, id = ${getUser().component2()}")
}

数据类自动产生的函数

/**
 * Data class gets next functions, generated automatically:
 * component functions, toString(), equals(), hashCode() and copy().
 * See http://kotlinlang.org/docs/reference/data-classes.html#data-classes
 */

data class User(val name: String, val id: Int)

fun main(args: Array) {
    val user = User("Alex", 1)
    println(user) // toString()

    val secondUser = User("Alex", 1)
    val thirdUser = User("Max", 2)

    println("user == secondUser: ${user == secondUser}")
    println("user == thirdUser: ${user == thirdUser}")

    // copy() function
    println(user.copy())
    println(user.copy("Max"))
    println(user.copy(id = 2))
    println(user.copy("Max", 2))
}

解析map

fun main(args: Array) {
    val map = hashMapOf()
    map.put("one", 1)
    map.put("two", 2)

    for ((key, value) in map) {
        println("key = $key, value = $value")
    }
}