惯用法

data class Customer(val name: String, val email: String)

fun foo(a: Int = 0, b: String = "") { ... }

val positives = list.filter { x -> x > 0 }

val positives = list.filter { it > 0 }

if ("john@example.com" in emailsList) { ... }

if ("jane@example.com" !in emailsList) { ... }

println("Name $name")

// 读取一个字符串, 如果输入不能转换为整数, 返回 null. 例如: Hi there!
val wrongInt = readln().toIntOrNull()
println(wrongInt)
// 输出结果为 null

// 读取一个能够转换为整数的字符串, 返回整数值. 例如: 13
val correctInt = readln().toIntOrNull()
println(correctInt)
// 输出结果为 13

when (x) {
    is Foo -> ...
    is Bar -> ...
    else   -> ...
}

val list = listOf("a", "b", "c")

val map = mapOf("a" to 1, "b" to 2, "c" to 3)

println(map["key"])
map["key"] = value

for ((k, v) in map) {
    println("$k -> $v")
}

for (i in 1..100) { ... }  // 终端封闭的(closed-ended)数值范围: 包括 100
for (i in 1..<100) { ... } // 终端开放的(open-ended)数值范围: 不包括 100
for (x in 2..10 step 2) { ... }
for (x in 10 downTo 1) { ... }
(1..10).forEach { ... }

val p: String by lazy { // 只在第一次访问时计算属性值
    // 在这里计算字符串值
}

fun String.spaceToCamelCase() { ... }

"Convert this to camelcase".spaceToCamelCase()

object Resource {
    val name = "Name"
}

@JvmInline
value class EmployeeId(private val id: String)

@JvmInline
value class CustomerId(private val id: String)

abstract class MyAbstractClass {
    abstract fun doSomething()
    abstract fun sleep()
}

fun main() {
    val myObject = object : MyAbstractClass() {
        override fun doSomething() {
            // ...
        }

        override fun sleep() { // ...
        }
    }
    myObject.doSomething()
}

val files = File("Test").listFiles()

println(files?.size) // 如果 files 不为 null, 这里会打印 size 值

val files = File("Test").listFiles()

// 简单的 fallback 值:
println(files?.size ?: "empty") // 如果 files 为 null, 这里会打印 "empty"

// 如果要通过一个代码段来计算更加复杂的 fallback 值, 可以使用 `run`
val filesSize = files?.size ?: run {
    val someSize = getSomeSize()
    someSize * 2
}
println(filesSize)

val values = ...
val email = values["email"] ?: throw IllegalStateException("Email is missing!")

val emails = ... // 可能为空
val mainEmail = emails.firstOrNull() ?: ""

val value = ...

value?.let {
    ... // 这个代码段将在 data 不为 null 时执行
}

val value = ...

val mapped = value?.let { transformValue(it) } ?: defaultValue
// 如果 value 为 null, 会 transform 处理结果为 null, 则返回 defaultValue

fun transform(color: String): Int {
    return when (color) {
        "Red" -> 0
        "Green" -> 1
        "Blue" -> 2
        else -> throw IllegalArgumentException("Invalid color param value")
    }
}

fun test() {
    val result = try {
        count()
    } catch (e: ArithmeticException) {
        throw IllegalStateException(e)
    }

    // 使用 result
}

fun foo(param: Int) {
    val result = if (param == 1) {
        "one"
    } else if (param == 2) {
        "two"
    } else {
        "three"
    }
}

fun arrayOfMinusOnes(size: Int): IntArray {
    return IntArray(size).apply { fill(-1) }
}

fun theAnswer() = 42

fun theAnswer(): Int {
    return 42
}

fun transform(color: String): Int = when (color) {
    "Red" -> 0
    "Green" -> 1
    "Blue" -> 2
    else -> throw IllegalArgumentException("Invalid color param value")
}

class Turtle {
    fun penDown()
    fun penUp()
    fun turn(degrees: Double)
    fun forward(pixels: Double)
}

val myTurtle = Turtle()
with(myTurtle) { // 描绘一个边长 100 像素的正方形
    penDown()
    for (i in 1..4) {
        forward(100.0)
        turn(90.0)
    }
    penUp()
}

val myRectangle = Rectangle().apply {
    length = 4
    breadth = 5
    color = 0xFAFAFA
}

val stream = Files.newInputStream(Paths.get("/some/file.txt"))
stream.buffered().reader().use { reader ->
    println(reader.readText())
}

//  public final class Gson {
//     ...
//     public <T> T fromJson(JsonElement json, Class<T> classOfT) throws JsonSyntaxException {
//     ...

inline fun <reified T: Any> Gson.fromJson(json: JsonElement): T = this.fromJson(json, T::class.java)

var a = 1
var b = 2
a = b.also { b = a }

fun calcTaxes(): BigDecimal = TODO("Waiting for feedback from accounting")