Kotlin-02.惯用语法/语法习惯(Idioms)

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

data class自动生成以下方法:
    类的所有属性变量都有getters方法
    类的var变量还有setters方法
    equals()
    hashCode()
    toString()
    copy()
    类的所有属性变量都有component1()、 component2()

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

val positives = list.filter { x -> x > 0 }
或更短:
val positives = list.filter { it > 0 }

println("Name $name")

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

for ((k, v) in map) {
    println("$k -> $v")
}
k、v 可以改成任意名字

for (i in 1..100) { …… }  // 闭区间：包含 100
for (i in 1 until 100) { …… } // 半开区间：不包含 100
for (x in 2..10 step 2) { …… } // 步长2: 2 4 6 8 10
for (x in 10 downTo 1) { …… } // 递减: 10 9..1
if (x in 1..10) { …… }

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

访问时才加载
val p: String by lazy {
    // 计算该字符串
}

fun String.spaceToCamelCase() {
    // 不改变String类，增加了String的函数
}
"Convert this to camelcase".spaceToCamelCase()

object Resource {
    val name = "Name"
}
object本身就是对象,很明显object不能像class多次创建实例对象

If not null 缩写
    val files = File("Test").listFiles()
    println(files?.size)

if null 执行一个语句
    val data = ……
    val email = data["email"] ?: throw IllegalStateException("Email is missing!")

if not null 执行一个语句
    val data = ……
    data?.let {
        …… // 假如data不为null, 代码会执行到此处
    }

标记Boolean可空null
    val b: Boolean? = ……
    if (b == true) {
        ……
    } else {
        // b 是 false 或者 null
    }

返回when结果   
    val result =  when (color) {
        "Red" -> 0
        "Green" -> 1
        "Blue" -> 2
        else -> throw IllegalArgumentException("Invalid color param value")
    }
  
返回try/catch结果
    val result = try {
        count()
    } catch (e: ArithmeticException) {
        throw IllegalStateException(e)
    }

返回if结果
    val result = if (param == 1) {
        "one"
    } else if (param == 2) {
        "two"
    } else {
        "three"
    }

fun theAnswer() = 42
等价于
fun theAnswer(): Int {
    return 42
}

单表达式函数与其它习惯用法一起简化代码，例如和 when 表达式一起使用：
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) { // 画一个 100 像素的正方形
    penDown()
    for(i in 1..4) {
        forward(100.0)
        turn(90.0)
    }
    penUp()
}

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

对fromJson方法参数简化
inline fun <reified T: Any> Gson.fromJson(json): T = this.fromJson(json, T::class.java)

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