kotlin基本语法（二）

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

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

var positives = list.filter { it > 0 }

println("Name $name")

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

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

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) { …… }
for (x in 10 downTo 1) { …… }
if (x in 1..10) { …… }

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

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

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

val p: String by lazy {    
    // 计算该字符串
 }

fun String.spaceToCamelCase() { …… }
"Convert this to camelcase".spaceToCamelCase()

object Resource {    
    val name = "Name" 
}

val files = File("Test").listFiles()
println(files?.size)

val files = File("Test").listFiles()
println(files?.size ?: "empty")

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

val emails = …… // 可能会是空集合 
val mainEmail = emails.firstOrNull() ?: ""

val value = ……
value?.let {    …… // 代码会执?到此处, 假如data不为null 
}

val value = ……
val mapped = value?.let { transformValue(it) } ?: defaultValueIfValueIsNull

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? 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) { // 画?个 100 像素的正?形
    penDown()
    for(i in 1..4) {
        forward(100.0)
        turn(90.0)
    } 
    penUp() 
}

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

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