原文地址

Creating DTOs (POJOs/POCOs) 创建DTOs

data class Customer(val name: String, val email: String)
提供一个带有以下功能的Customer类

•   getters (and setters in case of vars) for all properties
•   equals()
•   hashCode()
•   toString()
•   copy()
•   component1(), component2(), …, for all properties

Default values for function parameters 函数的默认值

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

Filtering a list 过滤一个集合

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

更简洁的方式：

val positives = list.filter { it > 0 }

String Interpolation 字符串插入

println("Name $name")

Instance Checks 实例检查

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

Traversing a map/list of pairs

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

Using ranges

for (i in 1..100) { ... } // closed range: includes 100
for (i in 1 until 100) { ... } // half-open range: does not include 100
for (x in 2..10 step 2) { ... }
for (x in 10 downTo 1) { ... } 
if (x in 1..10) { ... }

Read-only list

val list = listOf("a", "b", "c")
Read-only map
val map = mapOf("a" to 1, "b" to 2, "c" to 3)
Accessing a map
println(map["key"])
map["key"] = value

Lazy property

val p: String by lazy {
    // compute the string
}

Extension Functions

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

Creating a singleton

object Resource {
    val name = "Name"
}

If not null shorthand

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

If not null and else shorthand

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

Executing a statement if null

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

Execute if not null

val data = ...
data?.let {
    ... // execute this block if not null
}

Return on when statement

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

'try/catch' expression

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

    // Working with result
}

'if' expression

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

Builder-style usage of methods that return Unit

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

Single-expression functions

fun theAnswer() = 42
This is equivalent to
fun theAnswer(): Int {
    return 42
}

这能有效的和其他方言相结合，旨在更简洁的代码。例如when表达式

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

Calling multiple methods on an object instance ('with')

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() 
}

Java 7's try with resources

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

对于需要泛型类型信息的通用函数的方便形式

// public final class Gson { 
// ... // public <T> T fromJson(JsonElement json, Class<T> classOfT) throws JsonSyntaxException {
// ...
inline fun <reified T: Any> Gson.fromJson(json): T = this.fromJson(json, T::class.java)

Consuming a nullable Boolean

val b: Boolean? = ...
if (b == true) {
    ...
} else {
    // `b` is false or null
}