Kotlin编程范式

创建DTOs(POJOs/POCOs)

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

Customer类拥有以下方法：

• getters (and setters in case of var s) for all properties
• equals()
• hashCode()
• toString()
• copy()
• component1(),component2(), ..., for all properties (see Data classes)

函数参数缺省值

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

list filter

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

val positives = list.filter { it > 0 }

判断集合中是否存在某元素

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

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

模板字符串

println("Name $name")

实例类型判断

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

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map

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

遍历map

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

range循环

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) { ... }

延迟属性Lazy

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

val lazyValue: String by lazy {
    println("computed!")
    "Hello"
}

fun main(args: Array<String>) {
    println(lazyValue)
    println(lazyValue)
}

输出结果为

computed!
Hello
Hello

第一次调用get() 会执行已传递给 lazy() 的 lambda 表达式并记录结果， 后续调用get() 只是返回记录的结果。

Extension Function

fun String.spaceToCamelCase() { ... }

"Convert this to camelcase".spaceToCamelCase()

Extension function可以为任何一个Class加上function，却不必更改该Class的源码。

单例

object Resource {
    val name = "Name"
} 

初始化抽象class

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

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

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

if-not-null简写

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

println(files?.size) // size is printed if files is not null

if-not-null-else 简写

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

println(files?.size ?: "empty") // if files is null, this prints "empty"

if-null执行语句

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

获取集合（集合可能为空）中第一个元素

val emails = ... // might be empty
val mainEmail = emails.firstOrNull() ?: ""

if not null执行

val value = ...

value?.let {
    ... // execute this block if not null
}

if not null map操作

val value = ...

val mapped = value?.let { transformValue(it) } ?: defaultValue
// defaultValue is returned if the value or the transform result is null.

when返回语句

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

try-catch

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

    // Working with result
}

if表达式

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

builder风格

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

Single-expression 方法

fun theAnswer() = 42

等同于

fun theAnswer(): Int {
    return 42
}

与when结合使用

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

使用with执行实例的多个方法

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

val myTurtle = Turtle()
with(myTurtle) { //draw a 100 pix square
    penDown()
    for (i in 1..4) {
        forward(100.0)
        turn(90.0)
    }
    penUp()
}

配置对应的属性

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

Java7的try-with-resources

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

泛型

class Box<T>(t : T) {
    var value = t
}
fun main(){
    val intBox = Box<Int>(1)
    val stringBox = Box<String>("Evan")
    println(intBox.value)
    println(stringBox.value)
}

Nullable Boolean

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

交换变量

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

TODO

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

参考资料

kotlin-idioms