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
}
