Scala
Scala
https://twitter.github.io/scala_school/zh_cn/
- 表达能力：函数是一等公民、闭包
- 简洁：类型推断、函数创建的文法支持
- Java互操作性：可重用Java库、可重用Java工具、没有性能惩罚
启动解释器
sbt console
Basics
表达式、值、函数（匿名函数）
scala> 1 + 1
res0: Int = 2
scala> val two = 1 + 1
two: Int = 2
scala> var name = "steve"
name: java.lang.String = steve
scala> name = "marius"
name: java.lang.String = marius
scala> def addOne(m: Int): Int = m + 1
addOne: (m: Int)Int
scala> val three = addOne(2)
three: Int = 3
scala> def three() = 1 + 2
three: ()Int
scala> three()
res2: Int = 3
scala> three
res3: Int = 3
scala> (x: Int) => x + 1
res2: (Int) => Int = <\function1>
scala> res2(1)
res3: Int = 2
scala> val addOne = (x: Int) => x + 1
addOne: (Int) => Int = <\function1>
scala> addOne(1)
res4: Int = 2
def timesTwo(i: Int): Int = {
println("hello world")
i * 2
}
scala> { i: Int =>
println("hello world")
i * 2
}
res0: (Int) => Int = <\function1>
部分应用（Partial application）
scala> def adder(m: Int, n: Int) = m + n
adder: (m: Int,n: Int)Int
scala> val add2 = adder(2, _:Int)
add2: (Int) => Int = <\function1>
scala> add2(3)
res50: Int = 5
柯里化函数
scala> def multiply(m: Int)(n: Int): Int = m * n
multiply: (m: Int)(n: Int)Int
scala> multiply(2)(3)
res0: Int = 6
scala> val timesTwo = multiply(2) _
timesTwo: (Int) => Int = <\function1>
scala> timesTwo(3)
res1: Int = 6
scala> (adder _).curried
res1: (Int) => (Int) => Int = <\function1>
可变长度参数
def capitalizeAll(args: String*) = {
args.map { arg =>
arg.capitalize
}
}
类
scala> class Calculator {
| val brand: String = "HP"
| def add(m: Int, n: Int): Int = m + n
| }
defined class Calculator
scala> val calc = new Calculator
calc: Calculator = Calculator@e75a11
scala> calc.add(1, 2)
res1: Int = 3
scala> calc.brand
res2: String = "HP"
构造函数
class Calculator(brand: String) {
/**
* A constructor.
*/
val color: String = if (brand == "TI") {
"blue"
} else if (brand == "HP") {
"black"
} else {
"white"
}
// An instance method.
def add(m: Int, n: Int): Int = m + n
}
scala> val calc = new Calculator("HP")
calc: Calculator = Calculator@1e64cc4d
scala> calc.color
res0: String = black
函数 vs 方法
scala> class C {
| var acc = 0
| def minc = { acc += 1 }
| val finc = { () => acc += 1 }
| }
defined class C
scala> val c = new C
c: C = C@1af1bd6
scala> c.minc // calls c.minc()
scala> c.finc // returns the function as a value:
res2: () => Unit = <\function0>
http://stackoverflow.com/questions/2529184/difference-between-method-and-function-in-scala
继承
class ScientificCalculator(brand: String) extends Calculator(brand) {
def log(m: Double, base: Double) = math.log(m) / math.log(base)
}
class EvenMoreScientificCalculator(brand: String) extends ScientificCalculator(brand) {
def log(m: Int): Double = log(m, math.exp(1))
}
scala> abstract class Shape {
| def getArea():Int // subclass should define this
| }
defined class Shape
scala> class Circle(r: Int) extends Shape {
| def getArea():Int = { r * r * 3 }
| }
defined class Circle
scala> val s = new Shape
<\console>:8: error: class Shape is abstract; cannot be instantiated
val s = new Shape
^
scala> val c = new Circle(2)
c: Circle = Circle@65c0035b
特质（Traits）
trait Car {
val brand: String
}
trait Shiny {
val shineRefraction: Int
}
class BMW extends Car {
val brand = "BMW"
}
class BMW extends Car with Shiny {
val brand = "BMW"
val shineRefraction = 12
}
- 优先使用特质。一个类扩展多个特质是很方便的，但却只能扩展一个抽象类。
- 如果你需要构造函数参数，使用抽象类。因为抽象类可以定义带参数的构造函数，而特质不行。例如，你不能说trait t(i: Int) {}，参数i是非法的。
类型
trait Cache[K, V] {
def get(key: K): V
def put(key: K, value: V)
def delete(key: K)
}
def remove[K](key: K)
Basics continued
相关连接
- https://twitter.github.io/scalaschool/zhcn/basics.html
