scala 学习笔记-持续更新中
学习列表：
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几乎一切乎都是表达式
scala> 1 + 1 res0: Int = 2
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复合表达式——{}
花括号用于创建复合表达式，复合表达式的返回值是最后一个表达式
scala> { | println("hello world") | "hi" | } hello world res0: String = hi -
常量
使用val,不能重复给常量赋值
scala> val value = 100 + 1 value: Int = 101 scala> value = 200// 给常量重复赋值将报错 <console>:11: error: reassignment to val value = 200 ^ -
变量
使用var,可以修改变量的值
scala> var value = 100 value: Int = 100 scala> value = 200 value: Int = 200 -
类型推断
scala支持类型推断,类型声明置后,编译器会推断其类型,会比java更加简洁
scala> val m: Int = 100// 不建议 m: Int = 100 scala> var n: String = "hi"// 不建议 n: String = hi scala> val m = 100// 推荐 m: Int = 100 scala> var n = "hi"// 推荐 n: String = hi -
函数
使用def创建函数(与python一样),需要为函数参数指定类型签名,函数返回值类型可以不写(递归函数必须声明返回值类型),让编译器进行类型推断
scala> def sum(a: Int, b: Int): Int = a + b// 中规中矩的写法 sum: (a: Int, b: Int)Int scala> sum(2, 3) res8: Int = 5 scala> def fun(x: Int) = x*x// 不声明返回值类型 fun: (x: Int)Int scala> fun(5) res9: Int = 25 // 使用复合表达式 scala> def mul(x: Int, y: Int) = { | println(x.toString) | println(y.toString) | x * y | } mul: (x: Int, y: Int)Int scala> mul(2, 3) 2 3 res12: Int = 6 // 无返回值的函数可以不用=,代表返回值为Unit类似java中void scala> def hi() { | println("hi world!") | } hi: ()Unit scala> hi// 无参函数可以不使用括号调用 hi world! -
匿名函数
使用 =>
scala> (x: Int) => x*x res14: Int => Int = <function1> scala> res14(5) res15: Int = 25可以将匿名函数进行赋值
scala> val fun = (x: Int) => x*x fun: Int => Int = <function1> scala> fun(3) res27: Int = 9同样可以使用{}定义复杂的匿名函数
scala> var m = {x: Int => | println("hi") | val temp = x*x | temp + 1 | } m: Int => Int = <function1> scala> m(4) hi res28: Int = 17 -
部分应用(partially applied function)
你可以使用下划线“_”(通配符)部分应用一个函数，结果将得到另一个函数。
scala> def add(x: Int, y: Int) = x+y add: (x: Int, y: Int)Int scala> add(4, _:Int) res1: Int => Int = <function1> scala> res1(3) res2: Int = 7 -
柯里化函数(curried)
有时会有这样的需求：允许别人一会在你的函数上应用一些参数，然后又应用另外的一些参数。
scala> def mul(x: Int)(y:Int) = x*y mul: (x: Int)(y: Int)Int scala> mul(2)_ res3: Int => Int = <function1> scala> res3(5) res4: Int = 10多参函数柯里化
scala> def mul3(x: Int, y: Int, z: Int) = x*y*z mul3: (x: Int, y: Int, z: Int)Int scala> (mul3 _).curried res8: Int => (Int => (Int => Int)) = <function1> scala> res8(2) res9: Int => (Int => Int) = <function1> scala> res9(3) res10: Int => Int = <function1> scala> res10(4) res11: Int = 24 -
变长参数
向方法传入任意多个同类型的参数,比如任意多个整数求和
scala> def sum(nums: Int*) = (nums reduceLeft ((x: Int, y: Int) => x+y)) sum: (nums: Int*)Int scala> sum(1,2,3,4,5,6,7,8,9,10) res19: Int = 55 -
参数限定
scala可以使用require进行函数参数限制,类似java assert
scala> def sq(x: Int) = { | require(x >= 0, "input must > 0!") | math.sqrt(x) | } sq: (x: Int)Double scala> sq(2) res0: Double = 1.4142135623730951 scala> sq(-1) java.lang.IllegalArgumentException: requirement failed: input must > 0! at scala.Predef$.require(Predef.scala:219) at .sq(<console>:11) ... 33 elided -
隐式类型转换(implicit)
在同一作用域下,自动进行类型的转换,与该函数名无关,仅与其输入输出类型有关
scala> implicit def fun(x: String) = x.toInt fun: (x: String)Int scala> math.max("132", 13) res0: Int = 132 -
闭包(closure)
函数及其执行所需的上下文环境(“An object is data with functions. A closure is a function with data.” — John D. Cook)
scala> def fun() = { | var i = 0 | val lam = () => {i+=1;i} | lam | } fun: ()() => Int scala> val clo = fun() clo: () => Int = <function0> scala> clo() res0: Int = 1 scala> clo() res1: Int = 2 scala> clo() res2: Int = 3 -
传名参数(call by name,传值、传引用之外的另外一种参数传递方式)
传名的参数传递使用替换规则。
scala> def callByNameFun(x: => Int) = List(x, x)// =>开头为传名参数 callByNameFun: (x: => Int)List[Int] // 定义高阶函数createNum产生一个函数，使用闭包，每调用一次fun(),i自增加1 scala> def createNum() = { | var i = 0 | val fun = { | () => | i += 1 | i | } | fun | } createNum: ()() => Int scala> val f = createNum() f: () => Int = <function0> scala> callByNameFun(f()) res0: List[Int] = List(1, 2) scala> callByNameFun(f()) res1: List[Int] = List(3, 4) -
尾递归
尾递归函数返回值为本身或者结果,同一般的递归相比,可被直接转化为循环,栈的开销为O(1),scala中可以用 @tailrec 检测定义的函数是否为尾递归
scala> import scala.annotation.tailrec scala> @tailrec def fun(re: Int, index: Int): Int = { | if (index == 0) { | re | } else { | fun(re+index, index - 1) | } | } fun: (re: Int, index: Int)Int scala> fun(0, 100) res0: Int = 5050 scala> fun(0, 100) res1: Int = 5050 -
类
使用class关键字
scala> class User { | var name = "bernie" | var age = 23 | def show() = println(name + ":" + age.toString) | } defined class User scala> var user = new User() user: User = User@41112c13构造函数,构造函数不是特殊的方法,他们是除了类的方法定义之外的代码。
scala> class Person(pName: String, pAge: Int) { | var name = pName | var age = pAge | def show() = println(name + ":" + age.toString) | } defined class Person // 也可重载构造函数 scala> class Person(pName: String, pAge: Int) { | def this(pName: String) = this(pName, 18) | var name = pName | var age = pAge | } defined class Person
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抽象类
和java差不多,继承也使用extends
scala> abstract class AbstractFather { | def fun(x: Int): Int | } defined class AbstractFather -
特质(trait)
是一组属性与行为的组合,特质可以多扩展,在抽象类与特质之间优先选择特质
scala> trait Fly { | val wing: Int | def fly() = println("I can fly") | } defined trait Fly scala> trait Run { | val leg: Int | def run() = println("I can run") | } defined trait Run scala> class Bird extends Fly with Run { | val wing = 2 | val leg = 2 | def power() = { | fly() | run() | } | } defined class Bird scala> new Bird().power() I can fly I can run -
单例的语言级别实现——object
Util类就非常适合使用object定义
object Util { def echo() = println("Hello World!") } -
伴生对象与伴生类
名字相同的class与object,放在同一个文件下定义,能够访问彼此的私有成员
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泛型
使用中括号,类和方法都可以是泛型的
scala> class Gen[T] { | def show(x: T) = x.toString + "$" | } defined class Gen scala> val gen = new Gen[Int] gen: Gen[Int] = Gen@3a44ebcb scala> gen.show(100) res11: String = 100$ scala> def genFun[T](x: T) = x.toString + "&" genFun: [T](x: T)String scala> genFun(100) res13: String = 100& -
语法糖 apply update
为了方便你使用类,scala提供了apply 与 update语法糖
scala> object Arr { | val arr = Array(0,1,2,3,4,5) | def apply(i: Int) = arr(i) | def update(i: Int, value: Int) = {arr(i) = value} | } defined object Arr scala> Arr(1) res38: Int = 1 scala> Arr(1) = 100 scala> Arr.arr res40: Array[Int] = Array(0, 100, 2, 3, 4, 5)其实 Array 取值与更新就是实现了这两个方法
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scala数组 -
函数与对象de边界
函数可以看作一堆特性的集合(Funtion0 -> Funtion22),apply语法糖有助于统一对象和函数式编程的二重性,你可以传递类,并把它们当做函数使用,而函数本质上是类的实例
scala> object Fun extends Function1[Int, Int] { | def apply(x: Int) = x*x | } defined object Fun scala> Fun(3) res50: Int = 9 -
模式匹配
感觉像java中switch case,但功能更加强大
scala> val m = 1 m: Int = 1 scala> m match { | case 1 => 100 | case 2 => 200 | case _ => 300 | } res25: Int = 100 -
异常
异常也是表达式
scala> val re = try { | 1/0 | "oh yeah" | } catch { | case e: Exception => println("error happen") | "oh no" | } finally { | println("turn me") | } error happen turn me re: String = oh no -
集合
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scala集合
List(列表)、Set(集)、Tuple(元组)、Map(映射)、Option(选项)Tuple
元组是在不使用类的前提下,将元素组合起来形成简单的逻辑集合,读取方式使用下标,从1开始,可以使用->创建2元组
scala> val tuple = ("hi", "he", "ha", 12, 'a') tuple: (String, String, String, Int, Char) = (hi,he,ha,12,a) scala> tuple._1 res36: String = hi scala> 1 -> 2 res63: (Int, Int) = (1,2)Map
scala> Map(1 -> "one", 2 -> "two") res0: scala.collection.immutable.Map[Int,String] = Map(1 -> one, 2 -> two) scala> Map((1, "one"), (2, "two")) res1: scala.collection.immutable.Map[Int,String] = Map(1 -> one, 2 -> two)Option
表示可能包含值的容器,有Some与None两个子类,特征接口如下
trait Option[T] { def isDefined: Boolean def get: T def getOrElse(t: T): T }scala> val map = Map(1 -> "one", 2 -> "two") map: scala.collection.immutable.Map[Int,String] = Map(1 -> one, 2 -> two) scala> map.get(1) res2: Option[String] = Some(one) scala> map.get(3) res3: Option[String] = None scala> res2.get res4: String = one scala> res3.get java.util.NoSuchElementException: None.get at scala.None$.get(Option.scala:347) at scala.None$.get(Option.scala:345) ... 33 elided scala> res3.isDefined res6: Boolean = false scala> res3.getOrElse("three") res7: String = three -
集合操作符(操作符即函数)
map、foreach
map将产生一个新的集合,foreach无返回值,一般仅作遍历使用,取它的副作用
scala> (1 to 10) map (_ * 2) res21: scala.collection.immutable.IndexedSeq[Int] = Vector(2, 4, 6, 8, 10, 12, 14, 16, 18, 20) scala> (1 to 10) foreach (println(_)) 1 2 3 4 5 6 7 8 9 10reduceLeft、reduceRight(将集合进行函数计算，得到结果)
scala> (1 to 10) reduceLeft ((m: Int, n: Int) => m+n) res42: Int = 55 scala> (1 to 10) reduceRight ((m: Int, n: Int) => m+n) res43: Int = 55filter(白名单过滤)、partition(分割)
fliter将符合条件的元素产生一个集合返回,partition将其分隔成满足与不满足条件的两元组
scala> (1 to 10) filter (_%2 == 0) res24: scala.collection.immutable.IndexedSeq[Int] = Vector(2, 4, 6, 8, 10) scala> (1 to 10) partition (_%2 == 0) res25: (scala.collection.immutable.IndexedSeq[Int], scala.collection.immutable.IndexedSeq[Int]) = (Vector(2, 4, 6, 8, 10),Vector(1, 3, 5, 7, 9)) scala> res25._1 res26: scala.collection.immutable.IndexedSeq[Int] = Vector(2, 4, 6, 8, 10) scala> res25._2 res27: scala.collection.immutable.IndexedSeq[Int] = Vector(1, 3, 5, 7, 9)find、exists、count
find返回符合条件的第一个选项 Option,exists返回Boolean值,count返回满足条件的个数Int
scala> (1 to 10) find (_%2==0) res30: Option[Int] = Some(2) scala> (1 to 10) exists (_%2==0) res50: Boolean = true scala> (1 to 10) count (_ %2 ==0) res57: Int = 5zip
顾名思义就是像拉链一样彼此咬合生成新的集合,长短不一的时候以短为准
scala> Array(1, 2, 3) zip List("a", "b", "c", "d") res53: Array[(Int, String)] = Array((1,a), (2,b), (3,c))
