学习列表：
- 几乎一切乎都是表达式
scala> 1 + 1
res0: Int = 2
- 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
- 隐式参数(implicit)
可以为方法定义隐式的参数值，在不传值的情况下，编译器会从当前作用域自动检索满足类型要求的隐式参数进行填充，但是必须保证无二义性
scala> def func(implicit str: String) = println(str)
def func(implicit str: String): Unit
scala> implicit val x = "test"
val x: String = test
scala> func
test
scala> implicit val y = "yyds"
val y: String = yyds
scala> func // 出现二义性，存在多个满足类型要求的隐式参数
^
error: ambiguous implicit values:
both value x of type String
and value y of type String
match expected type String
- 隐式类型转换方法(implicit)
当编译器发现方法入参类型不满足要求时，会在当前作用域下自动寻找满足需求的隐式类型转换方法
scala> implicit def fun(x: String) = x.toInt
fun: (x: String)Int
scala> math.max("132", 13)
res0: Int = 132
- 隐式类(implicit)
支持扩展对象调用自身未定义的方法，编译器发现无该方法时会在当前作用域下自动寻找满足条件的隐式类
scala> class Animal {}
class Animal
scala> implicit class Fly(animal: Animal) {
| def fly() = println("I can fly")
| }
class Fly
scala> val animal = new Animal
val animal: Animal = Animal@715fa8c5
scala> animal.fly
I can fly
- 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
- 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
- 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
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
scala集合
Seq(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 "->" 符号创建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
10
reduceLeft、reduceRight(将集合进行函数计算，得到结果)
scala> (1 to 10) reduceLeft (_ + _)
res42: Int = 55
scala> (1 to 10) reduceRight (_ + _)
res43: Int = 55
fold
fold 支持自定义初始值的聚合计算
scala> (1 to 10).fold(10)(_ + _)
res10: Int = 65
filter(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、forall、count
find 返回符合条件的第一个选项 Option
exists 只要集合中有任意元素符合条件及返回 true，否则 false
forall 需要集合所有元素都满足条件才返回 true，否则 false
count 返回满足条件的元素个数Int
scala> (1 to 10) find (_%2==0)
res30: Option[Int] = Some(2)
scala> (1 to 10) exists (_%2==0)
res40: Boolean = true
scala> (1 to 10) forall (_ > 0)
res50: Boolean = true
scala> (1 to 10) count (_ %2 ==0)
res60: Int = 5
zip
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))
