Hello ScalaJINLIANG OU

2014 04

The First Example http://git.n.xiaomi.com/oujinliang/scala-intro

假设我们要解析这个文件，对符合条件的每一行构造一个对象 (Xmq), 构造一个列表，对某两列排序，然后对这个列表的每一个对象进行操作 (print):

http://git.n.xiaomi.com/oujinliang/scala-intro/blob/master/xmq.txt

Java Solution:

http://git.n.xiaomi.com/oujinliang/scala-intro/blob/master/src/main/java/org/jinou/javasample/ParseXmqNodes.java

Scala Solution:

http://git.n.xiaomi.com/oujinliang/scala-intro/blob/master/src/main/scala/org/jinou/scalasample/ParseXmqNodes.scala

About Scala Scalable Language 的缩写 By Martin Ordersky (Generic Java 作者，最初 javac 的参考实现的作者 )

James Gosling (Creator of Java):◦ "Which Programming Language would you use *now* on top of JVM, except Java?". ◦ The answer was surprisingly fast and very clear: - Scala.

James Strachan (Created Groovy in 2003):◦ I can honestly say if someone had shown me the Programming in Scala book by by Martin Odersky, Lex

Spoon & Bill Venners back in 2003 I'd probably have never created Groovy.◦ http://macstrac.blogspot.com/2009/04/scala-as-long-term-replacement-for.html

About Scala (features)oGeneral purpose Language

oJVM based (full inter-op with Java, community, libraries)

oFast

oStatically Typed (Type safe)

oObject Oriented

oFunctional

oConcurrency

oConcise and Expressive

oPowerful, highly Scalable,

oProductivity

Scala Is Used By Companies:

◦ Twitter, LinkedIn, Siemens, Amazon, SONY, Foursquare, …

Libraries and Frameworks◦ Akka, Kafka, Finagle, Spark, Play! Lift, …

Scala 是简洁的 Java Type less, do more.

Less code, fewer bugs.

一些东西是可以省略的 表达式后的分号

◦ 基本都是不需要的 ( 除非你想一行写多条，不建议 )

return 关键字◦ 大部分可以省略， 除了少数情况必须从函数中间返回

没有参数的函数调用和声明◦ a.toString => a.toString()

方法调用中间的 . ◦ 在某些 DSL 的场合◦ nameList foreach println => nameList.foreach(n => println(n))

类型 ( 见下页 )

Type Inference 类型推导， 静态语言写出动态语言的感觉 . 在能推导出类型的时候，可以省略类型 值或变量的类型

◦ val sum = 1 + 2 + 3 ◦ 和下面的等价， Compiler 推导出类型为 Int◦ val sum : Int = 1 + 2 + 3

◦ var names = List(“Steve”, “Mike”, “Bob”)◦ List[String]

◦ val map = Map(1234 -> Person(“Jim”, 18), 5678 -> Person(“Jack”, 22))◦ Map[Int, Person]

函数的返回类型 (def 是函数定义的关键字 )◦ def getName = “myName”

◦ 等价于 :◦ def getName() : String = “myName”

简洁的表达 简化的类定义：

class Xmq(chid:Int, start:Long, end:Long, host:String, port:Int, site:String) { override def toString = “xxxx”}val xmq = new Xmq(1, 1, 10000, “localhost”, 7915, “site0”)

利用高价函数进行简化val list = List(1, 2, 3, 4, 5, 6)val hasTwo = list.exists(_ == 2) // trueval even = list.filter(_ % 2 ==0) // List(2, 4, 6)

转换val id = “12345”.toInt

val list = Array(“a”, “b”, “c”).toList

val set = list.toSet

其它有用的特性 Tuple:

◦ 有的时候你需要组合两个或三个对象，但创建一个类又太 heavy.◦ val (host, port) = getHostAndPort(xxxx)

Default Parameter value, Named Parameter

Raw String:◦ Java 中字符串转义，拼接太麻烦了 , ◦ 可读性很重要"""\[(\d+),\s*(\d+),\s*(\d+),\s*"(.+)",\s*(\d+),\s*"(.+)".*\]""? (Scala) Or"\\[(\\d+),\\s*(\\d+),\\s*(\\d+),\\s*\"(.+)\",\\s*(\\d+),\\s*\"(.+)\".*\\]" (Java)

val usage = """Usage: 1. xxxx 2. yyyy 3. xxxx """

XML case class Book(val name: String, val price: Int)

object XmlSample {

def main(args: Array[String]) {

val xml = getBooksXml("computer", 2, List(Book("C Language", 10), Book("Java Memory Model", 40)))

println(xml)

println(xml \ "@category") // get the attribute

}

def getBooksXml(category: String, version: Int, books: List[Book]): Node =

<books category={category} version={version.toString}> { books.map(getBookXml) } </books>

def getBookXml(book: Book) = <book name={book.name} price={book.price.toString} />

}

<books version="2" category="computer"> <book name="C Language" price="10"></book><book name="Java Memory Model" price="40"></book> </books>computer

Scala is Object Oriented It’s PURE OO

一切都是对象 , Scala 是面向对象的，没有 Java 中的所谓静态等非对象的东西( 想想 Java 有哪些不是 pure OO 的？基本类型，静态类？）◦ 1 + 2 ◦ 1.+(2)

OO 有哪些特性？哪些原则？

Class/Object/Trait object vs. class

◦ class 和 Java 中的概念一样，用来创建对象的◦ object is singleton !◦ 忘记各种复杂的 singleton 的构造方式吧，忘记 double-checked locking

Trait◦ Interface in Java, 可以有预定义的方法 (Java 8 ?)◦ Mixin ( 解决 Java 中无法多继承的问题 )

object IdGenerator { private val acc = new AtomicInteger(0) def nextId() = acc.incrementAndGet}

val newId = IdGenerator.nextId()

Scala is Functional Function is first-class citizen

◦ 单独声明一个函数： def foo(a: Int) = {

a * 2

}

foo(3)

◦ 函数作为变量 val plus2 = (x: Int) => x + 2

plus2(3) // 5

◦ 函数作为参数，返回值都可以

Higher Order Function a higher-order function is a function that does at least one of the following:

◦ takes one or more functions as an input◦ outputs a function

val plus2 = (x: Int) => x + 2

val nums = List(1,2,3,4)

// higher order functions

val double = nums.map(_*2)  //◦ or nums.map(n => n * 2)  // List(2,4,6,8)

val allPlus2 = nums.map(plus2)   // List(3,4,5,6)

val reduce = nums.reduce(_ + _) // 10

val result = nums.filter(_ > 2) // List(3,4)

Higher-order function example def withClient[T](f: Client => T) = {

val client = getClient

try {

f(client)

} finally {

client.close

}

}

val i: Int = withClient( _.getIntValue() )

val s: String = withClient( _.getStringValue() )

Pattern Match Java 的 switch/case 太弱了。

Scala 可以匹配 ( 基本类型， String, 正则表达式， List, case 对象… )

def what(any: Any) = any match {

case i: Int => "I'm int"

case s: String if s.startsWith("s") => "Starts with s "

case "hello" => "This is a hello"

case _ => "what ever "

}

what(123)

what(“hello”)

what(“scala”)

其他高级货o 并发， actor, Akka, 并发 collections

o Generic, variance, co-variance

o Implicit conversions

o Partially applied functions

o Currying

o For comprehensions

o Macro

o…

使用 ScalaIDE:◦ Eclipse (http://scala-ide.org, 官方支持， 很好用 ) ◦ IntelliJ IDEA (with the Scala plugin， JetBrains 官方插件 , 很好用 )◦ NetBeans (with the Scala plugin， 没用过 )

Build Tool:◦ Sbt (simple build tool, Scala 项目默认都使用的 )◦ Maven (with plugin) 我在一些项目里使用这个，混合 Java + Scala ◦ Ant..

Test ◦ 可以使用 Junit◦ 也可以使用专门给 Scala 设计的 : specs, scala test …

Libraries and Frameworks◦ 找得到 Scala 的用 Scala 的，找不到就用 Java 的！◦ https://wiki.scala-lang.org/display/SW/Tools+and+Libraries

缺点 编译速度

◦ 使用 IDE 的增量编译

二进制不兼容◦ 使用版本化

语法越来越复杂◦ 易上手，难精通◦ 开始不要使用高级功能◦ 应用开发和库开发需要使用的技能是不同的◦ 不要炫技

用的人少， 招聘难◦ 学吧 , 用 Scala 的人越来越多了。

学习 Scala 开始：

◦ http://docs.scala-lang.org/tutorials/scala-for-java-programmers.html ◦ http://twitter.github.io/scala_school/ (http://twitter.github.io/scala_school/zh_cn/index.html )

全面学习◦ Programming in Scala

Effective Scala◦ http://twitter.github.io/effectivescala/index.html