Scala introduction

Scala 初探
亦思科技鄭紹志
vito@is-land.com.tw

WHO AM I
• 鄭紹志
• IS-LAND Technical Manager
• HareDB
• Hadoop + HBase
• Scala + Spark

WHY SCALA
WHY NOT SCALA
• Scala - The good, the bad and the very ugly
• http://www.slideshare.net/Bozho/scala-the-good-the-bad-and-the-very-ugly
•Why Scala
• http://www.slideshare.net/al3x/why-scala-presentation
•Today Gist: http://tinyurl.com/z8xx6ab

SCALABLE LANGUAGE
• Runs on JVM
• Concise, 簡潔
• Statically typed, 靜態型別
• Object Oriented Programming, 物件導向開發
• Functional Programming, 函數式開發

SCALA INSTALL
• Scala 2.10.x - Spark 1.0 - 1.6 都使⽤ 2.10.x
• Download Scala 2.10.x
• http://www.scala-lang.org/download/all.html
• 解壓縮 & 設定路徑
• http://www.scala-lang.org/download/install.html

SCALA INTERPRETER
• scala: Read-Evaluate-Print-Loop(REPL)
[~]$ scala
Welcome to Scala version 2.11.7 (Java HotSpot(TM) 64-Bit
Server VM, Java 1.7.0_80).
Type in expressions to have them evaluated.
Type :help for more information.
scala> val name = "Vito"
name: String = Vito
scala> println( s"Hello, $name" )
Hello, Vito
scala> println( name.getClass )
class java.lang.String
scala>

SCALA INTERPRETER
• One line execute
• Run script
• Run main class
• Shell script
• Load ﬁle: :load
• Paste mode: :paste
$ scala -e 'val name="Vito"; println(s"Hello,$name")'
$ scala hello.scala
$ scala HelloWorld.scala
$ ./script.sh

Value & Variable
• 定義常數與變數
• 優先考慮使⽤ val
var price1 = 100 
price1 = 200 
 
val price2 = 100 
price2 = 200 // error: reassignment to val

Type Inference
• 推斷使⽤的型別
val name = "Vito" 
val n1 = 100 
val n2 = 100L 
val n3: Int = 100 
 
val n4 = 20.6F 
val list = Seq("a", "b", "c") 
 
import java.util._ 
val list1 = new ArrayList[String]()

STRING
//  
val s1 = "Hello" 
//  
val s2 = """object HelloWorld extends App { 
println("Hello, World!") 
} 
""" 
// string interpolation 
val s3 = s"[$s1] source code:n$s2" 
 
// no escape 
val s4 = raw"Hello n world." 
val s5 = "Hello"(1) 
val s6 = "Hello".charAt(1)

TUPLE
• Tuple2 - Tuple22
val t2a = ("a", 2) 
val t2b = Tuple2("a", 2) 
val t2c = new Tuple2("a", 2) 
val t2d = "a" -> 2 
val t5 = ("a", 100, "c", 20.5, (1,2,3)) 
 
val v1 = t2a._1 == t2a._1 // true 
val v2 = t5._5 == (1,2,3) // true

OBJECT ORIENTED
//  
// . ( ) 
"Hello World" split " " 
"Hello World".split(" ") 
 
// operator  
"a" + "b" + "c" 
"a".+("b").+("c")

IMPORT CLASS
• 預設 import
• java.lang._
• scala._
• Java *, Scala _
• scala.Predef object
import java.util.{List, ArrayList}
import java.util.{List=>JList, HashMap=>JHashMap}
import java.util.{Date=>_, _}

IMPORT CLASS
• Import anywhere
import org.apache.spark.{SparkContext, SparkConf, Logging}
class MyClass { 
def printDate() = { 
val date = new Date() 
println(date) 
} 
def printFilePath() = { 
import java.io.File 
val file = new File("data.txt") 
println(file.getAbsolutePath) 
} 
}

OBJECT
• Scala 沒有 Java 的 static 宣告, 可⽤ object 代替
• 想⽤ Singletion pattern 就⽤ object
• Companion Objects: 同名的 class 與 object 同時
存在
object Student { 
def sayHi = println("Hi~") 
} 
Student.sayHi

OBJECT
class Student(val name: String) { 
override def toString = s"[Student]: $name" 
} 
 
object Student { 
def apply(names: String*) = { 
names.map { n => new Student(n) } 
} 
def count(ss: Seq[Student]) = ss.size 
} 
 
val ss = Student("Vito", "John") 
Student.count(ss)

RETURN VALUE
• method 傳回值不須加 return
• Every statement(expression) yields a value
val a = 5 
val b = if (a>2) 0 else 1 
val data = Seq(1,2,3,4,5) 
val data1 = for ( 
num <- data 
if num % 2 == 0 
) yield { 
num + 10 
} // data1: List(12, 14)

RETURN VALUE
• Every statement(expression) yields a value
def calc(a: Int, b: Int) = try { 
a / b 
} catch { 
case _: Throwable => 0 
} 
 
val a = calc(100, 0)
calc: (a: Int, b: Int)Int
a: Int = 0

Anonymous Functions
val data = Seq.range(1, 5) 
val data2 = data.map( (s: Int) => s*2 ) 
val data3 = data.map( (s) => s*2 ) 
val data4 = data.map( _*2 )
data: Seq[Int] = List(1, 2, 3, 4)
data2: Seq[Int] = List(2, 4, 6, 8)

Anonymous Functions
val multi = (s: Int) => s * 2 
val multi2 = new Function1[Int, Int]() { 
override def apply(v1: Int): Int = v1 * 2 
} 
val data2 = data.map(multi) 
val data3 = data map multi 
val data4 = data map multi2
multi: Int => Int = <function1>
multi2: Int => Int = <function1>

Function ? Method ?
• ⽬前不必追究
• (想追究的😆)可從這裡先開始, 再 Google
• http://stackoverﬂow.com/q/2529184

map
val multi = (s: Int) => s * 2 
val data2 = data.map( _*2 ) 
val data3 = data map multi 
multi: Int => Int = <function1>

Reduce
val data2 = data.reduce(_+_) 
val data3 = data.reduce { (v1, v2) => 
println( (v1, v2) ) 
v1 + v2 
}
data2: Int = 10
(1,2)
(3,3)
(6,4)
data3: Int = 10

PRACTICE
• 看懂第⼀個 Spark scala sample code 使⽤的語法
• SparkPi.scala

CLASS
// Scala 
// Both getter and setter 
class Student1(var name: String) 
 
// Only getter 
class Student2(val name: String) 
 
// private 
class Student3(val name: String, private val
id: Int)

CASE CLASS
• 同時建⽴ class 與 object
• 為 class 建⽴ getter & setter
• 為 object 建⽴ apply & unapply method
case class Student(val id: Int, val name:
String) 
 
val s = Student(100, "Vito") 
Student.unapply(s)

Pattern Matching
• 第⼀眼看起來很像 switch...case...
val b = true 
b match { 
case true => println("true") 
case false => println("false") 
}

Pattern Matching
• 實際與 switch 差很⼤, example: Source

class extends & with
• Scala 沒有 Java 的 interface
• Trait - 結合 interface 與 class
class Person {} 
trait PlayGame {} 
trait ReadBook {}
 
class Student extends Person
with PlayGame with ReadBook { 
}
 
class LittleBoy extends Person with PlayGame { 
}

SBT
• The interactive build tool, run task from shell
• Installing sbt
• Spark
• Maven is recommented for packaging Spark
• SBT is supported for day-to-day development

SBT
• Commands
• run, run-main
• test, testOnly
• compile
• reload
• 威⼒強⼤的 ~
• Demo

SCALA 實務
• Scala 還蠻難的
• 直接使⽤ Java SDK & 第三⽅ Libraries
• SBT project 可混合 Java & Scala
• 好的 IDE 帶你快速上路: Intellij IDEA
• Spark 是分散式的 Scala

SCALA 實務
• Build tool: recommand SBT
• Testing: Scalatest, Specs2, JUnit
• SBT PermGen space resolve
• http://www.scala-sbt.org/0.13/docs/Manual-Installation.html
• Book

Scala introduction

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