scala for java programmers

Scalaf o r J a v a P r o g r a m m e r s

Hello, Scala!

Hello, World!

object HelloWorld { def main(args: Array[String]) = println("Hello, World!")}

Agenda

About Scala

Features

Motivation

Scalable language. To be scalable in the sense that the same concepts can describe small as well as large parts.

A unified and generalized object-oriented and functional programming language provides scalable support.

From Inventor

Motivation

More productive as “better Java” with existing Java resources.

Several concepts not in Java bring more posibilities to you.

For Java programmers

the Programming Language

Language Specification

Runtime

Scaladoc

Runtime

scalac

.scala

.class

the compiler for .NET is out of date

JVMscala

Running on JVM

Compiler generates class files

Influencers

Java C#

Smalltalk

Haskell

Algol Simula

Eiffel

SML F#

Erlang

IswimLisp

Python

Inventor

Martin Odersky

Professor of programming methods at EPFL in Switzerland

Designer of Java generics

photo#1

Object, Operator

val apple = new Appleval orange = new Orangeprintln(apple + orange)

Object, Operator

val apple = new Appleval orange = new Orangeprintln(apple + orange)

(A) "Apple@xxxxOrange@xxxx"(B) 2(C) Compile error(D) Runtime exception(E) Others

Features

Static typing & Suitable type inference

Object-oriented & Functional

Collaborating with Java

Vorbose JavaType declaration

public String suffix(String str) { int pos = str.indexOf("-"); return str.substring(pos);}

Vorbose JavaType declaration

def suffix(str: String) = { val pos = str.indexOf("-") str.substring(pos)}

Vorbose JavaJavaBeans

public class JavaBeans { private String name; public String getName() { return name; } public void setName(String name) { this.name = name; }}

Vorbose JavaJavaBeans

case class ScalaCase(name: String)

Functional Programming

variables

method parameters

method returns

Functions are the first-class values

Consider a method signature sending mails to appropriate addresses from listing

def send() { for(c: Contact <- listing()) { mail(c) }}

age==20

age==20 def send(age: Int)

age==20

35<=age, age<=60

def send(age: Int)

age==20

35<=age, age<=60

def send(age: Int)

def send(minAge: Int, maxAge: Int)

age==20

35<=age, age<=60

age<30, gender:Male

def send(age: Int)

age==20

35<=age, age<=60

age<30, gender:Male

def send(age: Int)

def send(minAge: Int, maxAge: Int, g: Gender)

age==20

35<=age, age<=60

age<30, gender:Male

gender:Female, in Tokyo

def send(age: Int)

age==20

35<=age, age<=60

age<30, gender:Male

gender:Female, in Tokyo

def send(age: Int)

def send(minAge: Int, maxAge: Int, g: Gender, pref: String)

def send(p: Contact => Boolean) { for(c: Contact <- listing()) { if (p(c)) mail(c) }}

Apply condition function to method

y = f(x)Avoiding side effects

Conscious of immutability

Partially applied Function

def sum(a: Int, b: Int, c: Int) = a + b + cval f1 = sum _val f2 = sum(1, _: Int, 3)

def main(args: Array[String]) { println(f1(1, 2, 3)) // 6 println(f2(2)) // 6 println(f2(5)) // 9}

Curried Function

def sum(a: Int)(b: Int) = a + b

def main(args: Array[String]) { println(sum(1)(2)) // 3}

Curried Function

def withResource(r: Closeable)(op: => Unit) { try { op } finally { r.close() }} def main(args: Array[String]) { val writer = new FileWriter("test.txt") withResource(writer) { writer.write("foo bar") }}

a special form of an abstract classwhich can be used as mixins

to enrich interface

class MyInt(val n: Int) extends Ordered[MyInt] { def compare(that: MyInt) = this.n - that.n}

val n1 = new MyInt(1)val n2 = new MyInt(2)println(n1 < n2)println(n1 > n2)println(n1 <= n2)println(n1 >= n2)

stackable modifications

abstract class Sale { def price(): Double}

class BasicSale extends Sale { def price() = 100;}

trait OffTenPercent extends Sale { abstract override def price() = { super.price.toDouble * 0.9 }}

trait OffTwenty extends Sale { abstract override def price() = { super.price - 20 }}

stackable modifications

class UnbelievableSale extends BasicSale with OffTenPercentclass UnbelievableSale2 extends BasicSale with OffTwentyclass UnbelievableSale3 extends BasicSale with OffTenPercent with OffTwentyclass UnbelievableSale4 extends BasicSale with OffTwenty with OffTenPercent

def main(args: Array[String]) { println((new BasicSale).price) // 100.0 println((new UnbelievableSale).price) // 90.0 println((new UnbelievableSale2).price) // 80.0 println((new UnbelievableSale3).price) // 70.0 println((new UnbelievableSale4).price) // 72.0}

-10%-¥20-10%, -¥20 -¥20, -10%

Pattern Match

case class ScalaCase(name: String)

def test(x: Any) = x match { case 1 => println("1") case str: String => println(str) case ScalaCase(name) => println("name: " + name) case _ => println("other")}

def main(args: Array[String]) { test(1) // 1 test("Hello!") // Hello! test(ScalaCase("apple")) // name: apple test(2.5) // other}

Scala in Action

Collection Operation

getting the highest score in 2013

case class Student(year: Int, score: Int)

def students(): List[Student] = List(Student(2013, 92), Student(2012, 98), Student(2013, 70))def students2(): List[Student] = Student(2013, 92)::Student(2012, 98)::Student(2013, 70)::Nil

def highestScoreIn2013() = students() .filter(s => s.year == 2013) .map(s => s.score) .foldLeft(0)((a, b) => max(a, b))

actorA

actorB

actorC

actorD

No shared data

Exchanging messages

Concurrency model

Actor (Akka)

object Actors { implicit val system = ActorSystem("MySystem") val a, b = actor(new Act { become { case ("ping", actor: ActorRef) => { println("received ping") Thread.sleep(1000) actor ! ("pong", self) } case ("pong", actor: ActorRef) => { println("received pong") Thread.sleep(1000) actor ! ("ping", self) } } }) def main(args: Array[String]) { a ! ("ping", b) }}

received pingreceived pongreceived pingreceived pongreceived pingreceived pong...

Practical Scala

Killer framework Play (+)

Java on the bench (+)

Interactive shell (+)

There are many concepts (-)

the compiler needs more resources (-)

Backward compatibility? (-)

Next Step

Implicit conversions

Extractor

Abstract type

Type parameter & variance

Reference

An Overview of the Scala Programming Language Second Edition (pdf)

A Scala Tutorial for Java programmers (pdf)

Programming in Scala, Second Edition (Japanese version: Scala スケーラブルプログラミング第2版)

This work is licensed under the Creative Commons Attribution-NonCommercial 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc/3.0/.

photo#1: http://en.wikipedia.org/wiki/File:Mark_Odersky_photo_by_Linda_Poeng.jpg

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