scala for java programmers

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

Hello, Scala!

Copyright © 2013 Akira Koyasu. Some rights reserved.

Hello, World!

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Hello, World!

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

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Agenda

About Scala

Features

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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

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Motivation

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

Several concepts not in Java bring more posibilities to you.

For Java programmers

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the Programming Language

Language Specification

API

Runtime

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API

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Scaladoc


Runtime

scalac

.scala

.class

the compiler for .NET is out of date

JVMscala

Running on JVM

Compiler generates class files

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Influencers

Java C#

Smalltalk

Haskell

Algol Simula

Eiffel

SML F#

Erlang

IswimLisp

Python

Ruby

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Inventor

Martin Odersky

Professor of programming methods at EPFL in Switzerland

Designer of Java generics

photo#1

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Object, Operator

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

?

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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

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Features

Static typing & Suitable type inference

Object-oriented & Functional

Trait

Collaborating with Java

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Vorbose JavaType declaration

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

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Vorbose JavaType declaration

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

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Vorbose JavaJavaBeans

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

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Vorbose JavaJavaBeans

case class ScalaCase(name: String)

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Functional Programming

variables

method parameters

method returns

Functions are the first-class values

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Consider a method signature sending mails to appropriate addresses from listing

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

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age==20

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age==20 def send(age: Int)

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age==20

35<=age, age<=60

def send(age: Int)

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age==20

35<=age, age<=60

def send(age: Int)

def send(minAge: Int, maxAge: Int)

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age==20

35<=age, age<=60

age<30, gender:Male

def send(age: Int)


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age==20

35<=age, age<=60

age<30, gender:Male

def send(age: Int)


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

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age==20

35<=age, age<=60

age<30, gender:Male

gender:Female, in Tokyo

def send(age: Int)



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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)

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

Apply condition function to method

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y = f(x)Avoiding side effects

Conscious of immutability

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Partially applied Function

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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

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

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


Curried Function

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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") }}


Trait

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

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Trait

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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)


Trait

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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 }}

-10%

-¥20

¥100


Trait

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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

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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

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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))


Actor

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actorA

actorB

actorC

actorD

No shared data

Exchanging messages

Concurrency model


Actor (Akka)

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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

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Practical Scala

Killer framework Play (+)

Java on the bench (+)

Interactive shell (+)

There are many concepts (-)

the compiler needs more resources (-)

Backward compatibility? (-)

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Next Step

Implicit conversions

Extractor

Abstract type

Type parameter & variance

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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)

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Notes

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/.

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

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scala for java programmers

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