An Introduction to Scala

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An Introduction to ScalaDallas, TXMarch 07, 2012

An Introduction to Scala

2March 7, 2012

Agenda …

The four W’s plus an H

• What: Scala Intro

• When: Scala History

• Who: Scala Implementations

• Why: A Case for Scala

• How: The Basics of Scala

– Getting Up and Running

– Classes and Objects

– Functional Programming

– Traits and Mixins

– Java in Scala

– Scala in Java


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Agenda …

Goals

• Have an understanding of what Scala is

• Have an interest in learning more

• Go install Scala!


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What: Scala Intro …

What is Scala?

Scala is a general purpose programming language designed to express common programming patterns in a concise, elegant and type-safe way. It smoothly integrates features of object-oriented and functional languages, enabling Java and other programmers to be more productive. Code sizes are typically reduced by a factor of two to three when compared to an equivalent Java application.

• Origin of name is two-fold

– Scala is Italian for staircase. Scala is considered a “step up” from other languages

– SCAlable Language

• Seamless integration with Java. Runs in the JVM

• Proven performer in high transaction, highly scalable environments

• Developers can follow an imperative or functional style

• ThoughtWorks has put Scala in the “trial” radar category (July 2011)


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When: Scala History …

Scala History … 1995 to 2012

Brief history of Scala• Developed by Martin Odersky

– 1995 he learned of Java and wrote functional language that compiled to Java bytecode - Pizza

– Pizza evolved into what we now recognize as Java generics

• Sun approached Odersky in 1997 to write Java 1.1 compiler

• Odersky led javac development from Java 1.1 through 1.4

• In 1999, Odersky joined EPFL to conduct research into improving functional and OO languages

• Design of Scala began in 2001 and first release was in 2003

• Early releases of compiler written in Java

• Version 2.0.0 introduced a completely rewritten compiler in Scala

• Current version 2.9.1 released in August 2011


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Who: Scala Implementations …

Scala is finding a home in high transaction environments

• Twitter Kestrel – Twitter’s message queue server

– Ported from RoR

• FourSquare – Message queue, website, mobile website and RESTful API

• LinkedIn – Public and backend RESTful API’s

– Ported from RoR

• Novell – Pulse, a cloud-based, real-time collaboration platform for the enterprise


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Why: A Case for Scala …

Is Scala a fit?

Upside of Scala• Ease of integration with existing Java environments

• When a functional style is utilized, scales easily

• Performance is equivalent, and in some cases, better than Java

• Good candidate for medium risk greenfield projects

• Learning curve is not massive, as Scala follows Java syntax

Downside of Scala• Toolset is immature

– IDE integration is weak, but improving

– Java profiling tools can be used, but difficult

• Overcoming Java footprint will be difficult. It’s comfortable and works. It’s the COBOL/C+ of our generation

• User acceptance still low due to lack of knowledge and reasons listed above


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How: The Basics of Scala ...

What you need to know to get started!

• Getting Up and Running

• Building Blocks

• Classes and Objects

• Functional Programming

• Traits and Mixins

• Java in Scala

• Scala in Java


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How: The Basics of Scala …

Getting up and running

Required• Java 1.5 or greater

• Scala 2.9.1 distribution

Optional• SBT – Simple Build Tool

• IDE Plugin

– ScalaIDE (Eclipse – must use Helios)

– Scala Plugin for IntelliJ IDEA

– Scala Plugin for NetBeans

Notes:

1) Installation of Scala and SBT involve expanding compressed file and adding to PATH

2) IDE installation varies by tool; some dependency on IDE release number

3) ScalaIDE officially supported by Typesafe


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Building Blocks

Declarations - var• A var is similar to a non-final variable in Java

• If type is not declared, it will be inferred from the assigned object

• Once initialized, it can be reassigned (mutable), but type cannot change

• Object can be assigned at any time

class VarA(n: Int) { var value = n} class VarB(n: Int) { val value = new VarA(n)} object ExampleVar { var x = new VarB(5) x = new VarB(6) // x.value = new VarA(7) x.value.value = 7}

Example


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Building Blocks

Declarations - val• A val is similar to a final variable in Java

• Once initialized, it can not be reassigned (immutable)

• Object must be initialized at declaration

• Object cannot be reassigned, but internal state can be modified

class ValA(n: Int) { var value = n} class ValB(n: Int) { val value = new ValA(n)} object ExampleVar { val x = new ValB(5) x.value.value = 6}

Example


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Building Blocks

Declarations - def• A def is used to define a function

• A comma-separated list of parameters in parentheses follows the name

• The type of the return value must be defined following the parameters

• If a return type is not included, you are defining a “procedure”

• The equals sign hints to the fact a function defines an expression resulting in a value

• The final value of the control block is the return value

class ExampleDef(n: Int, a: Int, b: Int) { val value = n + max(a,b) def max(x: Int, y: Int): Int = { if (x > y) x else y }}

Example


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How: The Basics of Scala … Classes and Objects …

Classes

Class• A class in Scala looks similar to a class in Java – minus the boilerplate code

• Public by default

• Getters and setters defined by variable declaration

class Coordinate() { var degrees = 0.0 var minutes = 0.0 var seconds = 0.0 def asDegrees = degrees + minutes/60.0 + seconds/60.0/60.0 def asRadians = asDegrees.toRadians} object Example extends App { val latitude = new Coordinate latitude.degrees = 32.0 latitude.minutes = 57.0 latitude.seconds = 30.762}

Example


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Classes

Class – Primary Constructor• More concise

• val has getter only

• Primary constructor creates the fields

class Coordinate(val degrees: Double, val minutes: Double, val seconds: Double) { def asDegrees = degrees + minutes/60.0 + seconds/60.0/60.0 def asRadians = asDegrees.toRadians} object Example extends App { val latitude = new Coordinate(32.0, 57.0, 30.762) val longitude = new Coordinate(96.0, 49.0, 25.1076) println(latitude.degrees) // prints 32.0}

Example


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Classes

Class – Auxiliary Constructor• Auxiliary constructor is created as a def this

• Must start with a call to a previously defined auxiliary or primary constructor

class Coordinate(val degrees: Double, val minutes: Double, val seconds: Double) { def this(degrees: Double, minutes: Double) = this(degrees, minutes, 0.0) def this(degrees: Double) = this(degrees, 0.0) def this() = this(0.0) def asDegrees = degrees + minutes/60.0 + seconds/60.0/60.0 def asRadians = asDegrees.toRadians} object Example extends App { val latitude = new Coordinate(32.0) val longitude = new Coordinate(96.0, 49.0) println(latitude.minutes) // prints 0.0 println(longitude.minutes) // prints 49.0}

Example


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Objects

Object• Creates a singleton of a class

• Can be used as a home for miscellaneous functions

• No constructor parameters

object CrederaLatitude extends Coordinate { this.degrees = 32.0 this.minutes = 57.0 this.seconds = 30.762 def printCoordinate = println(this.asDegrees)}class Coordinate(val degrees: Double, val minutes: Double, val seconds: Double) { def this(degrees: Double, minutes: Double) = this(degrees, minutes, 0.0) def this(degrees: Double) = this(degrees, 0.0) def this() = this(0.0)

def asDegrees = degrees + minutes/60.0 + seconds/60.0/60.0 def asRadians = asDegrees.toRadians} object Example extends App { CrederaLatitude.printCoordinate // prints 32.958545}

Example


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How: The Basics of Scala … Functional Programming …

Functional programming… defined

Definition• Treats computations as the evaluation of mathematical functions

• Avoids state and mutable data

• Calling a function twice, with the same arguments, should produce the same results both times

• First-class functions are functions that either take other functions as arguments or return them as results

• In Scala, a function literal (the definition) is compiled to a functional value (a class)

• Functional values can be assigned to a val or var. If assigned to a var, it is mutable.

var increase = (x: Int) => x + 1val result0 = increase(10) // results in 11

Example


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Functional programming… defined

Definition• Treats computations as the evaluation of mathematical functions

• Avoids state and mutable data

• Calling a function twice, with the same arguments, should produce the same results both times

• First-class functions are functions that either take other functions as arguments or return them as results

• In Scala, a function literal (the definition) is compiled to a functional value (a class)

• Functional values can be assigned to a val or var. If assigned to a var, it is mutable.

var increase = (x: Int) => x + 1val result0 = increase(10) // results in 11 increase = (x: Int) => x + 10val result1 = increase(10) //results in 20

Example


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Passing functional values

Passing as a value• Functional values can be passed as a value into another function

• Common use is to make calls on the immutable collection List

val someNumbers = List(-22,-19,-12,-9,-2,0,1,5,9,32)val printList = (x: Int) => println(x)println("print all numbers: ")someNumbers.foreach(printList) println("-----------------------------------")

Example


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Passing functional values

Passing as a value• Functional values can be passed as a value into another function

• Common use is to make calls on the immutable collection List

val someNumbers = List(-22,-19,-12,-9,-2,0,1,5,9,32)val printList = (x: Int) => println(x)println("print all numbers: ")someNumbers.foreach(printList) println("-----------------------------------")println("print only positive numbers:") val filterList = (x: Int) => x > 0someNumbers.filter(filterList).foreach(printList)

Example


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How: The Basics of Scala … Traits and Mixins …

Traits and Mixins

Trait• Scala developers saw inherent problems with Java interfaces

• Scala solution is a combination of Java interfaces and Ruby mixins called traits

• Like an object, traits do not have constructors

• Added to class via the extends keyword

• Additional traits can be “mixed in” via the with keyword


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How: The Basics of Scala … Java in Scala …

Integration

Java in Scala• Add jar to your CLASSPATH

• Import package or class

• Implement an object using the class

import org.joda.time._ trait CurrentTime { val now = new DateTime()}


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How: The Basics of Scala … Java in Scala … Scala in Java …

Integration

Java in Scala• Add jar to your CLASSPATH



Scala in Java• Compile to a jar

• Add jar to your CLASSPATH



import com.credera.tech.scala.*; public class Hello { public static void main(String[] args) { System.out.println("Hello, this is java."); CrederaOffice.printOfficeLocation(); }}

import org.joda.time._ trait CurrentTime { val now = new DateTime()}


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Web Resources

March 7, 2012

Appendix … Web References ...

Resources

Description Link

Scala http://www.scala-lang.org

SBT – Simple Build Tool http://github.com/harrah/xsbt

ScalaIDE for Eclipse http://scala-ide.org

Scala Plugin for IntelliJ IDEA http://confluence.jetbrains.net/display/SCA/Scala_Plugin+for+IntelliJ+IDEA

Scala Plugin for NetBeans http://wiki.netbeans.org/Scala

ThoughtWorks Technology Radar http://thoughtworks.com/radar

Indeed.com Scala Job Trends http://indeed.com/jobtrends?q=scala&l=&relative=1

Twitter’s Scala Experience http://web2expo.com/webexsf2009/public/schedule/detail/6110

LinkedIn’s Scala Experience http://infoq.com/articles/linkedin-scala-jruby/voldement

Java Interfaces Discussion http://www.codecommit.com/blog/scala/scala-for-java-refugees-part-5


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An Introduction to Scala

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