Scala coated JVM

Scala coated JVM
@ Joint meeting of Java User Group Scotland
and Scala Scotland

Stuart Roebuck
stuart.roebuck@proinnovate.com

The basics…

• Created by Martin Odersky (EPFL)
• JVM
• Object oriented and functional
• ‘scalable language’
• Scala 1.0—late 2003
• Scala 2.8.0—July 2010

“If I were to pick a language to
use today other than Java, it
would be Scala”
James Gosling

Commercial users of Scala
• Twitter—Scala back end Ruby front end
• LinkedIn
• Foursquare—Scala and Lift
• Siemens—Scala and Lift
• SAP
• EDF
• Sony Pictures (ImageWorks)
• Nature Magazine
• TomTom
• …and Google

Try this at home!

• Scala home: http://www.scala-lang.org/
• Downloadable for Mac, Linux & Windows
• Shell interpreter: scala
• Compilers: scalac and fsc
• Documentation generator scaladoc
• Plugins for Eclipse, Netbeans, IntelliJ
• Popular build tool “sbt” (simple-build-tool)

Email extraction shell script
#! /bin/sh
exec scala "$0" "$@"
!#

import scala.io.Source

val email = """[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+.[a-zA-Z]{2,4}""".r
val filtered = Source.stdin.getLines.
flatMap( email.findAllIn(_)).
map(_.toLowerCase).toSet.toList.sortWith(_<_)

filtered.foreach{ println(_) }

How does Scala diﬀer
from Java?

• Everything is an object • Pattern matching and
Extractors
• First-class functions
(‘closures’) • XML literals
• Singleton objects • Case classes
• Mixin composition • Lazy evaluation
with Traits • Tuples

Everything is an object
“Answer = ” + 6 * 4

“Answer = ”.+((6).*(4))

First class functions
def time(f: => Unit): Double = {
val start = System.nanoTime
f
val end = System.nanoTime
(end - start) / 1000000.0
}

val timeTaken = time {
Thread.sleep(100)
}

Singleton Objects (Java)
public class Singleton {

private Singleton() {
}

private static class SingletonHolder {
public static final Singleton INSTANCE = new Singleton();
}

public static Singleton getInstance() {
return SingletonHolder.INSTANCE;
}

}

Singleton Objects (Scala)
object Singleton {
val name = “This is a Singleton Object”
}

Traits / Mix-in Composition
class Executor(f: () => Unit) {
def exec() { f() }
}
trait Logging extends Executor {
override def exec() {
println("Executing...")
super.exec()
}
}
trait Timing extends Executor {
override def exec() {
val start = System.currentTimeMillis
super.exec()
val end = System.currentTimeMillis
printf("==> Time taken: %d ms%n", end-start)
}
}

val e = new Executor(() => println("Hello")) with Timing with Logging
e.exec
Executing...
Hello
==> Time taken: 0 ms

Pattern Matching
def intToString(value: Any) = value match {
case x:Int => x.toString
case (x:Int) :: y => x.toString
case Some(x:Int) => x.toString
case _ => ""
}

scala> intToString(23)
res1: java.lang.String = 23

scala> intToString(List(23,45))

scala> intToString(Some(11))

scala> intToString(Some("String"))
res4: java.lang.String =

Demo 2
e Scala REPL(Read Eval Print Loop)

BigInteger / BigInt Factorial
import java.math.BigInteger

def factorial(x: BigInteger): BigInteger =
if (x == BigInteger.ZERO)
BigInteger.ONE
else
x.multiply(factorial(x.subtract(BigInteger.ONE)))

def factorial(x: BigInt): BigInt =
if (x == 0) 1 else x * factorial(x - 1)

BigInt Deﬁnition
class BigInt(val bigInteger: BigInteger) extends java.lang.Number {

override def hashCode(): Int = this.bigInteger.hashCode()

override def equals (that: Any): Boolean = that match {
case that: BigInt => this equals that
case that: java.lang.Double => this.bigInteger.doubleValue == that.doubleValue
case that: java.lang.Float => this.bigInteger.floatValue == that.floatValue
case that: java.lang.Number => this equals BigInt(that.longValue)
case that: java.lang.Character => this equals BigInt(that.charValue.asInstanceOf[Int])
case _ => false
}

def equals (that: BigInt): Boolean = this.bigInteger.compareTo(that.bigInteger) == 0
def compare (that: BigInt): Int = this.bigInteger.compareTo(that.bigInteger)
def <= (that: BigInt): Boolean = this.bigInteger.compareTo(that.bigInteger) <= 0
def >= (that: BigInt): Boolean = this.bigInteger.compareTo(that.bigInteger) >= 0
def < (that: BigInt): Boolean = this.bigInteger.compareTo(that.bigInteger) < 0
def > (that: BigInt): Boolean = this.bigInteger.compareTo(that.bigInteger) > 0
def + (that: BigInt): BigInt = new BigInt(this.bigInteger.add(that.bigInteger))
…

Implicit conversion
scala> factorial(10)
res0: BigInt = 3628800

implicit def int2bigInt(i: Int): BigInt = BigInt(i)

def factorial(x: BigInt): BigInt = …

scala> factorial(int2bigInt(10))
res0: BigInt = 3628800

Pattern matching
scala> val Email = """([a-zA-Z0-9._%+-]+)@([a-zA-Z0-9.-]+.[a-zA-Z]
{2,4})""".r
Email: scala.util.matching.Regex = ([a-zA-Z0-9._%+-]+)@([a-zA-
Z0-9.-]+.[a-zA-Z]{2,4})

scala> val Email(name,address) = "stuart.roebuck@proinnovate.com"
name: String = stuart.roebuck
address: String = proinnovate.com

Demo 3
Building a Scala project with sbt

Build tools +
• Maven Plugin (no endorsement implied)—http://
scala-tools.org/mvnsites/maven-scala-plugin/
• simple-build-tool—http://code.google.com/p/
simple-build-tool/
• Apache Ant tasks for Scala—http://www.scala-
lang.org/node/98
• Apache Buildr—http://buildr.apache.org/
• JavaRebel—http://www.zeroturnaround.com/
jrebel/

Scala coated JVM

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