Scala - en bedre Java?

- en bedre Java?

Jesper Kamstrup Linnet
jesper@linnet-data.dk

Community Day 2010
27. maj 2010

Om mig

•Freelancekonsulent
• Java- og .NET-udvikler/arkitekt
• Sprogbegejstret

Agenda

•Hvad er Scala?
• En bedre Java?
• Ready for prime time?

Er Java dødt som sprog?

Kilde: InfoQ.com

Kort om Scalas historie

• Startet i 2001
• Skabt af Martin Odersky

Kendetegn

•Statisk typet
• Hybridsprog
• Skalabilitet i højsædet

Java-kompatibelt

• Eksisterende Java API’er
• Afvikles på JVM’en
• Genererer .class-ﬁler

Hybridsprog

=
Objektorienteret
+
Funktionsorienteret

Objektorienteret

1.to(5) Range(1, 2, 3, 4, 5)

Funktionsorienteret

val a = 10

val f = (x: Int) => x + 5

Immutability

• Centralt i funktionsprogrammering
• Vigtigt for parallelisering
• Letter kodelæsning

Hvad gør Scala mere
effektivt?

Syntaktisk sukker

Image: Suat Eman / FreeDigitalPhotos.net

Hello, world

println("Hello, world")

Kompakt syntaks (1)
Java Scala
public class Person {
private final String name;
class Person(
private final String address; val name: String,
public Person(String name,
val address: String);
String address) {
this.name = name;
this.address = address;
}

public String getName() {
return name;
}

public String getAddress() {
return address;
}

Kompakt syntaks (2)
Java Scala
case class Person(
public class Person {
private final String name;
private final String address;

public Person(String name, String address) {
this.name = name;
name: String,
address: String);
this.address = address;
}

public String getName() {
return name;
}

public String getAddress() {
return address;
}

@Override
public int hashCode() {
...
}

@Override
public boolean equals(Object obj) {
...
}

@Override
public String toString() {
...
}

Kompakt syntaks (3)
class Car {
var driven = 0.0

def drive(distance: Double) = driven += distance

def milesDriven = driven * 1.6
}

...

val car = new Car
car drive 10

copy method ala 2 .8
Sc

val person = Person("Jesper", "Kbh")

val newPerson = person.copy(address = "Aarhus")

Person(Jesper,Aarhus)

Collections
val list = List(1,2,3)
val map = Map(
"Jesper" -> 39,
"Peter" -> 55
)
val set = Set(1, 2, 5, 2)
val array = Array(1, 2, 3)

Hvad foretrækker du?
Java Scala
List<Integer> numbers = ... val numbers = ...

List<Integer> result = val result =
new ArrayList<Integer>(); numbers filter isEven
for (Integer number : numbers) {
if (isEven(number) {
result.add(number);
}
}

Anonyme funktioner
val list = List(1,2,3,4)

list filter isEven List(2, 4)

list filter { n => n % 2 == 0 }

list filter { _ % 2 == 0 }

Eksempler
val list = List(1,2,3,4)

list map (x => x * x) List(1, 4, 9, 16)
list sum 10
list mkString "," 1,2,3,4
list forall { _ < 5 } true
list partition isEven (List(2, 4),
List(1, 3))

Pattern matching (1)

value match {
case 1 => println("Tal")
case i: Int => printf("Tallet %d", i)
case "test" => println("Streng")
case (x, y) => printf("Et par, x=%s, y=%s", x, y)
case _ => println("Alt andet")
}

Lister
value match {
case List(_, _) => println("To elementer")
case List(1, rest @ _*) => println("1 og flere")
}

Regulære udtryk
val Danish = "Hej (.*)".r
val English = "Hi, (.*)".r

greeting match {
case Danish(name) => printf("Dansk: %s", name)
case English(name) => printf("Engelsk: %s", name)
}

Case classes

value match {
case Person(_, "Kbh") => println("Københavner")
case _ => println("Uden for København")
}

XML (1)

val personsXml =
<persons>
<person name="Jesper"><age>38</age></person>
<person name="Ulla"><age>{age}</age></person>
</persons>

XML (2)

val persons = personsXml "person"

val name = person "@name"

val names = personsXml "@name"

XML (3)

node match {
case <name>{name}</name> => println(name)
case _ => println("Andet")
}

Duck typing

“If it walks like a duck, and quacks like a
duck, then it is a duck”

Duck typing

public class Text extends ... {
public void setText (String string) {

public class Button extends ... {
public void setText (String string) {

Duck typing m. Scala

def update(control: { def setText(text: String) }) = {
control.setText("Hello, world")
}

type ControlWithText = { def setText(text: String) }

def update(control: ControlWithText) = {
control.setText("Hello, world")
}

Traits (1)
Som interface
trait Editable {
def isEditable(): Boolean
}

class EditablePerson extends Editable {
def isEditable() = true
}

Traits (2)
Deﬁnition af mixin

trait Persistable {
val entityManager: EntityManager = ...

def save = {
entityManager.persist(this)
}
}

Traits (3)
Statisk brug af mixin

class Car extends Vehicle with Persistable {
...
}

val car = new Car
car.save

Traits (4)
Dynamisk brug af mixin

class Bicycle extends Vehicle {
...
}

val bicycle = new Bicycle with Persistable
bicycle.save

Traits (5)
Overstyring
trait LoggingCollection extends
java.util.Collection[String] {

abstract override def add(e: String) = {
printf("Adding: %s", e)
super.add(e)
}
}

val coll = new java.util.ArrayList[String]
with LoggingCollection

Traits (6)
Eksempel: Observer
trait Subject {
type Observer = { def receiveUpdate(subject:Any) }

private var observers = List[Observer]()

def addObserver(observer:Observer) =
observers ::= observer

def notifyObservers =
observers foreach (_.receiveUpdate(this))
}

Parallelisering

•Højere abstraktionsniveau
• Aktørmodel
• Beskedudveksling
• Share-nothing

Simpel aktør

import scala.actors.Actor._

actor {
calculateStuff
}

// stuff in main thread

Beskedudveksling

val parrot = actor {
while (true) {
receive {
case msg =>
println("Msg: " + msg)
}
}
}

parrot ! "Hello, Polly"

Scala i den virkelige
verden

Masser af information

• http://scala-lang.org
• Bøger

• Tutorials og artikler

Hvorfor ikke Scala?

•Ny syntaks
• “Ungt” sprog
• Værktøjsunderstøttelse

Hvorfor Scala?

•Kompatibilitet med Java
• Stærkere syntaks
• I fremgang

Hvorfor Scala?
“You only fully comprehend
the awesomeness of #scala
when after weeks of being
pure scala you have to edit
some Java again...”
James Strachan (@jstrachan)

En bedre Java?
Java Scala

+ ?
J av a+

Konklusion

En bedre og mere
effektiv Java?

Kontakt

• Slides: http://bit.ly/scala-cd10

• jesper@linnet-data.dk
• http://twitter.com/jesper_linnet
• http://blog.kamstrup-linnet.dk

Scala - en bedre Java?

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