Fast Forward To Scala

Fast Forward to Scala

A quick introduction to Scala
for Java programmers

Martin Kneißl http://kneissl.eu/Members/martin/blog

Computers get faster all the time

The Future
10000 100000
Clockspeed
"Total Speed"
Memory 10000
1000

1000

100

100

10 Martin’s PCs 10
Dark
Ages
1 1
1985 1990 1995 2000 2005 2010 2015


73728
*
32-bit
850 MHz
*
4 cores

1 000 000 000 000 000 FLOP/s

Photo: Jay Simmons

ow y
H n
a s
m re
co s a ial
oe ent ur
d u
q f yo se?
se t o u
ar ram
p g
p ro

“As soon as you start sharing memory,
you are hosed” – Bruce Eckel

Photo: Adam Klepsteen

Don’t share!
Use messaging instead.

Don’t modify!
Create new values instead.

Object Functional
Oriented Tr
ming
ansfor alues
ulated able v
E ncaps tate i mmut
le s
mutab

a add: x a := a + x

a b a b
x

x

Object Functional
Oriented Tr
ming
ansfor alues
ulated able v
E ncaps tate Immut
le s
mutab

“Deep” Concise
Statically
Typed
Compatible

Photo: adriaan de man

I like static types

Photo: adriaan de man

e to wait
D on’t hav e to
un t il runtim
!
f ind this

Avoid RSI

Photo: Ginny Austin

Use Scala!

Photo: Ginny Austin

public abstract class Animal { abstract class Animal {

public abstract String getName(); def name: String

@Override public String toString() override def toString(): String = {
{
// . . . Calc. class name
// . . . Calc. class name
shortClassName + " " + name
return shortClassName +
" " + getName();
}
}
}
}

Bla bla class
Animal bla bla
bla . . . Class
Animal.

public class Cat extends Animal { class Cat (
val name: String
private final String name;
) extends Animal

public Cat(String name) {
this.name = name;
}

@Override public String getName() {
return name;
}
}

Bla a Cat is an Animal
with a fixed name, when
I make one I have to
give it a name and bla A cat is an
bla I can ask for the animal with a
name and will answer fixed name.
with the given name.

Uniform Access Principle
abstract class Animal {
Subclas
ses can
def name: String // method
access refine
a
represe nd
}

ntation
class Cat ( !
val name: String // immutable
) extends Animal

class Dog ( class Dog (name: String) extends
Animal {
var name: String // mutable private var _name: String = name
) extends Animal
def name : String = _name
def name_= (name: String) {
_name = name
}
}

public interface Iterable<T> { trait Iterable[+A] {

Iterator<T> iterator(); def elements: Iterator[A]
def map[B](f: A => B): Iterable[B] =
}
...
def flatMap[B](f: A => Iterable[B]):
Iterable[B] =
...

aits
def filter(p: A => Boolean):
Iterable[A] =

Tr
...
def partition(p: A => Boolean):
(Iterable[A], Iterable[A]) = {
...
}
...
} Traits: abstract and
Interfaces: only concrete methods
abstract methods
Rich interfaces!
Sparse
interfaces!

More on Types

Gener
ics tha
t jus t
work!
(vs 513
pages
of F.A.Q
.)

Variance Declaratio
ns

s
Abstract Type

Photo: Carl Lender, New Haven, CT

import java.io.File; import java.io.File
import RichFile.apply
File cwd = new File("."); val cwd = new File(".")
for (File f: for (f <- cwd / "src" ) {
new File(cwd,"src").listFiles()) { println(file)
value / is not a member of
System.out.println(f); }
java.io.File
}

object RichFile {
implicit def apply(f: File):
RichFile
= new RichFile(f)
}

import java.io.File
package files
import RichFile.apply
import java.io.File val cwd = new File(".")
class RichFile( for (f <- cwd / "src" ) {
private val underlying: File
) extends Iterable[File] { println(file)
}
def elements: Iterator[File] = {
val files = underlying.listFiles()
if (files == null)
Iterator.empty
else
files.elements
}

def /(child: String): File =
new File(underlying, child)

}

object RichFile {
implicit def apply(file: File): RichFile =
new RichFile(file)
}

Functional Programming
• Immutability
• Higher Order Functions
• Closures
• Pattern matching
• Recursion

Higher Order Functions
val listBuffer = new ListBuffer[File] val listBuffer = new ListBuffer[File]
for (file <- currentDir) { for (file <- currentDir) {
if (file.isDirectory) { if (file.getName.startsWith(p)) {
listBuffer += file listBuffer += file
} }
} }
println(listBuffer) println(listBuffer)

val listBuffer = new ListBuffer[File] val listBuffer = new ListBuffer[File]
for (file <- currentDir) { for (file <- currentDir) {
if (file.canRead) { if (predicate(file)) {
listBuffer += file listBuffer += file
} }
} }
println(listBuffer) println(listBuffer)

package java.io;
public interface FileFilter {
boolean accept(File pathname);
Templa
}
te Patt
here.listFiles(new FileFilter() {
ern
@Override
public boolean accept(File f) {
return f.isDirectory();
}
});

Act ionListener
mplate
Hiberna
teTemp Transa ctionTe
late

// in class RichFile
def collect(predicate: File=>Boolean): List[File] = {
val listBuffer = new ListBuffer[File]
for (file <- this)
if (predicate(file))
function
listBuffer += file
type
listBuffer.toList
}

def fileIsDirectory(file: File): Boolean = file.isDirectory
println(currentDir.collect(fileIsDirectory))
function value
println(currentDir.collect((f: File) => f.isDirectory))
function literal
println(currentDir.collect(f => f.isDirectory))

println(currentDir.collect(_.isDirectory))

Altern atively
def listFiles (predicate: File=>Boolean): Array[File] =
underlying.listFiles(new FileFilter {
def accept(f: File) = predicate(f)
})

val currentDir = new File(".")
println(currentDir.listFiles((f: File) => f.isDirectory))

But…

for (file <- this)
if (predicate(file))
listBuffer += file
listBuffer.toList
}

println(currentDir.collect(fileIsDirectory))

println(currentDir.collect((f: File) => f.isDirectory))

println(currentDir.collect(f => f.isDirectory))

println(currentDir.collect(_.isDirectory))

“collect” is
for (file <- this)
if Iterable.filter
(predicate(file))
listBuffer += file
listBuffer.toList
}

println(currentDir.filter(fileIsDirectory))

println(currentDir.filter((f: File) => f.isDirectory))

println(currentDir.filter(f => f.isDirectory))

println(currentDir.filter(_.isDirectory))


println(currentDir.filter(_.isDirectory))

println(currentDir.filter(_.getName.startsWith(p))

println(currentDir.filter(_.canRead))

val numbers = 1 to 3
assert( numbers sameElements List(1, 2, 3) )

assert( numbers.map(_ * 2) sameElements List(2, 4, 6) )

assert( numbers.reduceLeft(_ + _) == 1 + 2
+ 3 )

assert( numbers.foldLeft("")(_ + _) == "" + 1
+ 2
+ 3 )

assert( ("" /: numbers)(_ + _) == "" + 1
+ 2
+ 3 )

Function Literals and Closures
// variable i is bound
(i: Int) => i * 2

// variable i is bound, factor is unbound
// “open” expression
(i: Int) => factor * i

// “open” expression is closed by binding factor
val factor = 2
val closure = (i: Int) => factor * i

// closure object remembers the factor
assert( closure(3) == 6 )

Closures aren’t new!
Lisp
Scheme Smalltalk

Python Ruby
Perl
PHP Haskell OCaml

JavaScript C#
Groovy

Functional
Source: Seite „Closure“. In: Wikipedia, Die freie Enzyklopädie. Bearbeitungsstand: 7. Juni 2009,

Other paradigm
09:48 UTC. URL: http://de.wikipedia.org/w/index.php?title=Closure&oldid=60866571 (Abgerufen:
11. Juni 2009, 22:49 UTC)

int x = 7; val x = 7;
switch (x) { x match {
case 1: case 1 =>
System.out.println("one"); println("one")
break;
case 2: case 2 =>
System.out.println("two"); println("two")
break;
default: case _ =>
System.out.println("many"); println("many")
break; }
}

int x = 7; val x = 7;
switch (x) { x match {
case 1: case 1 =>
System.out.println("one"); println("one")
break;
case 2: case 2 =>
System.out.println("two"); println("two")
break;
default: case _ =>
System.out.println("many"); println("many")
break; }
}

val x = 7;
val text = x match {
case 1 => "one"
case 2 => "two"
case _ => "many"
}
println(text)

var force = false
var help = false
val opt = "-help";
opt match {
case "-force" => force = true
case "-help" => help = true
case _ => throw new …
}

sealed abstract class Shape

case class Circle(radius: Double) extends Shape

case class Rectangle(x: Double, y: Double) extends
Shape

def area(shape: Shape): Double = shape match {
case Circle(r) => r * r * PI
case Rectangle(x,y) => x * y
}

println(area(Circle(3.0)))

Exception in thread "main"
java.lang.NullPointerException

Exception in thread "main"
java.lang.NullPointerException
def isEven(i: Int) = i % 2 == 0

val found = List(1, 2, 3).find(isEven)

// won’t compile
// val half = found / 2

found match { value / is not a
case None => member of Option[Int]
“I call it my billion- println("No even value")

dollar mistake. case Some(i) =>
println("First even falue: " + i)
}
It was the invention
of the null reference
in 1965. “
-- Tony Hoare

Some(null)

Photo: Anna Humphreys, UK

case object Fly

Messag
val tweetie = new Bird
e-Pass
tweetie.start() Concurr ing
ency:
tweetie ! Fly Erlang
Style
Actors
class Bird extends Actor {
def act {
loop {
receive {
case Fly => println("Flying")
case other => println("What? " + other)
}
}
}
}

Tools
• Compiler: scalac
• Interpreter (“REPL”): scala
• IDE: Eclipse, Netbeans, IDEA
• Build Systems: Ant, Maven, SBT

Eclipse plugin usable, but far from perfect!

Users
• (message routing middleware)

• (energy trading)
(Sponsor of Eclipse plugin)

• (community site)

• Sony Pictures (middleware)

Books
Programming in Scala – Available Now
by Martin Odersky, Lex Spoon, and Bill Venners

Beginning Scala – Available Now
by David Pollak.
Published by Apress and also available from Amazon.

The Definitive Guide to Lift – Available Now
Scala-based Web Framework
By Derek Chen-Becker, Tyler Weir, Marius Danciu

Programming Scala – Available August, 2009
("Rough cuts" available now)
By Alex Payne and Dean Wampler

Other Resources

• http://www.scala-lang.org/

• Mailinglisten, Blogs

• #scala on IRC

• #scala on twitter

Fast Forward To Scala

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