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

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

Computers get faster all the time
73728
*
32-bit
850 MHz
*
4 cores
1 000 000 000 000 000 FLOP/s

Photo by Kamil Dratwa

Photo by John Trif, Melbourne

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

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)

Higher Order Functions
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

Higher Order Functions
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

Higher Order Functions
// 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
// in class RichFile
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…

Higher Order Functions
// in class RichFile
def collect(predicate: File=>Boolean): List[File] = {
val listBuffer = new ListBuffer[File]
for (file <- this)
if (predicate(file))
listBuffer += file
listBuffer.toList
}
def fileIsDirectory(file: File): Boolean = file.isDirectory
println(currentDir.collect(fileIsDirectory))
println(currentDir.collect((f: File) => f.isDirectory))
println(currentDir.collect(f => f.isDirectory))
println(currentDir.collect(_.isDirectory))

Higher Order Functions
// in class RichFile
def collect(predicate: File=>Boolean): List[File] = {
val listBuffer = new ListBuffer[File]
“collect” is
for (file <- this)
if Iterable.filter
(predicate(file))
listBuffer += file
listBuffer.toList
}
def fileIsDirectory(file: File): Boolean = file.isDirectory
println(currentDir.filter(fileIsDirectory))
println(currentDir.filter((f: File) => f.isDirectory))
println(currentDir.filter(f => f.isDirectory))
println(currentDir.filter(_.isDirectory))

Higher Order Functions
println(currentDir.filter(_.isDirectory))
println(currentDir.filter(_.getName.startsWith(p))
println(currentDir.filter(_.canRead))

Higher Order Functions
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)

Pattern Matching

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

“As soon as you start sharing memory,
you are hosed” – Bruce Eckel
Photo: Adam Klepsteen

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

Avoid RSI
Photo: Ginny Austin

Use Scala!
Photo: Ginny Austin