Workshop Scala

Scala workshop
Created by
Fredrik Vraalsen (fredrik@vraalsen.no)
and
Alf Kristian Støyle (alf.kristian@gmail.com)

Adapted
Bert Van Vreckem (bert.vanvreckem@hogent.be)

If I were to pick a language to use
today other than Java, it would be
Scala.

James Gosling

Scala, it must be stated, is the current heir
apparent to the Java throne. No other language
on the JVM seems as capable of being a
"replacement for Java" as Scala, and the
momentum behind Scala is now unquestionable.

Charlies Olivier Nutter - JRuby lead

Though my tip though for the long term
replacement of javac is Scala. I'm very
impressed with it! I can honestly say if
someone had shown me the Programming
in Scala book by by Martin Odersky, Lex
Spoon & Bill Venners back in 2003 I'd
probably have never created Groovy.

James Strachen

public class Person {
private int age;
private String name;

public Person(int age, String name) {
this.age = age;
this.name = name;
}

public int getAge() {
return this.age;
}

public void setAge(int age) {
this.age = age;
}

public String getName() {
return this.name;
}

public void setName(String name) {
this.name = name;
}
}

class Person(var age: Int,
var name: String)

List<Person> persons = ...
List<Person> adults = new LinkedList<Person>();
List<Person> kids = new LinkedList<Person>();
for (Person person : persons) {
if (person.getAge() < 18) {
kids.add(person);
} else {
adults.add(person);
}
}

val (kids, adults) =
persons.partition(_.age < 18)

String s = "!em esreveR";
System.out.println(s.reverse());

val s = "!em esreveR"
println(s.reverse)

=> Reverse me!

BufferedReader reader = null;
try {
reader = new BufferedReader(new FileReader("f.txt"));
System.out.println(reader.readLine());
} finally {
if (reader != null) {
try {
reader.close();
} catch (IOException e) {
// Exception on close, ignore
}
}
}

using(new BufferedReader(new FileReader("f.txt"))) {
reader => println(reader.readLine())
}

I will never forget these words: "With great power
comes great responsibility." This is my gift, my
curse. Who am I? I'm Spider-man.

val myList = List(1, 2, 3)
val res = (10 /: myList)(_+_)

=> ??

Scala
● Object oriented and functional
● Statically typed
● Java compatible
● Complies to Java bytecode (and CLR)
● Existing libraries/frameworks
● Better Java

Todays schedule
● Basic syntax
● REPL, IDEs and setup
● First class functions
● Pattern matching
● OO and traits
● Functional programming
● Higher order functions

Type definitions
Scala Java

s: String String s
i: Int int i / Integer i

Variables
Scala: Java:

val s = "Hello World" public final String s =
"Hello World";

var i = 1
public int i = 1;

private var j = 3
private int j = 3;

Methods (3)
Scala: Java:

override def toString = ... @Override
public String toString()
{...}

Classes and constructors
Scala: Java:

class Person(val name: public class Person {
String) private final String
name;
public Person(String
name) {
this.name = name;
}
public String getName() {
return name;
}
}

Traits (= Interface + Mixin)
Scala: Java:

trait Shape { interface Shape {
def area: Double public double area();
} }

class Circle extends Object public class Circle extends
with Shape Object implements Shape

No “static” in Scala
Scala: Java:

object PersonUtil { public class PersonUtil {
val ageLimit = 18 public static final int
AGE_LIMIT = 18;

def countPersons( public static int
persons: List[Person]) = countPersons(
... List<Person> persons)
} {
...
}
}

While-loops
Scala: Java:

while (true) { while (true) {
... ...
} }

Exceptions
Scala: Java:

throw new throw new
Exception(“...”) Exception(“...”)

try { try {
} catch { } catch (IOException e) {
case e: IOException ...
=> ... } finally {
} finally { }
}

Varargs
def foo(values: String*){ } public void foo(String... values){ }

foo("bar", "baz") foo("bar", "baz");

val arr = Array("bar", "baz") String[] arr =
new String[]{"bar", "baz"}
foo(arr: _*) foo(arr);

(Almost) everything is an expression
val res = if (foo) x else y

val res = for (i <- 1 to 10) yield i
// List(1, ..., 10)

val res = try { x } catch { ...; y }
finally { } // x or y

Generics
Scala: Java:

List[String] List<String>

Tuples
Scala: Java:

val tuple: Tuple2[Int, Pair<Integer, String> tuple
String] = = new Pair<Integer,
String>(1, “apple”)
(1, “apple”)

val quadruple =
... yeah right... ;-)
(2, “orange”, 0.5d,
false)

Packages
Scala: Java:

package mypackage package mypackage;
... ...

Imports
Scala: Java:

import java.util.{List, import java.util.List
ArrayList}
import java.util.ArrayList

import java.io._
import java.io.*
import scala.util.Sorting._ ???

import java.sql.{Date => ???
SDate}

REPL: Read-Eval-Print Loop
● Command line shell for on-the-fly execution of
Scala statements
●
$ cd ${SCALA_HOME}/bin
$ scala
● Windows, e.g. C:Program FilesScala 2.8.0
● Linux, e.g. /opt/scala or /usr/local/scala

IDE
● They are all !#$&§? compared to what you are
used to with Java support
● Netbeans (very good) but bad in other areas...
● IntelliJ IDEA (pretty good) but slow compilation,
bonus community edition is free (with Scala)
● Eclipse (not all that good) but very fast when
working

Netbeans 6.9.x installation
● Download plugins
http://sf.net/projects/erlybird/files/nb-scala/6.9v1.1.0/
● In NetBeans "Tools" | "Plugins",
● click on "Downloaded" tab title,
● click on "Add Plugins..." button,
● choose the directory where the Scala plugins are unzipped,
● select all listed *.nbm files, following the instructions.
● Set ${SCALA_HOME} or %SCALA_HOME% environment variable
● Edit ${NETBEANS_HOME}/etc/netbeans.conf
● Add "-J-Dscala.home=/opt/scala" to
netbeans_default_options
● More info: http://wiki.netbeans.org/Scala

Tasks (20 min)
● Run REPL
● Windows: %scala_home%/bin/scala
● Linux: scala or /opt/scala/bin/scala
● Execute a few statements
● Copy & unpack
'HeliumPublicCursussenNavorming
ScalaScalaTraining.zip'
● into 'DocumentenNetbeansProjects' (Windows)
● Into '/home/student/NetbeansProjects' (Linux)

Tasks (20 min)
● Run unit test in IDE (JUnit 4)
● Open the 'ScalaTraining' project in NetBeans
● Try to compile and test the project (should work)
● scalaexamples.intro.MyFirstTest
– Create a failing test
– Make it run
● Make the two classes scalaexamples/intro/HelloWorld.scala
print “Hello world”. What is the difference?
● Remove comments from @Test for the methods in
scalaexamples/intro/CreateStuffTest.scala. Make tests pass.

First class functions

val even = Function[Int, Boolean] {
def apply(i: Int) = i % 2 == 0
}

val even: (Int => Boolean) = (i: Int) => i % 2 == 0
val even = (i: Int) => i % 2 == 0

even.apply(42) // true
even(13) // false

First class functions
val numbers = List(1, 2, 3, 4, 5)

numbers.filter(even) // List(2, 4)

numbers.filter((i: Int) => i > 2) // List(3, 4, 5)
numbers.filter(i => i > 2) // List(3, 4, 5)
numbers.filter(_ > 2) // List(3, 4, 5)

Collections

numbers.filter(i => i > 2) // List(3, 4, 5)
numbers.find(i => i > 2) // Some(3)
numbers.exists(i => i > 2) // true
numbers.forall(i => i > 2) // false

numbers.map(i => i * 2) // List(2, 4, 6, 8, 10)

numbers.foldLeft(0) { (a, b) => a + b } // 15

Closures

val people = List(Person(“Alf”), Person(“Fredrik”))

var name = “Fredrik”
val nameFilter = (p: Person) => p.name == name

people.filter(nameFilter) // Person(“Fredrik”)
name = “Alf”
people.filter(nameFilter) // Person(“Alf”)

Tasks (30 min)
● Open the 'first-class-functions' project
● Tests in package
scalaexamples.firstclassfunctions
● Add @Test to one and one method
● Follow instructions in the code
● Make the tests pass

myObject match {
case 1 => println("First was hit")
case 2 => println("Second was Hit")
case _ => println("Unknown")
}

myObject match {
case i: Int => println("Found an int")
case s: String => println("Found a String")
case _ => println("Unknown")
}

myObject match {
case i: Int => println("Found an int")
case other => println("Unknown " + other)
}

myObject match {
case i: Int if i == 1 => println("Found an int")
case other => println("Unknown " + other)
}

val res = myObject match {
case i: Int if i == 1 => "Found an int"
case s: String => "Found a String"
case other => "Unknown " + other
}

val res = myObject match {
case (first, second) => second
case (first, second, third) => third
}

val mathedElement = list match {
case List(firstElement, lastElement) => lastElement
case List(firstElement, _ *) => firstElement
case _ => "failed"
}

def length(list: List[_]): Int =
list match {
case Nil => 0
case head :: tail => 1 + length(tail)
}

public static Integer getSecondOr0(List<Integer> list) {
if (list != null && list.size() >= 2) {
return list.get(1);
} else {
return 0;
}
}

def second_or_0(list:List[Int]) = list match {
case List(_, x, _*) => x
case _ => 0
}

Case classes
● Class types that can be used in pattern
matching
● Generated into your class:
● equals
● hashCode
● toString
● getters (and optionally setters)
● ++

abstract class Person(name: String)
case class Man(name: String) extends Person(name)
case class Woman(name: String, children: List[Person])
extends Person(name)

p match {
case Man(name) => println("Man with name " + name)
case Woman(name, children) => println("Woman with name "
+ name + " and with " +
children.size + " children")
}

val regex = """(d+)(w+)""".r

val myString = ...

val res: String = myString match {
case regex(digits, word) => digits
case _ => "None"
}

val regex = """(d+)(w+)""".r

val myString = ...

val res: Option[String] = myString match {
case regex(digit, word) => Some(digit)
case _ => None
}

The Option type, never again
NullPointerException
● Option has two possible values
● Some(value)
● None

val someOption: Option[String] = Some("value")
val noOption: Option[String] = None

def getValue(s: Any): Option[String]

getValue(object) match {
case Some(value) => println(value)
case None => println("Nothing")
}

val result = getValue(object).getOrElse("Nothing")

Tasks (30 min)
● Open the 'pattern-matching' project
● Tests in package
scalaexamples.patternmatching
● Follow instructions in the code
● Make the tests pass

Object orientation and
traits

Annotations – not marker interfaces

@serializable class Person

@SerialVersionUID(1) class Person

@cloneable class Person

@remote class Service

object
● object is a “singleton” class
● Call may look like static method calls in Java
● Can inherit from other classes and traits
● Can be passed as a reference

object MyObject {
def foo = "bar"
}

var baz = MyObject.foo
val personObject = MyObject
baz = personObject.foo

Companion object
● Can read the companion class' private fields
● Has to be in the same source file

class Person(private val age: Int)

object Person {
def getPersonAge(p: Person) = p.age
}

val personInstance = new Person(30)
val age = Person.getPersonAge(personInstance)

Magical apply

class Person private(val age: Int)

object Person {
def apply(age: Int) = new Person(age)
}

var personInstance = Person.apply(30)
personInstance = Person(30)

Not built in, clever use of apply

val myList = List(1, 2, 3)
val mySet = Set(1, 2, 3)
val myMap = Map(1 -> "one", 2 -> "two")

Constructors
● Always one primary constructor
● Parameters are automatically instance variables
● Class “body” is the primary constructors content
● Auxiliary constructors MUST call or chain to primary
constructor
class MyClass(myString: String, myInt: Int)

val myOtherInt = 10
println("In main body")

}

Auxiliary constructors
class MyClass(myString: String, myInt: Int) {

def this(myString: String) = this(myString, 0)

def this() = {
this("foo")
println("In default constructor")
}

}

Inheritance

class MyClass(myString: String)

class MySubClass(myString: String, myInt: Int)
extends MyClass(myString) {

println("MyString: '" + myString +
"', MyInt: '" + myInt + "'")

}

Inheritance
● Single class inheritance
● Multiple trait mixins

Syntax

class Person extends AnyRef
with java.io.Serializable
with java.rmi.Remote

class Person extends java.io.Serializable
with java.rmi.Remote

Traits
● “Multiple inheritance done right”
● Implement methods
● Initialized fields
● Abstract methods
● Abstract fields
● Abstract types
● Does not have constructors
● Call to super → strict semantics

scala.Ordered trait

trait Ordered[A] {
def compare(that: A): Int

def < (that: A): Boolean = (this compare that) < 0
def > (that: A): Boolean = (this compare that) > 0
def <= (that: A): Boolean = (this compare that) <= 0
def >= (that: A): Boolean = (this compare that) >= 0
}

The Ordered trait

class Person(private val age: Int)
extends Ordered[Person] {

def compare(other: Person) =
this.age - other.age
}

val person1 = new Person(21)
val person2 = new Person(31)
person1 < person2 // true
person1 <= person2 // true
person1 >= person2 // false

“Dynamic mixins”

class Person(age: Int) {
override def toString = "my age is " + age
}

trait MyTrait {
override def toString = "I am sure " +
super.toString
}

val person = new Person(30) with MyTrait
println(person)

=> I am sure my age is 30

is-a vs has-a
● Inheritance as usual
● Easier to implement has-a
● Inheritance vs composition

Tasks (30 min)
● Open 'oo-traits' project
● Tests under scalaexamples
● companionobject
● inheritance
● traits
● Follow instructions in code
● Make tests pass

“Functional programming”
● First class functions
● Pattern matching
● Higher order functions

Functional programming
● Purity
● Mathematical functions have no side effects
● f(x) = 2x + 3
● y = sin(x)
● Mathematical functions always give same result
for same input
● Only immutable objects (and object graphs)

In practice
● All fields must be immutable
● All methods must return something
● No side-effects from method calls
●
println("...") is also a side effect!

List
● head :: tail
● Prepends to the head of the list
● Other operations (filter, map, remove,
partition...) returns a new List instance

scala.collection.immutable
● ...or scala.collection.mutable
● Pick your poison!

Scala 2.8
● Case classes get copy methods
● Constructors and methods get
● named parameters
def resize(width: Int, height: Int) = { ... }
resize(height = 42, width = 120)
● default parameters
def f(elems: List[Int], x: Int = 0,
cond: Boolean = true)
f(List(1))
f(Nil, cond = false)

Strive to be pure
● Parallelization and concurrency (no
synchronization)
● Usually easier to find errors (a lot less state)
● Easier to test

Refactoring imperative style code
def printMultiTable () {
var i = 1 // row
while (i <= 10) {
var j = 1 // column
while (j <= 10) {
val prod = (i * j).toString
var k = prod.length // padding
while (k < 4) { 1 2 3 4 5 6 7 8 9 10
print(" ") 2 4 6 8 10 12 14 16 18 20
k += 1 3 6 9 12 15 18 21 24 27 30
} 4 8 12 16 20 24 28 32 36 40
print(prod) 5 10 15 20 25 30 35 40 45 50
j += 1 6 12 18 24 30 36 42 48 54 60
7 14 21 28 35 42 49 56 63 70
}
8 16 24 32 40 48 56 64 72 80
println() 9 18 27 36 45 54 63 72 81 90
i += 1 10 20 30 40 50 60 70 80 90 100
}
}

Refactoring imperative style code
● Side effect: printing table
● Functional style: return String
● Harder to test
● How to test result of println()?
● While loop & vars
● Functional style: val, for expressions, helper
functions
● Helper functions can be tested separately

Functional style multiplication table

def makeRowSeq(row: Int) =
for (col <- 1 to 10) yield {
val prod = (row * col).toString
val padding = " " * (4 - prod.length)
padding + prod
}

def makeRow(row: Int) = makeRowSeq(row).mkString

def multiTable = {
val tableSeq =
for (row <- 1 to 10)
yield makeRow(row)
tableSeq.mkString("n")
}

A closer look at Collections
● Overview of collection traits

http://www.decodified.com/scala/collections-api.xml

Traversable
● One abstract method:
def foreach[U](f: Elem => U)
● 50+ concrete methods
● Addition, mapping, conversion, copying, size info,
element retrieval, sub-collection retrieval,
subdivision, element tests, folds, string operations,
view

http://www.scala-lang.org/api/
http://www.scala-lang.org/docu/files/collections-api/

Iterable
● Abstract method iterator
● Default implementation of foreach:
def foreach[U](f:Elem => U): Unit = {
val it = iterator
while (it.hasNext) f(it.next())
}
● Subclasses may override
● Some concrete methods
● Subcollections, "zippers", comparison

Seq, IndexedSeq, LinearSeq
● Sequence:
● iterable that has a length,
● elements have fixed index position, starting with 0
● Operations:
● Indexing (apply), search, addition, update, sorting,
reversal, comparison, set operations
● IndexedSeq, LinearSeq
● No new operations, but different performance
– LinearSeq: efficient head, tail
– IndexedSeq: efficient apply, length

Sets, SortedSet
● Set = iterable that contain no duplicates
● Operations for tests, addition, removal, set
operations
● SortedSet
● Iterator/foreach visit elements in given ordering
● Default implementation: binary tree

Maps
● Map = collection of pairs of keys and values
e.g. Map("x" -> 24, "y" -> 25, "z" -> 26)
● Operations for lookup, addition/update, removal,
subcollections, transformations

Immutable collections

http://www.decodified.com/scala/collections-api.xml

Higher order functions
● Functions which take functions as parameters
and/or return functions

Short summary of first class functions:
val even: (Int => Boolean) = (i: Int) => i % 2 == 0

Same type definition:
def test(numbers: List[Int], f: Int => Boolean) = ...

Call:
test(List(1, 2, 3), (i: Int) => i % 2 == 0)


def test(numbers: List[Int],
f: Int => Boolean) =
numbers.map(tall => f(tall))

// List[Boolean]

Functions with several parameters must list them
in parenthesis:

def test(l: List[String],
f: (Int, String) => Boolean)

call-by-value vs. call-by-name
● by-value: expressions are evaluated before
being passed to the function

● by-name: expressions evaluated inside function
● nice when computationally expensive
● possible to create nice APIs

call-by-value vs. call-by-name
Example: Logging

def thisTakesTime = {
println(“Slow computation”)
“result”
}
logger.debug(thisTakesTime())

call-by-value
def debug(s: String) {
println(“debug”)
if (logLevel <= DEBUG) println(s)
}

// Slow computation
// debug
// result

call-by-name
def debug(s: => String) {
println(“debug”)
if (logLevel <= DEBUG) println(s)
}

// debug
// Slow computation
// result

def using[T <: { def close() }, A]
(closeable: T)
(f: T => A) = {
try {
f(closeable)
} finally {
if (closeable != null) {
try {
closeable.close()
}
catch {
case e: Exception =>
// Do something clever!?
}
}
}
}

Tasks (30 min)
● Open 'higher-order-functions' project
● Tests in scalaexamples.higherorderfunctions
● Implement missing functions in PersonFilter
and so on.
● Follow instructions in code
● Make tests pass

Advanced topics
● Type inference
● Implicit conversions
● Extractors
● Annotations
● XML
● Parallel programming with Actors
● Domain Specific Languages (DSLs)
● GUI programming with Scala Swing

Exercises
● Solutions to exercises:
http://github.com/javaBin/scala-training-code/zipball/solutions
● 99 Scala Problems:
http://aperiodic.net/phil/scala/s-99/

Bert's Scala bookmarks:
http://www.delicious.com/bertvv/scala

A lot of blogs

http://www.planetscala.com/

Mailing lists

scala@listes.epfl.ch
scala-announce@listes.epfl.ch
scala-user@listes.epfl.ch
scala-debate@listes.epfl.ch
scala-tools@listes.epfl.ch
scala-internals@listes.epfl.ch

http://www.scala-lang.org/node/199

Several books

+++
http://www.scala-lang.org/node/959

Creative Commons

http://programming-scala.labs.oreilly.com/index.html

Creative Commons
Attribution 3.0 Unported

Scala Training Code
http://github.com/javaBin/scala-training-code
git clone git://github.com/javaBin/scala-training-code

Scala Training Slides
http://github.com/javaBin/scala-training-slides
git clone git://github.com/javaBin/scala-training-slides.git

Workshop Scala

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