Download Scala

Scala
or
”The Free Lunch is Over”
Andreas Enbohm
Capgemini Sverige AB
Why Scala

The free lunch is over...

Today – 2 Cores

2010 maybe 12 (AMD), in 5 years maybe 16

Difficult to utilize several CPUs with Java

Huge API in current Java – some argue it may collape due to its own weight

Scala = ScalableLanguage

Designed to fit todays and some of tomorrows programming paradigms

Actors – Easier to utilize than memory synchronization (more about this later)

Nice (less boiler plate code) syntax and ’type safe duck typing’

And much more...
© 2004 Capgemini - All rights reserved
2008, July – Andreas Enbohm
Scala - Features

OO and functional language
”a general purpose programming language designed to express common programming patterns in a concise,
elegant, and type-safe way”

Functional language – (often) no side effect, avoids state and mutable data, i.e functions can run independently and
without knowlegde of eachother
def sum(a:Int,b:Int) = a+b

Open Source

Compact code, ca 50 % less code than Java

Static typed language (but with ’kind of’ duck typing, more about this later)

Pattern matching, singel inheritance, immutable collections (like Java Strings), XML as FCC

SUnit, Lift (web framework), Support for Maven, Ant, Plugins for Eclipse, Netbeans, IntelliJ

Why not Ruby, Groovy, F#?
- Fully interoperable with Java (call Java classes whenever you want)
- Refactoring (static typed)
- runs on world’s best VM!
- very simple to inherit from Java classes
- Javas annotations, threads, for each, it all works
- Very fast (600x faster than groovy, lift 6x faster than Rails?)
© 2004 Capgemini - All rights reserved
2008, July – Andreas Enbohm
Scala – Compared to Java

Defining a class
public class Person
{
private String firstName;
private String lastName;
class Person(var firstName: String, var lastName: String) {
}
public Person(String firstName, String lastName) {
this.firstName = firstName;
this.lastName = lastName;
}
public String getFirstName()
{
return firstName;
}
public void setFirstName(String firstName)
{
this.firstName = firstName;
}
public String getLastName()
{
return lastName;
}
public void setLastName(String lastName)
{
this.lastName = lastName;
}
}

Everything in Scala is an Object! This also includes functions
© 2004 Capgemini - All rights reserved
2008, July – Andreas Enbohm
Scala – Compared to Java

Defining variables - ; and type can often be omitted
val myInt: Int = 1;
val anotherInt = 2

Creating objects
val anObject = Object //Omit () if you want just ’cluttering’ your code
val aList = List(”John Doe”, ”Jane Doe”, ”Baby Doe”)

List is of type List[String] by default
List<String> aList = new ArrayList<String>(); //Java syntax for creating a list-object
val aList = List //Scala syntax for creating corresponding list
val aList = List[String]

//Also works, notice just ONE generic definiton
List objects in Scala contains several useful functions to access elements in the List
println("Hi, " +aList.tail.head) //Prints “Hi, Jane Doe”

Ommiting some dots…
import java.util.{Date,Locale}
import java.text.DateFormat
import java.text.DateFormat._
val now = new Date
val df = getDateInstance(LONG, Locale.FRANCE)
println( df format now) //same as df.format(now)
© 2004 Capgemini - All rights reserved
2008, July – Andreas Enbohm
Scala – Compared to Java

Exceptions
- Scala has no checked exception (unlike Java but like C#)
- If used with Java, use annotation @throws(classOf[IOException])
try { doSomething(params) }
catch {
case ex: IOException => println("Oops! Some file is probably missing")
case ex: NullPointerException => println(“Dohh!! Must initialize the variable ")
}

Using pattern matching and functions
object MatchTest1 extends Application {
def matchTest(x: Int): String =
x match {
case 1 => "one"
case 2 => "two"
case _ => "many"
}
println(matchTest(1)) //Prints “one”
println(matchTest(99)) //Prints “many”
}

Scala uses ’traits’ which can be compared to interfaces. Can have implementations (but no state)
© 2004 Capgemini - All rights reserved
2008, July – Andreas Enbohm
Duck Typing in Scala

”If it walks like a duck and quacks like a duck, then I would call it a duck.” – Wise Guy
class Duck {
public void quack() {
print(”Duck Quacking!”)
}
class Person {
public void quack() {
print(”Person Quacking!”)
}
}
Duck d = new Duck();
Person p = new Person();
testQuack(d);
testQucak(p);
testQuack(duckableObject) {
duckableObject.quack();
//Lets hope this work!!!
}

Statically typed language don’t offer this flexibility (Java, C#)
- i.e. no ’Duck Typing’

Downside – what happens if object does not have a ’quack’-method -> RuntimeException!

In Java, we must add an interface with method ’quack()’ – impossible to add an interface without changing the class!
© 2004 Capgemini - All rights reserved
2008, July – Andreas Enbohm
Duck Typing in Scala

However Scala offers ’Structual Typing’ – a.k.a. Type safe duck typing

Consider following example (Scala syntax)
class File(name: String) {
def getName(): String = name
def open() { /*..*/ }
def close() { println("close file") }
}
def testPrintName(aFileObject: { def getName(): String }) {
println(aFileObject.getName)
}
testPrintName(new File("test.txt"))
testPrintName(new java.io.File("test.txt"))

Scala offers structual typing, i.e. def getName() in test

The structual type ’getName()’ checks at compile time that ’aFileObject’ has a getName() method if not – compile
error

If getName() is needed more than one, use traits.
trait HasName {
def getName() : String
}
def testPrintName(HasName f) = { println(f.getName) }
© 2004 Capgemini - All rights reserved
2008, July – Andreas Enbohm
Actors

”Don't bring the grape juice into the living room“ – Brian Goetz

Java threads
- shared memory, i.e. only one thread can operate on a shared resource concurrently
- expensive to create, i.e. every thread has a high memory consumption
- context switch. i.e a Java thread maps to a OS thread (limited numbers ~3K)
- context switch make page faults - > slow due to thread needs to re-read data from RAM
- difficult to use, i.e. deadlocks, raise conditions, live locks, NASA Mars explorer experied live lock
- synchronize, i.e. not just a a monitor/lock but also write to main memory (no registers) ~100 slower!
- Java was designed for single processors, not multi-cores!

Actor based concurrency
- concurrenct processes communicates by exchanging messages
- asynchronous message passing
- ’share-nothing’-model
- Erlang early to implement this style of concurrency
- Erlang based web server ~80000 clients, Apache ~4000 clients

Two flavours in Scala;
- Thread based actors – high level abstraction of threads which replaces error-prone shared memory access
- Thread less actors – offers enourmous scalability. Lightweight processes are used as actors with much less overhead than a usual thread.
’Piggy-backs’ on calling threads stack trace (continuation closure)
© 2004 Capgemini - All rights reserved
2008, July – Andreas Enbohm
Downsides

”With great power comes great complexity” – James Gosling

New languages difficult to introduce to organisations

Not ready for prime time?
© 2004 Capgemini - All rights reserved
2008, July – Andreas Enbohm
Questions?
http://www.scala-lang.org/
© 2004 Capgemini - All rights reserved
2008, July – Andreas Enbohm