Strengths of Scala Java replacement? The answer doesn’t matter! Brief intro to Scala - Razvan Cojocaru – Nov’08 & Mar’09

Overview Brief syntax – at least 30% off Java Smart compiler != dumb programmer Compiles into bytecode, runs on JVM  access to all Java libraries and, more importantly, APIs C++ is back! …mean as ever! Multiple inheritance, operator overloading, true generics, pass functions around, simple syntax Scala is better: true multi-paradigm Functional programming Scalable language Not for the faint of heart!

Brief syntax – at least 30% off Java

Smart compiler != dumb programmer

Compiles into bytecode, runs on JVM  access to all Java libraries and, more importantly, APIs

C++ is back! …mean as ever!

Multiple inheritance, operator overloading, true generics, pass functions around, simple syntax

Scala is better: true multi-paradigm

Functional programming

Scalable language

Not for the faint of heart!

Smart Compiler Makes up constructors, getX/setX stuff class Point (x:Int,y:Int) Figures out types for values/variables and functions etc def getX = x Lots of shortcuts in the syntax, i.e no {} for method bodies

Makes up constructors, getX/setX stuff

class Point (x:Int,y:Int)

Figures out types for values/variables and functions etc

def getX = x

Lots of shortcuts in the syntax, i.e no {} for method bodies

Smart compiler != dumb programmer Class Point { int x; int y; public Point (int ax, int ay) { this.x = ax; this.y = ay; } public int getX () { return x; } public int getY () { return y; } public int setX (int x) { if (x <= 0) throw new IllegalArgumentException(“blah”); else this.x=x } public int setY (int y) { this.y=y } } Class Point ( var x:int, var y:int) def x_= (ax:Int) = require (ax > 0); x=ax To overwrite the assign, is actually a little more complicated: def + (p:Point) = new Point (x+p.x, y+p.y) Guess who ^^^ Guess who ^^^

Class Point {

int x;

int y;

public Point (int ax, int ay) {

this.x = ax;

this.y = ay;

}

public int getX () { return x; }

public int getY () { return y; }

public int setX (int x) {

if (x <= 0)

throw new IllegalArgumentException(“blah”);

else

this.x=x }

public int setY (int y) { this.y=y }

}

Class Point ( var x:int, var y:int)

def x_= (ax:Int) =

require (ax > 0); x=ax

To overwrite the assign, is actually a little more complicated:

def + (p:Point) =

new Point (x+p.x, y+p.y)

Constructor/field generation class SimplerPoint ( private var ix : Int, private var iy :Int) { def x = ix def y = iy def x_= ( ax :Int) { require( ax >0); ix = ax } def y_= ( ay :Int) { require( ay >0); iy = ay } } class Point (ax:Int, ay:Int) { private[this] var ix :Int = ax private[this] var iy :Int = ay def x = ix def y = iy def x_= (ax:Int) { require(ax>0); ix =ax } def y_= (ay:Int) { require(ay>0); iy =ay } } // check this out !!! class Point (ax:Int, ay:Int) { require(ax>0); // class body is in fact c-tor private[this] var ix :Int = ax def x_= (ax:Int) { require(ax>0); ix =ax } } Ix/iy are both constructor and fields Classic: ax/ay are just args for constructor

class SimplerPoint (

private var ix : Int,

private var iy :Int)

{

def x = ix

def y = iy

def x_= ( ax :Int) { require( ax >0); ix = ax }

def y_= ( ay :Int) { require( ay >0); iy = ay }

}

class Point (ax:Int, ay:Int)

{

private[this] var ix :Int = ax

private[this] var iy :Int = ay

def x = ix

def y = iy

def x_= (ax:Int) { require(ax>0); ix =ax }

def y_= (ay:Int) { require(ay>0); iy =ay }

}

// check this out !!!

class Point (ax:Int, ay:Int)

{

require(ax>0); // class body is in fact c-tor

private[this] var ix :Int = ax

def x_= (ax:Int) { require(ax>0); ix =ax }

}

Traits Like multiple inheritance More like “polymorphic composition”…if that pairing makes sense  Simplify coding a lot, for classes with multiple “traits”. No interfaces There are abstract classes, though An abstract trait (all methods abstract) is equivalent to a Java interface

Like multiple inheritance

More like “polymorphic composition”…if that pairing makes sense 

Simplify coding a lot, for classes with multiple “traits”.

No interfaces

There are abstract classes, though

An abstract trait (all methods abstract) is equivalent to a Java interface

Traits - features As opposed to interfaces, traits can implement methods No “controllers” etc No replication of code. With single inheritance, you decide on a base class and then REWRITE every time the methods of other implemented interfaces “ idiot controller” syndrome: keep algorthms in different classes, to avoid rewriting them, see next slide TODO: read about the “linearization” to avoid diamond inheritance Same as “mixin” in Ruby Trait vs. Class ? Semantics …

As opposed to interfaces, traits can implement methods

No “controllers” etc

No replication of code.

With single inheritance, you decide on a base class and then REWRITE every time the methods of other implemented interfaces

“ idiot controller” syndrome: keep algorthms in different classes, to avoid rewriting them, see next slide

TODO: read about the “linearization” to avoid diamond inheritance

Same as “mixin” in Ruby

Trait vs. Class ? Semantics …

Traits vs interfaces interface Centered { public Point getCenter () { return x; } } class IdiotController { public void offset (Centered s, Point offsetBy) {…} } class MyShape extends Drawable implements Centered { //… public Point getCenter () { return c; } } //…in a java file far far away: New IdiotController().offset (myShape, new Point (3,4)) trait Centered () { var center:Point def offset (offsetBy:Point) = center += offsetBy } Class MyShape extends Drawable with Centered { } //…in a scala file anywereh in the galaxy myShape.offset ((3,4)) Guess who ^^^ Guess who ^^^

interface Centered {

public Point getCenter () { return x; }

}

class IdiotController {

public void offset (Centered s, Point offsetBy) {…}

}

class MyShape extends Drawable implements Centered {

//…

public Point getCenter () { return c; }

}

//…in a java file far far away:

New IdiotController().offset (myShape, new Point (3,4))

trait Centered () {

var center:Point

def offset (offsetBy:Point) = center += offsetBy

}

Class MyShape extends Drawable with Centered {

}

//…in a scala file anywereh in the galaxy

myShape.offset ((3,4))

The screwyness of it trait AnotherCentered { def center:Point def offset (offsetBy:Point) = center += offsetBy } class AnotherShape extends AnotherCentered { override val center:Point = new Point(0,0) } // type parameters Class MyContainer [T :> SomeBaseClass] { var internal = new List[T] def += (t:T) = internal += t } Values/variables and functions share the same namespace Generics Uses [] instead of () (use () instead of [] as well) Operator overloading

trait AnotherCentered {

def center:Point

def offset (offsetBy:Point) = center += offsetBy

}

class AnotherShape extends AnotherCentered {

override val center:Point = new Point(0,0)

}

// type parameters

Class MyContainer [T :> SomeBaseClass] {

var internal = new List[T]

def += (t:T) = internal += t

}

Values/variables and functions share the same namespace

Generics

Uses [] instead of ()

(use () instead of [] as well)

Operator overloading

The screwyness of it (2) var greeting = &quot;“ for (i <- 0 until args.length) greeting += (args(i) + &quot; &quot;) // It’s the same as: val range = 0.until(args.length) for (i <- range) greeting += (args(i) + &quot; &quot;) // should use this instead…remember Java // Callback<T>? … ok…now forget it! args.foreach (x => greetings += x) expect (true) { “ Samba pa ti” == new String (“Samba pa ti”) } . For “for” is not “for” But, “equals” is actually “equals”

var greeting = &quot;“

for (i <- 0 until args.length)

greeting += (args(i) + &quot; &quot;)

// It’s the same as:

val range = 0.until(args.length)

for (i <- range)

greeting += (args(i) + &quot; &quot;)

// should use this instead…remember Java

// Callback<T>? … ok…now forget it!

args.foreach (x => greetings += x)

expect (true) {

“ Samba pa ti” == new String (“Samba pa ti”)

}

.

For “for” is not “for”

But, “equals” is actually “equals”

Functional…ity Functional language – Lisp family Function literals Lambda stuff and currying It’s fashionable, sounds cool and it’s even useful sometimes… Syntactic sweetness

Functional language – Lisp family

Function literals

Lambda stuff and currying

It’s fashionable, sounds cool and it’s even useful sometimes…

Syntactic sweetness

Functional…ity // a function typedef type Fun = (String,String) => String // a function literal assigned to a value/variable val aFun = (x,y) => x+y aFun ( aFun (“this”, “is”), “cool”) // function literal passed to a function: myArray.foreach ( x => println x ) myArray.sort (<) myArray.sort ( (x,y) => x < y ) Functions themselves are objects! You can pass them to other functions, Assign to variables etc … remember C++ pointers to functions) Guess who ^^^ Guess who ^^^

// a function typedef

type Fun = (String,String) => String

// a function literal assigned to a value/variable

val aFun = (x,y) => x+y

aFun ( aFun (“this”, “is”), “cool”)

// function literal passed to a function:

myArray.foreach ( x => println x )

myArray.sort (<)

myArray.sort ( (x,y) => x < y )

Functions themselves are objects!

You can pass them to other functions,

Assign to variables etc

… remember C++ pointers to functions)

The Cool and the Screwy Abstract VAL implements a DEF … or vice-versa Partially applied functions Extend the language syntax Interpreted AND/OR compiled Can setup SCALA-only desktop environment with scripted etc…

Abstract VAL implements a DEF

… or vice-versa

Partially applied functions

Extend the language syntax

Interpreted AND/OR compiled

Can setup SCALA-only desktop environment with scripted etc…

Quirks No statics funny implementation of singletons instead … as “companion objects” Can’t define as many constructors as you want…use factories instead, which is also perfect usage of the singletons above…

No statics

funny implementation of singletons instead

… as “companion objects”

Can’t define as many constructors as you want…use factories instead, which is also perfect usage of the singletons above…

Quirks // as a value val abs = if (x < 0) 0–x else x // or as a function – remember? val absfun = (x:int) => if (x < 0) 0–x else x myValue.asInstanceOf [OtherClass] implicit def itos (x:Int) : String = String.valueOf(x) // check this out: def repeat[T] (n: Int) ( what: => T ): List[T] = ... repeat(5) { println(“I will be quiet in class.&quot;))) } Every statement block returns a value Typecasts : Implicit type conversions Special syntax: Guess who ^^^ Guess who ^^^

// as a value

val abs = if (x < 0) 0–x else x

// or as a function – remember?

val absfun = (x:int) => if (x < 0) 0–x else x

myValue.asInstanceOf [OtherClass]

implicit def itos (x:Int) : String = String.valueOf(x)

// check this out:

def repeat[T] (n: Int) ( what: => T ): List[T] = ...

repeat(5) { println(“I will be quiet in class.&quot;))) }

Every statement block returns a value

Typecasts :

Implicit type conversions

Special syntax:

Apply() and bound functions //simple OBJECT to time a statement… note this is not a class !!! apply() behaves differently object Time { def apply[T] ( action: => T ): (T, Long) = { startTimer() val resp = action() val time = stopTimer() (resp, time) } // using it: val (response, ms) = Time(Http.get(&quot;http://scala-blogs.org/&quot;)) // it works because the http.get(xxx) above is a bound function not an actual call Time(x) is defined via the apply() method Special syntax to bind tuples The Http.get() is NOT a call but a bound function Guess who ^^^ Guess who ^^^

//simple OBJECT to time a statement… note this is not a class !!! apply() behaves differently

object Time { def apply[T] ( action: => T ): (T, Long) = {

startTimer()

val resp = action()

val time = stopTimer()

(resp, time)

}

// using it:

val (response, ms) = Time(Http.get(&quot;http://scala-blogs.org/&quot;))

// it works because the http.get(xxx) above is a bound function not an actual call

Time(x) is defined via the apply() method

Special syntax to bind tuples

The Http.get() is NOT a call but a bound function

UnitTests class TestRazElement extends JUnit3Suite { def testA = expect (&quot;roota&quot;) { doc a &quot;name&quot; } def testXpe = expect (&quot;11&quot;) { doc xpe &quot;/root/parent[@name='1']/child[@name='11']&quot; a &quot;name&quot; } } Several ways to write tests. Compatible with JUnit, NGUnit etc Guess who ^^^ Guess who ^^^

class TestRazElement extends JUnit3Suite {

def testA =

expect (&quot;roota&quot;) { doc a &quot;name&quot; }

def testXpe =

expect (&quot;11&quot;) { doc xpe &quot;/root/parent[@name='1']/child[@name='11']&quot; a &quot;name&quot; }

}

Several ways to write tests. Compatible with JUnit, NGUnit etc

Apply() and bound functions //simple OBJECT to time a statement… note this is not a class !!! apply() behaves differently object Time { def apply[T] ( action: => T ): (T, Long) = { startTimer() val resp = action() val time = stopTimer() (resp, time) } // using it: val (response, ms) = Time(Http.get(&quot;http://scala-blogs.org/&quot;)) // it works because the http.get(xxx) above is a bound function not an actual call Time(x) is defined via the apply() method Special syntax to bind tuples The Http.get() is NOT a call but a bound function Guess who ^^^ Guess who ^^^

//simple OBJECT to time a statement… note this is not a class !!! apply() behaves differently

object Time { def apply[T] ( action: => T ): (T, Long) = {

startTimer()

val resp = action()

val time = stopTimer()

(resp, time)

}

// using it:

val (response, ms) = Time(Http.get(&quot;http://scala-blogs.org/&quot;))

// it works because the http.get(xxx) above is a bound function not an actual call

Time(x) is defined via the apply() method

Special syntax to bind tuples

The Http.get() is NOT a call but a bound function

String pattern matching // note the triple double quotes “ “ “ val pat = &quot;&quot;&quot;(w+):(w+) (.*)&quot;&quot;&quot;.r // bind 3 vals at once val pat(who, cmd, args) = “john:go someplace” // or just iterate for (s <- pat findAllIn input) println(s) Same patterns as java Special syntax to bind multiple variables at once Guess who ^^^ Guess who ^^^

// note the triple double quotes “ “ “

val pat = &quot;&quot;&quot;(w+):(w+) (.*)&quot;&quot;&quot;.r

// bind 3 vals at once

val pat(who, cmd, args) = “john:go someplace”

// or just iterate

for (s <- pat findAllIn input)

println(s)

Same patterns as java

Special syntax to bind multiple variables at once

What Next? Read more (I like what the stuff the links on the next page point to). There’s a lot more to Scala than what I had the time and interest to show here! Start playing with Scala in Eclipse (plugin kinda sucks) or NetBeans (great plugin, especially on a dark background) or vi(m). Have Fun!

Read more (I like what the stuff the links on the next page point to). There’s a lot more to Scala than what I had the time and interest to show here!

Start playing with Scala in Eclipse (plugin kinda sucks) or NetBeans (great plugin, especially on a dark background) or vi(m).

Have Fun!

Links Good brief Scala intro: http://www.codecommit.com/blog/scala/roundup-scala-for-java-refugees The scala wiki: http:// scala.sygneca.com / Download new verions: http://www.scala-lang.org / More goodies: http://www.devoxx.com/download/attachments/1705916/D8_U_08_06_01.pdf http://www.slideshare.net/michael.galpin/introduction-to-scala-for-java-developers-presentation http://www.slideshare.net/jboner/pragmatic-real-world-scala-45-min-presentation

Good brief Scala intro:

http://www.codecommit.com/blog/scala/roundup-scala-for-java-refugees

The scala wiki:

http:// scala.sygneca.com /

Download new verions:

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

More goodies:

http://www.devoxx.com/download/attachments/1705916/D8_U_08_06_01.pdf

http://www.slideshare.net/michael.galpin/introduction-to-scala-for-java-developers-presentation

http://www.slideshare.net/jboner/pragmatic-real-world-scala-45-min-presentation