JBUG 11 - Scala For Java Programmers

By : Adi Baron, “Tikal” For Java Programmers

"Which Programming Language would you use *now* on top of JVM, except Java?" “ Scala”

Since 2003 Runs on JVM Pragmatic High Level Statically Typed Seamless Java interoperability Scalable EPFL Object Oriented Functional Production

A language that grows on you

To the Point

Not cluttered

Not syntax

var capital = Map( "US" -> "Washington" , "France" -> "Paris" ) capital += ( "Japan" -> "Tokyo" ) println(capital( "France" ))

Growing new types import java.math.BigInteger; public BigInteger factorial (BigInteger x) { if (x == BigInteger. ZERO ) { return BigInteger. ONE ; } return x.multiply(factorial( x.subtract(BigInteger. ONE ))); } def factorial(x: BigInt ): BigInt = if (x == 0) 1 else x * factorial(x - 1)

Growing new control constructs

Scala Actors

Implemented on top of threads

Two basic operations – Send / Receive

recipient ! msg ← async receive { case Msg1 => … // handle Msg1 case Msg2 => … // handle Msg2 // … } ← mailbox

Growing new control constructs val checksum = actor { var sum = 0 loop { receive { case Data (bytes) => sum += hash(bytes) case GetSum (requester) => requester ! sum } } }

High-Level boolean nameHasUpperCase = false ; for ( int i = 0; i < name.length(); ++i) { if (Character.isUpperCase(name.charAt(i))) { nameHasUpperCase = true ; break ; } } val nameHasUpperCase = name.exists(_.isUpperCase)

Concise public class MyClass { private int id ; private String description ; public MyClass( int id, String description) { this . id = id; this . description = description; } public int getId() { return id ; } public void setId( int id) { this . id = id; } public String getDescription() { return description ; } public void setDescription(String description) { this . description = description; } } class MyClass (id: Int , description: String )

What makes it scalable?

is Object Oriented

Pure Object Oriented 1 + 2 1.+(2) 123.toString

Example – Rational Number

A number that can be expressed a ratio

n (numerator) & d (denominator) are integers

Operations – Add, Subtract, Multiply, Divide

n _ d class Rational (n: Int , d: Int ) class Rational (n: Int , d: Int ) { println( “Created “ + n + “/” + d) } scala> new Rational (1, 2) Created 1/2 res0: Rational = Rational@90110a

Implementing toString

class Rational (n: Int , d: Int ) { override def toString = n + “/” + d } scala> val x = new Rational (1, 3) x: Rational = 1/3 scala> val x = new Rational (5, 7) y: Rational = 5/7

Input Validation

class Rational (n: Int , d: Int ) { require(d != 0) override def toString = n + “/” + d } scala> new Rational (5, 0) res0: Rational = 5/0

Add Fields

class Rational (n: Int , d: Int ) { require(d != 0) val numer: Int = n val denom: Int = d override def toString = n + “/” + d def add(that: Rational ): Rational = new Rational ( numer * that.denom + that.numer * denom, denom * that.denom ) } class Rational (n: Int , d: Int ) { // This won’t compile require(d != 0) override def toString = n + “/” + d def add(that: Rational ): Rational = new Rational (n * that.d + that.n * d, d * that.d) }

Adding

scala> val oneHalf = new Rational (1, 2) oneHalf: Rational = 1/2 scala> val twoThirds = new Rational (2, 3) twoThirds: Rational = 2/3 scala> oneHalf add twoThirds res3: Rational = 7/6

A better version

class Rational (n: Int , d: Int ) { require(d != 0) val numer: Int = n val denom: Int = d def this (n: Int ) = this (n, 1) def +(that: Rational ): Rational = new Rational ( numer * that.denom + that.numer * denom, denom * that.denom ) override def toString = n + “/” + d }

Finally

scala> val x = new Rational (1, 2) x: Rational = 1/2 scala> val y = new Rational (2, 3) y: Rational = 2/3 scala> x + y res8: Rational = 7/6

Traits

Trait trait Dad { private var children: List [ Child ] = Nil def addChild(child: Child ) = children = child :: children def getChildren = children.clone }

Base class Man ( val name: String ) extends Human

Static mixin composition class Man ( val name: String ) extends Human with Dad val adi = new Man ( “Adi” ) adi.addChild( new Child ( “Yehonatan” )) adi.addChild( new Child ( “Lior” ))

Dynamic mixin composition class Man ( val name: String ) extends Human val adi = new Man ( “Adi” ) with Dad adi.addChild( new Child ( “Yehonatan” )) adi.addChild( new Child ( “Lior” ))

Composition class MrPotato (name: String ) extends MrPotatoHead with Eyes with Ears with Mouth with Nose with Hands with Hat with Shoes

is Functional

First Class Functions (x: Int ) => x + 1

Function literal

Compilation

Function class

Runtime – function value

Functions as values scala> val increase = (x: Int ) => x + 1 increase: (Int) => Int = <function> scala> increase(10) res0: Int = 11

=> converts the “thing” on the left (any integer x) to the thing pm the right (x + 1)

Functions as parameters scala> val someNumbers = List (-11, -10, -5, 0, 5, 10) someNumbers: List[Int] = List(-11, -10, -5, 0, 5, 10) scala> someNumbers.foreach((x: Int ) => println(x)) -11 -10 -5 0 5 10 scala> someNumbers.filter((x: Int ) => x > 0) res6: List[Int] = List(5, 10)

Short form of function literals scala> someNumbers.filter((x) => x > 0) res7: List[Int] = List(5, 10) scala> someNumbers.filter(x => x > 0) res8: List[Int] = List(5, 10)

Target typing

Leave the parentheses out

Placeholder syntax scala> someNumbers.filter(_ > 0) res9: List[Int] = List(5, 10)

“ Fill in” the missing...

scala> val f = _ + _ <console>:4: error: missing parameter type for expanded function ((x$1, x$2) => x$1.$plus(x$2)) val f = _ + _ ˆ

Type inference caution

scala> val f = (_: Int ) + (_: Int ) f: (Int, Int) => Int = <function>

Functions as closures (x: Int ) => x + more // how much more?

Functions as closures scala> (x: Int ) => x + more <console>:5: error: not found: value more (x: Int) => x + more ˆ scala> var more = 1 more: Int = 1 scala> val addMore = (x: Int ) => x + more addMore: (Int) => Int = <function> scala> addMore(10) res19: Int = 11

Functions as closures def makeIncreaser(more: Int ) = (x: Int ) => x + more scala> val inc1 = makeIncreaser(1) inc1: (Int) => Int = <function> scala> val inc9999 = makeIncreaser(9999) inc9999: (Int) => Int = <function> scala> inc1(10) res24: Int = 11 scala> inc9999(10) res25: Int = 10009

Functional Data Structures

List - Creating

Prepends new elements

Append time grows linearly

List (1, 2, 3) 1 :: 2 :: 3 :: Nil

List - Basics scala> val list = List (1, 2, 3) list: List[Int] = List(1, 2, 3) scala> list.head res0: Int = 1 scala> list.tail res1: List[Int] = List(2, 3) scala> list.isEmpty res2: Boolean = false

List – High level operations scala> val list = List (1, 2, 3) list: List[Int] = List(1, 2, 3) scala> list.map(_ + 1) res0: List[Int] = List(2, 3, 4) scala> list.filter(_ < 2) res1: List[Int] = List(1) scala> list.exists(_ == 3) res2: Boolean = true scala> list.drop(2) res3: List[Int] = List(3) scala> list.reverse res4: List[Int] = List(3, 2, 1)

Other

Maps

Sets

Trees

Stacks

& Java

Unit Testing - JUnit import junit.framework.TestCase import junit.framework.Assert.assertEquals import junit.framework.Assert.fail import Element.elem class ElementTestCase extends TestCase { def testUniformElement() { val ele = elem( 'x' , 2, 3) assertEquals(2, ele.width) assertEquals(3, ele.height) try { elem('x', -2, 3) fail() } catch { case e: IllegalArgumentException => // expected } } }

Unit Testing - TestNG import org.testng.annotations.Test import org.testng.Assert.assertEquals import Element.elem class ElementTests { @ Test def verifyUniformElement() { val ele = elem( 'x' , 2, 3) assertEquals(ele.width, 2) assertEquals(ele.height, 3) } @ Test { val expectedExceptions = Array (classOf[ IllegalArgumentException ]) } def elemShouldThrowIAE() { elem( 'x' , -2, 3) } }

Credits

Jonas Bonér Scalable Solutions &