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Scala
f o r J a v a P r o g r a m m e r s
Hello, Scala!
Copyright © 2013 Akira Koyasu. Some rights reserved.
Hello, World!
3
Copyright © 2013 Akira Koyasu. Some rights reserved.
Hello, World!
object HelloWorld {
def main(args: Array[String])
= println("Hello, World!")
}
3
Copyright © 2013 Akira Koyasu. Some rights reserved.
Agenda
About Scala
Features
4
Copyright © 2013 Akira Koyasu. Some rights reserved.
Motivation
Scalable language. To be scalable in the sense
that the same concepts can describe small as
well as large parts.
A unified and generalized object-oriented and
functional programming language provides
scalable support.
From Inventor
5
Copyright © 2013 Akira Koyasu. Some rights reserved.
Motivation
More productive as “better Java” with
existing Java resources.
Several concepts not in Java bring more
posibilities to you.
For Java programmers
6
Copyright © 2013 Akira Koyasu. Some rights reserved.
the Programming
Language
Language Specification
API
Runtime
7
Copyright © 2013 Akira Koyasu. Some rights reserved.
API
8
Scaladoc
Copyright © 2013 Akira Koyasu. Some rights reserved.
Runtime
scalac
.scala
.class
the compiler for .NET
is out of date
JVM
scala
Running on JVM
Compiler generates
class files
9
Copyright © 2013 Akira Koyasu. Some rights reserved.
Influencers
Java C#
Smalltalk
Haskell
Algol Simula
Eiffel
SML F#
Erlang
Iswim
Lisp
Python
Ruby
10
Copyright © 2013 Akira Koyasu. Some rights reserved.
Inventor
Martin Odersky
Professor of
programming methods at
EPFL in Switzerland
Designer of Java
generics
photo#1
11
Copyright © 2013 Akira Koyasu. Some rights reserved.
Object, Operator
val apple = new Apple
val orange = new Orange
println(apple + orange)
?
12
Copyright © 2013 Akira Koyasu. Some rights reserved.
Object, Operator
val apple = new Apple
val orange = new Orange
println(apple + orange)
?
(A) "Apple@xxxxOrange@xxxx"
(B) 2
(C) Compile error
(D) Runtime exception
(E) Others
12
Copyright © 2013 Akira Koyasu. Some rights reserved.
Features
Static typing & Suitable type inference
Object-oriented & Functional
Trait
Collaborating with Java
13
Copyright © 2013 Akira Koyasu. Some rights reserved.
Vorbose Java
Type declaration
public String suffix(String str) {
	 int pos = str.indexOf("-");
	 return str.substring(pos);
}
14
Copyright © 2013 Akira Koyasu. Some rights reserved.
Vorbose Java
Type declaration
def suffix(str: String) = {
val pos = str.indexOf("-")
str.substring(pos)
}
14
Copyright © 2013 Akira Koyasu. Some rights reserved.
Vorbose Java
JavaBeans
public class JavaBeans {
	 private String name;
	 public String getName() {
	 	 return name;
	 }
	 public void setName(String name) {
	 	 this.name = name;
	 }
}
15
Copyright © 2013 Akira Koyasu. Some rights reserved.
Vorbose Java
JavaBeans
case class ScalaCase(name: String)
15
Copyright © 2013 Akira Koyasu. Some rights reserved.
Functional
Programming
variables
method parameters
method returns
Functions are the first-class values
16
Copyright © 2013 Akira Koyasu. Some rights reserved.
Functional
Programming
Consider a method signature sending mails to
appropriate addresses from listing
def send() {
for(c: Contact <- listing()) {
mail(c)
}
}
17
Copyright © 2013 Akira Koyasu. Some rights reserved.
Functional
Programming
18
Copyright © 2013 Akira Koyasu. Some rights reserved.
Functional
Programming
age==20
18
Copyright © 2013 Akira Koyasu. Some rights reserved.
Functional
Programming
age==20 def send(age: Int)
18
Copyright © 2013 Akira Koyasu. Some rights reserved.
Functional
Programming
age==20
35<=age, age<=60
def send(age: Int)
18
Copyright © 2013 Akira Koyasu. Some rights reserved.
Functional
Programming
age==20
35<=age, age<=60
def send(age: Int)
def send(minAge: Int, maxAge: Int)
18
Copyright © 2013 Akira Koyasu. Some rights reserved.
Functional
Programming
age==20
35<=age, age<=60
age<30, gender:Male
def send(age: Int)
def send(minAge: Int, maxAge: Int)
18
Copyright © 2013 Akira Koyasu. Some rights reserved.
Functional
Programming
age==20
35<=age, age<=60
age<30, gender:Male
def send(age: Int)
def send(minAge: Int, maxAge: Int)
def send(minAge: Int, maxAge: Int, g: Gender)
18
Copyright © 2013 Akira Koyasu. Some rights reserved.
Functional
Programming
age==20
35<=age, age<=60
age<30, gender:Male
gender:Female, in Tokyo
def send(age: Int)
def send(minAge: Int, maxAge: Int)
def send(minAge: Int, maxAge: Int, g: Gender)
18
Copyright © 2013 Akira Koyasu. Some rights reserved.
Functional
Programming
age==20
35<=age, age<=60
age<30, gender:Male
gender:Female, in Tokyo
def send(age: Int)
def send(minAge: Int, maxAge: Int)
def send(minAge: Int, maxAge: Int, g: Gender)
def send(minAge: Int, maxAge: Int, g: Gender, pref: String)
18
Copyright © 2013 Akira Koyasu. Some rights reserved.
Functional
Programming
def send(p: Contact => Boolean) {
for(c: Contact <- listing()) {
if (p(c)) mail(c)
}
}
Apply condition function to method
19
Copyright © 2013 Akira Koyasu. Some rights reserved.
Functional
Programming
y = f(x)
Avoiding side effects
Conscious of immutability
20
Copyright © 2013 Akira Koyasu. Some rights reserved.
Partially applied
Function
21
def sum(a: Int, b: Int, c: Int) = a + b + c
val f1 = sum _
val f2 = sum(1, _: Int, 3)
def main(args: Array[String]) {
println(f1(1, 2, 3)) // 6
println(f2(2)) // 6
println(f2(5)) // 9
}
Copyright © 2013 Akira Koyasu. Some rights reserved.
Curried Function
22
def sum(a: Int)(b: Int) = a + b
def main(args: Array[String]) {
println(sum(1)(2)) // 3
}
Copyright © 2013 Akira Koyasu. Some rights reserved.
Curried Function
23
def withResource(r: Closeable)(op: => Unit) {
try {
op
} finally {
r.close()
}
}
	 	 	
def main(args: Array[String]) {
val writer = new FileWriter("test.txt")
withResource(writer) {
writer.write("foo bar")
}
}
Copyright © 2013 Akira Koyasu. Some rights reserved.
Trait
a special form of an abstract class
which can be used as mixins
24
Copyright © 2013 Akira Koyasu. Some rights reserved.
Trait
25
to enrich interface
class MyInt(val n: Int) extends Ordered[MyInt] {
def compare(that: MyInt)
= this.n - that.n
}
val n1 = new MyInt(1)
val n2 = new MyInt(2)
println(n1 < n2)
println(n1 > n2)
println(n1 <= n2)
println(n1 >= n2)
Copyright © 2013 Akira Koyasu. Some rights reserved.
Trait
26
stackable modifications
abstract class Sale {
def price(): Double
}
class BasicSale extends Sale {
def price() = 100;
}
trait OffTenPercent extends Sale {
abstract override
def price() = { super.price.toDouble * 0.9 }
}
trait OffTwenty extends Sale {
abstract override
def price() = { super.price - 20 }
}
-10%
-¥20
¥100
Copyright © 2013 Akira Koyasu. Some rights reserved.
Trait
27
stackable modifications
class UnbelievableSale
extends BasicSale with OffTenPercent
class UnbelievableSale2
extends BasicSale with OffTwenty
class UnbelievableSale3
extends BasicSale with OffTenPercent with OffTwenty
class UnbelievableSale4
extends BasicSale with OffTwenty with OffTenPercent
def main(args: Array[String]) {
println((new BasicSale).price) // 100.0
println((new UnbelievableSale).price) // 90.0
println((new UnbelievableSale2).price) // 80.0
println((new UnbelievableSale3).price) // 70.0
println((new UnbelievableSale4).price) // 72.0
}
-10%
-¥20
-10%, -¥20
-¥20, -10%
Copyright © 2013 Akira Koyasu. Some rights reserved.
Pattern Match
28
case class ScalaCase(name: String)
def test(x: Any) = x match {
case 1 => println("1")
case str: String => println(str)
case ScalaCase(name) => println("name: " + name)
case _ => println("other")
}
def main(args: Array[String]) {
test(1) // 1
test("Hello!") // Hello!
test(ScalaCase("apple")) // name: apple
test(2.5) // other
}
Copyright © 2013 Akira Koyasu. Some rights reserved.29
Scala in Action
Copyright © 2013 Akira Koyasu. Some rights reserved.
Collection
Operation
31
getting the highest score in 2013
case class Student(year: Int, score: Int)
def students(): List[Student] =
List(Student(2013, 92), Student(2012, 98), Student(2013, 70))
def students2(): List[Student] =
Student(2013, 92)::Student(2012, 98)::Student(2013, 70)::Nil
def highestScoreIn2013() = students()
.filter(s => s.year == 2013)
.map(s => s.score)
.foldLeft(0)((a, b) => max(a, b))
Copyright © 2013 Akira Koyasu. Some rights reserved.
Actor
32
actor
A
actor
B
actor
C
actor
D
No shared data
Exchanging messages
Concurrency model
Copyright © 2013 Akira Koyasu. Some rights reserved.
Actor (Akka)
33
object Actors {
implicit val system = ActorSystem("MySystem")
val a, b = actor(new Act {
become {
case ("ping", actor: ActorRef) => {
println("received ping")
Thread.sleep(1000)
actor ! ("pong", self)
}
case ("pong", actor: ActorRef) => {
println("received pong")
Thread.sleep(1000)
actor ! ("ping", self)
}
}
})
def main(args: Array[String]) {
a ! ("ping", b)
}
}
received ping
received pong
received ping
received pong
received ping
received pong
...
Copyright © 2013 Akira Koyasu. Some rights reserved.
Practical Scala
34
Copyright © 2013 Akira Koyasu. Some rights reserved.
Practical Scala
Killer framework Play (+)
Java on the bench (+)
Interactive shell (+)
There are many concepts (-)
the compiler needs more resources (-)
Backward compatibility? (-)
34
Copyright © 2013 Akira Koyasu. Some rights reserved.
Next Step
Implicit conversions
Extractor
Abstract type
Type parameter & variance
35
Copyright © 2013 Akira Koyasu. Some rights reserved.
Reference
An Overview of the Scala Programming
Language Second Edition (pdf)
A Scala Tutorial for Java programmers (pdf)
Programming in Scala, Second Edition
(Japanese version: Scala スケーラブルプロ
グラミング 第2版)
36
Copyright © 2013 Akira Koyasu. Some rights reserved.
Notes
This work is licensed under the Creative Commons Attribution-
NonCommercial 3.0 Unported License. To view a copy of this
license, visit http://creativecommons.org/licenses/by-nc/3.0/.
37
photo#1: http://en.wikipedia.org/wiki/File:Mark_Odersky_photo_by_Linda_Poeng.jpg
Feed backs Welcome!
http://twitter.com/akirakoyasu
http://fb.me/akirakoyasu
Thank you!

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Scala for Java programmers

  • 1. Scala f o r J a v a P r o g r a m m e r s
  • 3. Copyright © 2013 Akira Koyasu. Some rights reserved. Hello, World! 3
  • 4. Copyright © 2013 Akira Koyasu. Some rights reserved. Hello, World! object HelloWorld { def main(args: Array[String]) = println("Hello, World!") } 3
  • 5. Copyright © 2013 Akira Koyasu. Some rights reserved. Agenda About Scala Features 4
  • 6. Copyright © 2013 Akira Koyasu. Some rights reserved. Motivation Scalable language. To be scalable in the sense that the same concepts can describe small as well as large parts. A unified and generalized object-oriented and functional programming language provides scalable support. From Inventor 5
  • 7. Copyright © 2013 Akira Koyasu. Some rights reserved. Motivation More productive as “better Java” with existing Java resources. Several concepts not in Java bring more posibilities to you. For Java programmers 6
  • 8. Copyright © 2013 Akira Koyasu. Some rights reserved. the Programming Language Language Specification API Runtime 7
  • 9. Copyright © 2013 Akira Koyasu. Some rights reserved. API 8 Scaladoc
  • 10. Copyright © 2013 Akira Koyasu. Some rights reserved. Runtime scalac .scala .class the compiler for .NET is out of date JVM scala Running on JVM Compiler generates class files 9
  • 11. Copyright © 2013 Akira Koyasu. Some rights reserved. Influencers Java C# Smalltalk Haskell Algol Simula Eiffel SML F# Erlang Iswim Lisp Python Ruby 10
  • 12. Copyright © 2013 Akira Koyasu. Some rights reserved. Inventor Martin Odersky Professor of programming methods at EPFL in Switzerland Designer of Java generics photo#1 11
  • 13. Copyright © 2013 Akira Koyasu. Some rights reserved. Object, Operator val apple = new Apple val orange = new Orange println(apple + orange) ? 12
  • 14. Copyright © 2013 Akira Koyasu. Some rights reserved. Object, Operator val apple = new Apple val orange = new Orange println(apple + orange) ? (A) "Apple@xxxxOrange@xxxx" (B) 2 (C) Compile error (D) Runtime exception (E) Others 12
  • 15. Copyright © 2013 Akira Koyasu. Some rights reserved. Features Static typing & Suitable type inference Object-oriented & Functional Trait Collaborating with Java 13
  • 16. Copyright © 2013 Akira Koyasu. Some rights reserved. Vorbose Java Type declaration public String suffix(String str) { int pos = str.indexOf("-"); return str.substring(pos); } 14
  • 17. Copyright © 2013 Akira Koyasu. Some rights reserved. Vorbose Java Type declaration def suffix(str: String) = { val pos = str.indexOf("-") str.substring(pos) } 14
  • 18. Copyright © 2013 Akira Koyasu. Some rights reserved. Vorbose Java JavaBeans public class JavaBeans { private String name; public String getName() { return name; } public void setName(String name) { this.name = name; } } 15
  • 19. Copyright © 2013 Akira Koyasu. Some rights reserved. Vorbose Java JavaBeans case class ScalaCase(name: String) 15
  • 20. Copyright © 2013 Akira Koyasu. Some rights reserved. Functional Programming variables method parameters method returns Functions are the first-class values 16
  • 21. Copyright © 2013 Akira Koyasu. Some rights reserved. Functional Programming Consider a method signature sending mails to appropriate addresses from listing def send() { for(c: Contact <- listing()) { mail(c) } } 17
  • 22. Copyright © 2013 Akira Koyasu. Some rights reserved. Functional Programming 18
  • 23. Copyright © 2013 Akira Koyasu. Some rights reserved. Functional Programming age==20 18
  • 24. Copyright © 2013 Akira Koyasu. Some rights reserved. Functional Programming age==20 def send(age: Int) 18
  • 25. Copyright © 2013 Akira Koyasu. Some rights reserved. Functional Programming age==20 35<=age, age<=60 def send(age: Int) 18
  • 26. Copyright © 2013 Akira Koyasu. Some rights reserved. Functional Programming age==20 35<=age, age<=60 def send(age: Int) def send(minAge: Int, maxAge: Int) 18
  • 27. Copyright © 2013 Akira Koyasu. Some rights reserved. Functional Programming age==20 35<=age, age<=60 age<30, gender:Male def send(age: Int) def send(minAge: Int, maxAge: Int) 18
  • 28. Copyright © 2013 Akira Koyasu. Some rights reserved. Functional Programming age==20 35<=age, age<=60 age<30, gender:Male def send(age: Int) def send(minAge: Int, maxAge: Int) def send(minAge: Int, maxAge: Int, g: Gender) 18
  • 29. Copyright © 2013 Akira Koyasu. Some rights reserved. Functional Programming age==20 35<=age, age<=60 age<30, gender:Male gender:Female, in Tokyo def send(age: Int) def send(minAge: Int, maxAge: Int) def send(minAge: Int, maxAge: Int, g: Gender) 18
  • 30. Copyright © 2013 Akira Koyasu. Some rights reserved. Functional Programming age==20 35<=age, age<=60 age<30, gender:Male gender:Female, in Tokyo def send(age: Int) def send(minAge: Int, maxAge: Int) def send(minAge: Int, maxAge: Int, g: Gender) def send(minAge: Int, maxAge: Int, g: Gender, pref: String) 18
  • 31. Copyright © 2013 Akira Koyasu. Some rights reserved. Functional Programming def send(p: Contact => Boolean) { for(c: Contact <- listing()) { if (p(c)) mail(c) } } Apply condition function to method 19
  • 32. Copyright © 2013 Akira Koyasu. Some rights reserved. Functional Programming y = f(x) Avoiding side effects Conscious of immutability 20
  • 33. Copyright © 2013 Akira Koyasu. Some rights reserved. Partially applied Function 21 def sum(a: Int, b: Int, c: Int) = a + b + c val f1 = sum _ val f2 = sum(1, _: Int, 3) def main(args: Array[String]) { println(f1(1, 2, 3)) // 6 println(f2(2)) // 6 println(f2(5)) // 9 }
  • 34. Copyright © 2013 Akira Koyasu. Some rights reserved. Curried Function 22 def sum(a: Int)(b: Int) = a + b def main(args: Array[String]) { println(sum(1)(2)) // 3 }
  • 35. Copyright © 2013 Akira Koyasu. Some rights reserved. Curried Function 23 def withResource(r: Closeable)(op: => Unit) { try { op } finally { r.close() } } def main(args: Array[String]) { val writer = new FileWriter("test.txt") withResource(writer) { writer.write("foo bar") } }
  • 36. Copyright © 2013 Akira Koyasu. Some rights reserved. Trait a special form of an abstract class which can be used as mixins 24
  • 37. Copyright © 2013 Akira Koyasu. Some rights reserved. Trait 25 to enrich interface class MyInt(val n: Int) extends Ordered[MyInt] { def compare(that: MyInt) = this.n - that.n } val n1 = new MyInt(1) val n2 = new MyInt(2) println(n1 < n2) println(n1 > n2) println(n1 <= n2) println(n1 >= n2)
  • 38. Copyright © 2013 Akira Koyasu. Some rights reserved. Trait 26 stackable modifications abstract class Sale { def price(): Double } class BasicSale extends Sale { def price() = 100; } trait OffTenPercent extends Sale { abstract override def price() = { super.price.toDouble * 0.9 } } trait OffTwenty extends Sale { abstract override def price() = { super.price - 20 } } -10% -¥20 ¥100
  • 39. Copyright © 2013 Akira Koyasu. Some rights reserved. Trait 27 stackable modifications class UnbelievableSale extends BasicSale with OffTenPercent class UnbelievableSale2 extends BasicSale with OffTwenty class UnbelievableSale3 extends BasicSale with OffTenPercent with OffTwenty class UnbelievableSale4 extends BasicSale with OffTwenty with OffTenPercent def main(args: Array[String]) { println((new BasicSale).price) // 100.0 println((new UnbelievableSale).price) // 90.0 println((new UnbelievableSale2).price) // 80.0 println((new UnbelievableSale3).price) // 70.0 println((new UnbelievableSale4).price) // 72.0 } -10% -¥20 -10%, -¥20 -¥20, -10%
  • 40. Copyright © 2013 Akira Koyasu. Some rights reserved. Pattern Match 28 case class ScalaCase(name: String) def test(x: Any) = x match { case 1 => println("1") case str: String => println(str) case ScalaCase(name) => println("name: " + name) case _ => println("other") } def main(args: Array[String]) { test(1) // 1 test("Hello!") // Hello! test(ScalaCase("apple")) // name: apple test(2.5) // other }
  • 41. Copyright © 2013 Akira Koyasu. Some rights reserved.29
  • 43. Copyright © 2013 Akira Koyasu. Some rights reserved. Collection Operation 31 getting the highest score in 2013 case class Student(year: Int, score: Int) def students(): List[Student] = List(Student(2013, 92), Student(2012, 98), Student(2013, 70)) def students2(): List[Student] = Student(2013, 92)::Student(2012, 98)::Student(2013, 70)::Nil def highestScoreIn2013() = students() .filter(s => s.year == 2013) .map(s => s.score) .foldLeft(0)((a, b) => max(a, b))
  • 44. Copyright © 2013 Akira Koyasu. Some rights reserved. Actor 32 actor A actor B actor C actor D No shared data Exchanging messages Concurrency model
  • 45. Copyright © 2013 Akira Koyasu. Some rights reserved. Actor (Akka) 33 object Actors { implicit val system = ActorSystem("MySystem") val a, b = actor(new Act { become { case ("ping", actor: ActorRef) => { println("received ping") Thread.sleep(1000) actor ! ("pong", self) } case ("pong", actor: ActorRef) => { println("received pong") Thread.sleep(1000) actor ! ("ping", self) } } }) def main(args: Array[String]) { a ! ("ping", b) } } received ping received pong received ping received pong received ping received pong ...
  • 46. Copyright © 2013 Akira Koyasu. Some rights reserved. Practical Scala 34
  • 47. Copyright © 2013 Akira Koyasu. Some rights reserved. Practical Scala Killer framework Play (+) Java on the bench (+) Interactive shell (+) There are many concepts (-) the compiler needs more resources (-) Backward compatibility? (-) 34
  • 48. Copyright © 2013 Akira Koyasu. Some rights reserved. Next Step Implicit conversions Extractor Abstract type Type parameter & variance 35
  • 49. Copyright © 2013 Akira Koyasu. Some rights reserved. Reference An Overview of the Scala Programming Language Second Edition (pdf) A Scala Tutorial for Java programmers (pdf) Programming in Scala, Second Edition (Japanese version: Scala スケーラブルプロ グラミング 第2版) 36
  • 50. Copyright © 2013 Akira Koyasu. Some rights reserved. Notes This work is licensed under the Creative Commons Attribution- NonCommercial 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc/3.0/. 37 photo#1: http://en.wikipedia.org/wiki/File:Mark_Odersky_photo_by_Linda_Poeng.jpg Feed backs Welcome! http://twitter.com/akirakoyasu http://fb.me/akirakoyasu