Scala for Java Programmers Guide to Features and Benefits

Scala
f o r J a v a P r o g r a m m e r s

Copyright © 2013 Akira Koyasu. Some rights reserved.
Hello, World!
3

Hello, World!
object HelloWorld {
def main(args: Array[String])
= println("Hello, World!")
}
3

Agenda
About Scala
Features
4

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

Motivation
More productive as “better Java” with
existing Java resources.
Several concepts not in Java bring more
posibilities to you.
For Java programmers
6

the Programming
Language
Language Speciﬁcation
API
Runtime
7

API
8
Scaladoc

Runtime
scalac
.scala
.class
the compiler for .NET
is out of date
JVM
scala
Running on JVM
Compiler generates
class ﬁles
9

Influencers
Java C#
Smalltalk
Haskell
Algol Simula
Eiffel
SML F#
Erlang
Iswim
Lisp
Python
Ruby
10

Inventor
Martin Odersky
Professor of
programming methods at
EPFL in Switzerland
Designer of Java
generics
photo#1
11

Object, Operator
val apple = new Apple
val orange = new Orange
println(apple + orange)
?
12

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

Features
Static typing & Suitable type inference
Object-oriented & Functional
Trait
Collaborating with Java
13

Vorbose Java
Type declaration
public String suffix(String str) {
int pos = str.indexOf("-");
return str.substring(pos);
}
14

Vorbose Java
Type declaration
def suffix(str: String) = {
val pos = str.indexOf("-")
str.substring(pos)
}
14

Vorbose Java
JavaBeans
public class JavaBeans {
private String name;
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
}
15

Vorbose Java
JavaBeans
case class ScalaCase(name: String)
15

Functional
Programming
variables
method parameters
method returns
Functions are the ﬁrst-class values
16

Functional
Programming
Consider a method signature sending mails to
appropriate addresses from listing
def send() {
for(c: Contact <- listing()) {
mail(c)
}
}
17

Functional
Programming
18

Functional
Programming
age==20
18

Functional
Programming
age==20 def send(age: Int)
18

Functional
Programming
age==20
35<=age, age<=60
def send(age: Int)
18

Functional
Programming
age==20
35<=age, age<=60
def send(age: Int)
def send(minAge: Int, maxAge: Int)
18

Functional
Programming
age==20
35<=age, age<=60
age<30, gender:Male
def send(age: Int)
18

Functional
Programming
age==20
35<=age, age<=60
age<30, gender:Male
def send(age: Int)
def send(minAge: Int, maxAge: Int, g: Gender)
18

Functional
Programming
age==20
35<=age, age<=60
age<30, gender:Male
gender:Female, in Tokyo
def send(age: Int)
18

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, g: Gender, pref: String)
18

Functional
Programming
def send(p: Contact => Boolean) {
for(c: Contact <- listing()) {
if (p(c)) mail(c)
}
}
Apply condition function to method
19

Functional
Programming
y = f(x)
Avoiding side effects
Conscious of immutability
20

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
}

Curried Function
22
def sum(a: Int)(b: Int) = a + b
println(sum(1)(2)) // 3
}

Curried Function
23
def withResource(r: Closeable)(op: => Unit) {
try {
op
} finally {
r.close()
}
}

val writer = new FileWriter("test.txt")
withResource(writer) {
writer.write("foo bar")
}
}

Trait
a special form of an abstract class
which can be used as mixins
24

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)

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

Trait
27
stackable modiﬁcations
class UnbelievableSale
extends BasicSale with OffTenPercent
class UnbelievableSale2
extends BasicSale with OffTwenty
extends BasicSale with OffTenPercent with OffTwenty
extends BasicSale with OffTwenty with OffTenPercent
println((new BasicSale).price) // 100.0
println((new UnbelievableSale).price) // 90.0
println((new UnbelievableSale2).price) // 80.0
}
-10%
-¥20
-10%, -¥20
-¥20, -10%

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")
}
test(1) // 1
test("Hello!") // Hello!
test(ScalaCase("apple")) // name: apple
test(2.5) // other
}

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))

Actor
32
actor
A
actor
B
actor
C
actor
D
No shared data
Exchanging messages
Concurrency model

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)
}
}
})
a ! ("ping", b)
}
}
received ping
received pong
received ping
received pong
received ping
received pong
...

Practical Scala
34

Practical Scala
Killer framework Play (+)
Java on the bench (+)
Interactive shell (+)
There are many concepts (-)
the compiler needs more resources (-)
Backward compatibility? (-)
34

Next Step
Implicit conversions
Extractor
Abstract type
Type parameter & variance
35

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

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
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Scala for Java Programmers Guide to Features and Benefits

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