Introducing scala

Introducing Scala

Meetu Maltiar
Principal Consultant
Knoldus

Agenda

Starting a Scala project
Scala as a language
Scala Collections
Scala Test

SBT

Build tool for Scala based projects
Scala based frameworks like Akka uses it
SBT build definition uses scala based DSL
Incremental compilation
Works with mixed Scala and Java based projects

SBT: Lets create a project
SBT Installation download jar and create a script
Instructions: xsbt wiki
Descend in directory where you wanna create the project
In terminal type sbt
Once the sbt is started enter following commands
set name := “ScalaKnolx”
set version := “1.0”
set scalaVersion := “2.9.1”
session save
exit

Open build.sbt and have a look!!

SBT: Eclipse IDE
Add Typesafe repo and sbteclipse plugin
In build.sbt
resolvers += "Typesafe Repository" at "http://repo.typesafe.com/typesafe/releases/"

libraryDependencies += "org.scalatest" %% "scalatest" % "1.6.1"

libraryDependencies += "junit" % "junit" % "4.9"

Create project/plugins.sbt and add sbteclipse plugin
addSbtPlugin("com.typesafe.sbteclipse" % "sbteclipse-plugin" % "2.0.0")

Execute sbt eclipse

It will generate eclipse related configs and now we are ready to import
this project in eclipse!!

Scala Introduction

Scala is a JVM based language

Scala combines FP and OO which makes it a scalable language

Scala has a REPL

Scala is interoperable with Java

Scala is a scripting language
It has a REPL.
Types can be inferred
Less Boilerplate

Scala> var capital = Map(“US” → “Washington”, “France” → “Paris”)

Capital: Map[String, String] = Map(US-> Washington, France->Paris)

Scala> capital += (“japan” → “Tokyo”)

Scala> capital(“France”)

Res2: String = Paris

Scala is OO
Every value is an object
Every operation is method call
Exceptions to this in java like statics and primitives are removed

Scala> (1).hashCode

Res1: Int = 1

Scala> (1).+(2)

Res2: Int = 3

Scala compared to Java
Scala adds Scala removes

+ pure object system - static members

+ operator overloading - primitive types

+ closures - break, continue

+ mixin composition with traits - special treatment of interfaces

+ existential types - wildcards

+ abstract types - raw types

+ pattern matching - enums

Scala cheat sheet (1): Definitions
Scala method definitions Java method definitions

def fun(x: Int) = { Int fun(int x) {
result return result
} }

def fun = result (no parameterless methods)

Scala variable definitions Java variable definitions

var x: Int = expression Int x = expression
val x: String = expression final String x = expression

Scala cheat sheet (2): Definitions
Scala Class and Object Java method definitions

class Sample(x: Int, p: Int) { class Sample {
def instMeth(y: Int): Int = x + y private final int x;
} public final int p;

object Sample { Sample(int x, int p) {
def staticMeth(x: Int, y: Int): Int = x * y this.x = x;
} this.p = p;
}

int instMeth(int y) {
return x + y;
}

static int staticMeth(int x, int y) {
return x * y;
}
}

Scala cheat sheet (3): Traits

Scala Trait Java Interface

trait T { Interface T {
var field = “!” Int abstractMth(String x)
}
def abstractMth(x: Int): Int
(no concrete methods)
def concMth(x: String) = x + field (no fields)
}

Scala mixin composition Java extension plus implementation

class C extends Super with T class C extends Super implements T

Scala HOF
Scala is also FP along-with OO
This means that Function is also an Object
They can be passed along as objects

private def higherOrderFunction(f: Int => Int, x:
Int): Int = {
f(x) + 1
}

Scala Pattern Match
All that is required is to add case keyword to each class that is to
be pattern matchable

Similar to switch except that Scala compares objects as
expressions
getInteger(4) match {
case 4 => println("four")
case _ => println("not four")
}

def getInteger(x: Int): Int = {
x
}

Scala Traits
They are fundamental unit of code reuse in Scala

They encapsulates method and field definitions, which can be
reused by mixing them in classes

Unlike class inheritance a class can mix any number of traits

Unlike Interfaces they can have concrete methods

Scala Collections

Class Person(val name: String, age: Int)

val people: Array[Person]

val(minors, adults) = people partition (_.age < 18)

Three concepts:
- pattern mach
- infix method call
- a function value

Scala way of Collections

De-emphasize destructive updates

Focus on transformers that map collections to collections

Have complete range of persistent collections

Collection Properties
Object-Oriented

Generic: List[T], Map[K, V]

Optionally persistent: scala.collections.immutable

Higher order: methods like foreach, map, filter

Uniform return type principle: operations return same

type as their left operand

Uniform Return Type Principle
scala> val ys = List(1,2,3)
ys: List[Int] = List(1,2,3)

scala> val xs: Seq[Int] = ys
xs: Seq[Int] = List(1,2,3)

scala> xs map(_ + 1)
res0: Seq[Int] = List(2,3,4)

scala> ys map(_ + 1)
res1: List[Int] = List(2,3,4)

Using Collections: Map and Filter
scala> val xs = List(1,2,3)
xs: List[Int] = List(1,2,3)

scala> val ys = xs map (x => x + 1)
xs: List[Int] = List(2,3,4)

scala> val ys = xs map(_ + 1)
ys: List[Int] = List(2,3,4)

scala> val zs = ys filter (_ % 2 == 0)
zs: List[Int] = List(2,4)

scala> val as = ys map (0 to _)
as: List[scala.collection.immutable.Range.Inclusive] =
List(Range(0,1), Range(0,1,2), Range(0,1,2,3))

Using Collections: flatMap and groupBy
scala> val bs = as.flatten
bs: List[Int] = List(0,1,0,1,2,0,1,2,3)

scala> val bs = ys flatMap(0 to _)
bs: List[Int] = List(0,1,0,1,2,0,1,2,3)

scala> val fruit = Vector(“apples”, “oranges”, “ananas”)
fruit: scala.collection.immutable.Vector[java.lang.String] =
Vector(“apples”, “oranges”, “ananas”)

scala> fruit groupBy (_.head)
res2: scala.collection.immutable.Map[char,
scala.collection.immutable.Vector[java.lang.String]] = Map(a->
Vector(apples, ananas), o -> Vector(oranges))

Using Collections: for notation

scala> for(x ← xs) yield x + 1 // map
res0: Seq[Int] = List(2,3,4)

scala> for(x ← res0 if x % 2 == 0) yield x // filter
res1: Seq[Int] = List(2,4)

scala> for(x ← xs; y ← 0 to x) yield y // flatMap
res2: Seq[Int] = List(0, 1, 0, 1, 2, 0, 1, 2, 3)

String also a collection
Even String is a collection that means that we can apply higher
order functions on it

scala> val aString = “hello world”
aString: java.lang.String = hello world

scala> aString map (_.toUpper)
res1: String = HELLO WORLD

Using Maps
scala> val m = Map(1 → “ABC”, 2 → “DEF”, 3 → “GHI”)
m: scala.collection.immutable.Map[Int, java.lang.String] = Map(1 →
ABC, 2 → DEF, 3 → GHI)

scala> m(2)
res1: java.lang.String = DEF

scala> m + (4 → “JKL”)
res2: scala.collection.immutable.Map[Int, java.lang.String] = Map(1
→ ABC, 2 → DEF, 3 → GHI, 4 → JKL)

scala> m map {case (k, v) => (v, k)}
res2: scala.collection.immutable.Map[java.lang.String, Int] =
Map(ABC → 1, DEF → 2, GHI → 3)

Scala Collection Hierarchy

All collection classes are in scala.collection or one of its sub-
packages mutable, immutable and generic

Root collections are in scala.collection define same interface as
immutable collections and mutable collections add some
modification operations to make it mutable

The generic package contains building block for implementing
collections

Commonality In Collections
All classes are quite common. For instance every collection can be
created by same uniform syntax
Set(1, 2, 3)
Seq(1, 2, 3)
Traversable(1, 2, 3)
Map(“x” → 24, “y” → 25)

Applies with specific collection implementations
List(1, 2, 3)
HashMap(“x” → 24, “y” → 25)

All these collections get displayed with toString in same way

Trait Traversable
Top of Collection Hierarchy. Its abstract method is foreach:
def foreach[U](f: Elem => U)

Traversable also provides lot of concrete methods they fall in following
categories

Everything is a library
Collections feel that they are language constructs

Language does not contain any collection related constructs
- no collection types
- no collection literals
- no collection operators

Everything is in library

They are extensible

Scala Test

Scala Test is an open source framework for Java platform

With ScalaTest we can test either Scala or Java code

Integrates with popular tools like jUnit, TestNG, Ant, Maven and SBT

Designed to do different styles of testing like Behavior Driven Design for
example

Scala Test Concepts

Three concepts:

Suite: A collection of tests. A test is anything which has a name and can
succeed or fail

Runner: ScalaTest provides a runner application and can run a suite of
tests

Reporter: As the tests are run, events are fired to reporter, it takes care
of presenting results back to user

Scala Test Is Customizable
Suite
<<trait>>

def expectedTestCount(Filter: Int)
def testNames: Set[String]
def tags: Map[String, Set[String]]
def nestedSuites: List[Suite]
def run(Option[String], Reporter, …)
def runNestedSuites(Reporter, …)
def runTests(Option[String]), Reporter
def runTest(Reporter, …)
def withFixture(NoArgTest)

Scala Test: under the hood
When you run a Test in Scala Test you basically invoke
run(Option[String], Reporter, …) on Suite object
It then calls runNestedSuites(Reporter, …)
And it calls runTests(Option[String], Reporter, …)

runNestedSuites(Reporter, …):
Invokes nestedSuites(): List[Suite] to get a list of nested suites

runTests(Option[String], Reporter, …) will call def testNames:
Set[String] to get set of test names to run. For each test it calls
runTest(Reporter, …) It wraps the test code as a Function object with a
name and passes it to the withFixture(NoArgTest) which actually runs the
test

Scala Test: Available Traits
Suite
FunSuite
Spec
FlatSpec
WordSpec
FeatureSpec
Assertions
ShouldMatchers
MustMatchers

Suite

Traits approach to writing tests. Create classes extending Suite
and define test methods

Test methods have names testXXXX. All methods must be public

Scala Test provides === operator. It is defined in Trait Assertions.
Allows the failure report to include both right and left values

FunSuite

For writing Functional Tests use FunSuite Fun => functional

“test” is a method defined in FunSuite Trait. Test name goes in
parentheses and test body goes in curly braces

The test code in curly braces is passed as a by-name parameter to
“test” method which registers for later execution

Assignment
Lets map the world. We have Continents and Countries

Make a Collection hierarchy to hold the above information

Write method on the collection hierarchy to get countries of a
continent

Write method on the collection hierarchy to get continent for a
country

Write tests using FunSuite to test the methods created above

Introducing scala

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