Scala traits training by Sanjeev Kumar @Kick Start Scala traits & Play, organized by Knoldus Software LLP in New Delhi

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Kick Start Scala Traits

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Agenda
● Scala Who & Why?
● First Steps
● Classes and Objects
● Control Structures
and Functions
● Inheritance and
Traits
● Pattern Matching

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In 2003, Martin Odersky and his team set
out to create a “scalable” language that
fused important concepts from functional
and object-oriented programming. The
language aimed to be highly interoperable
with Java, have a rich type system, and to
allow developers to express powerful
concepts concisely.

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Functional programming
contructs make it easy
to build interesting
things quickly from small
parts
Object oriented constructs
make it easy to structure
large systems and
adapt to new demands

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Object oriented
Everything is an object, pure OO
Not like java where we have primitives / static
fields and methods which are not members of any
object
1+2 is actually 1.+(2)
1 + 2
//> res1: Int(3) = 3
1 .+(2)
//> res2: Int(3) = 3

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Functional language
Functions are first class values
Encourages immutability where operations map
input values to output rather than change data in
place

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Expressive
val numbers = 1 to 10
//>Range(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
numbers filter { _ % 2 == 0 }
//> Vector(2, 4, 6, 8, 10)

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High Level
Finding an upper case character

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Statically Typed
Verifiable Properties
Safe Refactorings
Documentation
Cannot add boolean to List[Int]
Strong type inference
val x = Map(1->”somevalue”)

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1. Create a new worksheet called Lab1
2. Print “Hello Scala World”
3. See the program execute in Worksheet

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1 + 2
What do you see?
/> res1: Int(3) = 3

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Variables
val
var
Similar to final
Mutable

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Why does this code compile?
Type Inference
val msg:String = "Hello World!" //> msg : String = Hello World!

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Sometimes variables need to mutate
var description = "start" > description : java.lang.String = start
description = "end"

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Unit
A result of Unit indicates that the function returns no interesting
value
def justGreet(name:String) = println("Hello " + name) //>
justGreet: (name: String)Unit
Similar to void in Java

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Everything returns a value
val number = {
val x: Int = 7
val y: Int = 9
x + y
} //> number : Int = 16
if (1 < 10) 15 else 17

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Functions, first look
List(1,2,3).foreach(l=>println(l))
List(1,2,3).foreach(l=>println(l))
List(1,2,3).foreach(println(_))
List(1,2,3) foreach (println(_))
List(1,2,3) foreach (println)
List(1,2,3) foreach println
Foreach takes a function of
what needs to be done
List(1,2,3) foreach ((x:Int)=>println(2*x))

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Function Literal
What we pass inside of foreach is a function literal
List(1,2,3) foreach ((x:Int)=>println(2*x))

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Working with Arrays
val greetStrings = new Array[String](3) //> greetStrings :
Array[String] = Array(null, null, null)
greetStrings(0) = "Bank"
greetStrings(1) = "of"
greetStrings(2) = "America"
greetStrings(0) = "BANK"
Notice?
val greetStrings = List(1,2,3)
greetStrings(0) = "Bank"
greetStrings(1) = "of"
greetStrings(2) = "America"
greetStrings(0) = "BANK"

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Lists
cons
concatenation

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Tuples
Immutable like lists, but can have different types of elements
Why we cannot get values out of a Tuple just like list?

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Sets and Maps
By default, we are provided with the immutable collections

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What is happening here
var treasureMap = Map(1->"a", 2->"b")
treasureMap += (3->"c")
What happens if we change treasureMap to val?
Why?

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Understanding Functional Style
No vars
No methods with side effects – Unit
No while loops

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Singleton objects
● A singleton object is like a class and its sole
instance
● Use the keyword object to define a singleton
object:
object Foo {
val bar = "Bar"
}
Access singleton objects like Foo.bar

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Classes
● Classes are blueprints for objects
● class Bank – is a valid class definition
No Semicolon
No access modifier, public by default
No curly braces since no body
Create a new instance
new Bank

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class Bank(id:Int, name:String)
new Bank(1,"BoA")
//> res5: lab1.Bank =
lab1$anonfun$main$1$Bank$1@1372a7a
Parameters are val and cannot be accessed
from outside

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class Bank(id:Int, name:String)
val b = new Bank(1,"BoA")
b.id

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Add another parameter to bank
Create another instance of bank

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Auxilliary Constructors
class Bank(id: Int, name: String) {
def this(str: String) = this(1, str)
def this() = this(1, "")
}
Must immediately call another constructor
with this

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Accessing fields of a class
class Bank(id: Int, name: String) {
val kind = "Blue"
var address = "Delhi"
}
val b = new Bank(1, "BoA")
b.address = "Mumbai"
b.kind = "op"
immutable
mutable

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Making fields from class parameters
class Bank(val id: Int, var name: String) {
val kind = "Blue"
}
See if you can access id and name outside the class now

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Make a Bank class with the following immutable
parameters
Id, name, address, city
Create a bank instance and access its fields
Make a Account class with the following immutable
parameters
Id, name, amount
Create a Account instance and access its fields

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Define a debit method in the Account class
def debit(amount:Int):Int = {...}

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class Account(id: Int, name: String, amount: Int) {
def debit(debitAmount: Int): Int = amount - debitAmount
def -(debitAmount: Int): Int = amount - debitAmount
}
new Account(1, "VikasAccount", 100) - 20 //

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Default Parameters
class Account(id: Int, name: String, amount: Int = 0) {
def debit(debitAmount: Int): Int = amount - debitAmount
def -(debitAmount: Int): Int = amount - debitAmount
}
With default parameters,
new Account(1,"Another")

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Singleton Objects
● Can be thought to be like static in Java but
these are real objects which can inherit and be
passed around

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Companion Objects
● If a singleton object and a class or trait share
the same name, package and file, they are
called companions
private val kind = "Blue"
var address = "Delhi" + Bank.city
}
object Bank {
private val city = "Delhi"
val newBank = new Bank(1,"ABC")
newBank.kind
}
Class or trait can
access private
member of
Companion
object

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Create a companion object for Account
Create a method in account object to get the type
of account like Saving or Checking
Or
Any other method that you like

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Understanding Predef
Predef object provides definitions which are accessible in all
Scala compilation units without explicit qualification
Aliases for types such as Map, Set and List
Assertions – Require and Ensuring
Console IO – such as print, println, readLine, readInt. These
methods are otherwise provided by scala.Console

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Add a require assertion to the account that id has
to be > 0
Add any other preconditions for accounts

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require(id > 0)
private val kind = "Blue"
Bank.city
}
object Bank {
val city = "Delhi"
val newBank = new Bank(1, "ABC")
}
new Bank(0,"ll") //>
java.lang.IllegalArgumentException: requirement failed

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Case Classes
Add syntactic convenience to your class
Add factory method with
Name of class so we don't
Need to do new
All parameters in the
Parameter list get a val by
default
Natural implementation of
toString, hashCode and
equals to the class
Support pattern matching
Small price
In terms of
Size

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case class Bird(name: String)
val b = Bird("pigeon") //> b : lab1.Bird = Bird(pigeon)
val c = Bird("pigeon") //> c : lab1.Bird = Bird(pigeon)
b == c //> res7: Boolean = true

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Try converting the Bank and Account classes to
case classes
Remove vals, new, try out the toString, equals etc

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Functional Objects
Are objects that do not have mutable state

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What does it mean?
1/2 + 5/6 = 8/6
Rational numbers do not have mutable state
When we add two rational numbers we get a new rational
number back

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Why immutability is good?
Easier to reason Since there are no complex
state spaces that change over
time
Pass around freely
2 threads cannot corrupt
Make safe hashtable keys
Whereas for mutable objects
we need to make defensive
copies first
Once constructed, no-one can
change it
A mutable object may not be
found in the same hashset
once its state is mutated

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Built in control structures
Almost all Scala control structures would result in
a value
Without this programs must create temporary
variables to hold values calculated inside control
structures

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● Conventional way
● Scala way
var fileName = "default.txt"
if (1 < 2) fileName = "newDefault.txt"
val fileNameNew = if (1<2) "newDefault.txt" else "default.txt"
Using val

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Are called loops and
Not expressions because
They do not result in a value
Usually left out of functional
programming

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For expression
Without a yield does not result in a value
for (l <- List(1,2,3,4)) yield if (l%2==0) l else 1
//
> res10: List[Int] = List(1, 2, 1, 4)
With a yield
returns a value

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Try Catch
In Scala all exceptions are unchecked exceptions and we
need not catch any exception explicitly.
Technically an Exception throw is of the type Nothing
val n = 11
import java.io.FileNotFoundException
try {
val bb = if (n % 2 == 0) n / 2 else throw new Exception
} catch {
case ex: FileNotFoundException => // Missing file
case ex: Exception => println("caught exception")
} //> caught exception

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Match Expression
Unlike Java's Switch statement, match expression results in a value
val matchValue = n match {
case 11 => "eleven"
case _ => "I dont know"
}
//> matchValue : java.lang.String = eleven
println(matchValue)

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Functions
Allow us to code the functionality of the system
Most Functions are member of some object
Functions can be nested within other functions
We can define function literals and function values

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Local
Not accessible
outside

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Function Literals
Functions can be written as unnamed function literals
and passed as values!
Function literal
in source code
Compiled to
Function value
at runtime

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(x:Int)=> x * 2
Is a valid function literal, it can be
Assigned to a variable
Passed in another function

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val multiply = (x:Int) => x * 2
multiply (20) //> res11: Int = 40
multiply (25) //> res12: Int = 50
Function literals can be assigned to variables

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Short forms for function literals works when the
compiler can infer what it would be
List(1,2,3) foreach ((x)=>x*2)
val multiply = (x) => x * 2
This would fail

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Placeholder syntax, if the parameter appears
only one time within the function literal
List(1,2,3) foreach (_*2)
List(1,2,3) foreach ((x)=>x*2)

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Class Inheritance
class Animal
class Dog extends Animal
Omitting the extends keyword means that we are extending from
AnyRef

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class Animal(name:String)
class Dog(name:String) extends Animal(name)
Subclasses must immediately call the superclass constructor
Calling it without the super class constructor would not work

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final class Animal(name:String)
class Dog(name:String) extends Animal(name)
Defining the class as final restricts it from being subclassed

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class Animal(name: String) {
val kind = "carnivorous"
}
class Dog(name: String) extends Animal(name) {
override val kind = "vegetarian"
}
class Dog(name: String) extends Animal(name) {
override val kind = "vegetarian"
override val dangerous = true
}
Complains that it
overrides nothing
Hence helps us with
warning

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Abstract class
Omitting an
initialization or
implementation would
have to make a class
abstract
Cannot be initialized, they need to be
subclassed

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Scala Hierarchy
● All classes extend from a common superclass
called Any
● Methods of Any are universal methods
● Null and Nothing are common subclasses at
the bottom of the hierarchy

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Any
AnyVal AnyRef
Byte
Short
Char
Int
Long
Float
Double
Boolean
Unit
Instances of these
classes are written as
literals in Scala
42
Not new Int
AnyRef is a an alias
for java.lang.Object
All Scala classes also
inherit from special
marker trait called
ScalaObject which
makes execution of
Scala programs
efficient

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Traits
Fundamental unit of code reuse in Scala
A class can mix in any number of traits but inherit from one
SuperClass

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We can use it as
variable of type
Philosophical

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Traits
Declare fields
Can have state
Define functionality
Anything that can be done in
class definition can be done in
trait definition as well
Cannot have class parameters
(which are passed to primary
constructor of class)
Super calls are dynamically
bound

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Rich Interface
Has many methods, which makes
it convenient for caller
Thin Interface
Has less methods which
makes it easy for
implementors
Scala allows adding concrete implementations in traits which makes it suitable
for Rich interfaces + methods which need to be implemented by implementors
which is like a thin interface as well

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General Syntax
Matching order is top
to bottom
No need to give
break
As soon as the first
match is executed, it
breaks
If a pattern matches, result is given. If no pattern
matches then MatchError is thrown

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Wildcard pattern
val name = "vikas"
name match {
case "Sachin" => println("Sud")
case "Virender" => println("Virender")
case "Dhoni" => println("Dhoni")
case "Vikas" => println("Vikas")
case _ => println("Nothing matched")
}
Put wildcard match as the last alternative to prevent
MatchError
Wildcard match

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Copyright (c) 2013-14 Knoldus
Software LLP
This training material is only intended to be used by
people attending the Knoldus Scala training.
Unauthorized reproduction, redistribution, or use
of this material is strictly prohibited.

Scala traits training by Sanjeev Kumar @Kick Start Scala traits & Play, organized by Knoldus Software LLP in New Delhi

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