Scala traits training by Sanjeev Kumar @Kick Start Scala traits & Play, organized by Knoldus Software LLP in New Delhi
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Scala traits training by Sanjeev Kumar @Kick Start Scala traits & Play, organized by Knoldus Software LLP in New Delhi
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Scala traits training by Sanjeev Kumar @Kick Start Scala traits & Play, organized by Knoldus Software LLP in New Delhi
- 1. Knoldus Kick Start Scala Traits
- 3. Knoldus Agenda ● Scala Who & Why? ● First Steps ● Classes and Objects ● Control Structures and Functions ● Inheritance and Traits ● Pattern Matching
- 4. Knoldus Agenda ● Scala Who & Why? ● First Steps ● Classes and Objects ● Control Structures and Functions ● Inheritance and Traits ● Pattern Matching
- 5. Knoldus 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.
- 6. Knoldus
- 7. Knoldus
- 8. Knoldus 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
- 9. Knoldus 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
- 10. Knoldus Functional language Functions are first class values Encourages immutability where operations map input values to output rather than change data in place
- 12. Knoldus 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)
- 13. Knoldus High Level Finding an upper case character
- 14. Knoldus Statically Typed Verifiable Properties Safe Refactorings Documentation Cannot add boolean to List[Int] Strong type inference val x = Map(1->”somevalue”)
- 15. Knoldus 1. Create a new worksheet called Lab1 2. Print “Hello Scala World” 3. See the program execute in Worksheet
- 16. Knoldus 1 + 2 What do you see? /> res1: Int(3) = 3
- 17. Knoldus Agenda ● Scala Who & Why? ● First Steps ● Classes and Objects ● Control Structures and Functions ● Inheritance and Traits ● Pattern Matching
- 19. Knoldus Why does this code compile? Type Inference val msg:String = "Hello World!" //> msg : String = Hello World!
- 20. Knoldus Sometimes variables need to mutate var description = "start" > description : java.lang.String = start description = "end"
- 22. Knoldus 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
- 23. Knoldus 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
- 24. Knoldus 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
- 25. Knoldus 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))
- 26. Knoldus Function Literal What we pass inside of foreach is a function literal List(1,2,3) foreach ((x:Int)=>println(2*x))
- 27. Knoldus 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"
- 32. Knoldus Tuples Immutable like lists, but can have different types of elements Why we cannot get values out of a Tuple just like list?
- 33. Knoldus Sets and Maps By default, we are provided with the immutable collections
- 34. Knoldus What is happening here var treasureMap = Map(1->"a", 2->"b") treasureMap += (3->"c") What happens if we change treasureMap to val? Why?
- 35. Knoldus Understanding Functional Style No vars No methods with side effects – Unit No while loops
- 36. Knoldus Agenda ● Scala Who & Why? ● First Steps ● Classes and Objects ● Control Structures and Functions ● Inheritance and Traits ● Pattern Matching
- 37. Knoldus 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
- 39. Knoldus 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
- 40. Knoldus 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
- 41. Knoldus class Bank(id:Int, name:String) val b = new Bank(1,"BoA") b.id
- 42. Knoldus class Bank(id:Int, name:String) val b = new Bank(1,"BoA") b.id
- 43. Knoldus Add another parameter to bank Create another instance of bank
- 44. Knoldus 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
- 45. Knoldus Accessing fields of a class class Bank(id: Int, name: String) { val kind = "Blue" var address = "Delhi" def this(str: String) = this(1, str) def this() = this(1, "") } val b = new Bank(1, "BoA") b.address = "Mumbai" b.kind = "op" immutable mutable
- 46. Knoldus Making fields from class parameters class Bank(val id: Int, var name: String) { val kind = "Blue" var address = "Delhi" def this(str: String) = this(1, str) def this() = this(1, "") } See if you can access id and name outside the class now
- 47. Knoldus 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
- 48. Knoldus Define a debit method in the Account class def debit(amount:Int):Int = {...}
- 49. Knoldus 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 //
- 50. Knoldus 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")
- 51. Knoldus 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
- 52. Knoldus Singleton Objects ● Can be thought to be like static in Java but these are real objects which can inherit and be passed around
- 53. Knoldus Companion Objects ● If a singleton object and a class or trait share the same name, package and file, they are called companions class Bank(val id: Int, var name: String) { private val kind = "Blue" var address = "Delhi" + Bank.city def this(str: String) = this(1, str) def this() = this(1, "") } object Bank { private val city = "Delhi" val newBank = new Bank(1,"ABC") newBank.kind } Class or trait can access private member of Companion object
- 54. Knoldus 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
- 55. Knoldus 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
- 56. Knoldus Add a require assertion to the account that id has to be > 0 Add any other preconditions for accounts
- 57. Knoldus class Bank(val id: Int, var name: String) { require(id > 0) private val kind = "Blue" var address = "Delhi" Bank.city def this(str: String) = this(1, str) def this() = this(1, "") } object Bank { val city = "Delhi" val newBank = new Bank(1, "ABC") } new Bank(0,"ll") //> java.lang.IllegalArgumentException: requirement failed
- 58. Knoldus 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
- 59. Knoldus 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
- 60. Knoldus Try converting the Bank and Account classes to case classes Remove vals, new, try out the toString, equals etc
- 61. Knoldus Functional Objects Are objects that do not have mutable state
- 62. Knoldus 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
- 63. Knoldus 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
- 64. Knoldus Agenda ● Scala Who & Why? ● First Steps ● Classes and Objects ● Control Structures and Functions ● Inheritance and Traits ● Pattern Matching
- 65. Knoldus 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
- 66. Knoldus ● 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
- 67. Knoldus Are called loops and Not expressions because They do not result in a value Usually left out of functional programming
- 68. Knoldus 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
- 69. Knoldus 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
- 70. Knoldus 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)
- 71. Knoldus 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
- 72. Knoldus Regular
- 74. Knoldus 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
- 75. Knoldus (x:Int)=> x * 2 Is a valid function literal, it can be Assigned to a variable Passed in another function
- 76. Knoldus val multiply = (x:Int) => x * 2 multiply (20) //> res11: Int = 40 multiply (25) //> res12: Int = 50 Function literals can be assigned to variables
- 77. Knoldus 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
- 78. Knoldus 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)
- 79. Knoldus Agenda ● Scala Who & Why? ● First Steps ● Classes and Objects ● Control Structures and Functions ● Inheritance and Traits ● Pattern Matching
- 80. Knoldus Class Inheritance class Animal class Dog extends Animal Omitting the extends keyword means that we are extending from AnyRef
- 81. Knoldus 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
- 82. Knoldus final class Animal(name:String) class Dog(name:String) extends Animal(name) Defining the class as final restricts it from being subclassed
- 83. Knoldus 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
- 84. Knoldus Abstract class Omitting an initialization or implementation would have to make a class abstract Cannot be initialized, they need to be subclassed
- 85. Knoldus 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
- 86. Knoldus
- 87. Knoldus 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
- 88. Knoldus
- 89. Knoldus Traits Fundamental unit of code reuse in Scala A class can mix in any number of traits but inherit from one SuperClass
- 90. Knoldus We can use it as variable of type Philosophical
- 91. Knoldus 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
- 92. Knoldus 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
- 93. Knoldus Agenda ● Scala Who & Why? ● First Steps ● Classes and Objects ● Control Structures and Functions ● Inheritance and Traits ● Pattern Matching
- 94. Knoldus Agenda ● Scala Who & Why? ● First Steps ● Classes and Objects ● Control Structures and Functions ● Inheritance and Traits ● Pattern Matching
- 95. Knoldus 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
- 96. Knoldus 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
- 97. Knoldus 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.