OOP I — Classes, Constructors, Inheritance, Polymorphism

Key concepts

• class and data class
• Primary constructor (in class header)
• Secondary constructors
• Custom get() / set()
• open class / open fun — inheritance is opt-in
• override and super
• Runtime polymorphism

Lectures in this day

1. 01

   Class and Object Definitions

   A class is a blueprint that says what a thing has (properties) and what it can do (functions). An object is an actual thing made from that blueprint. A `data class` is a shortcut for classes that mostly just hold data — Kotlin writes the boring bits for you.

   Real life · One blueprint of a house (the class) can be used to build many real houses (the objects).
   kotlin

   Copy

   0
   data class Book(val title: String, val author: String, val pages: Int)
   
   1
   val b = Book("Deep Work", "Cal Newport", 296)
   println(b)                          2
   
   3
   val short = b.copy(pages = 120)
   println(short)

   Practice · 3 quick questions

   1. Q1.A class is best described as…

      A.A blueprintB.A numberC.A file

   2. Q2.An object is…

      A.The blueprintB.An instance made from a classC.A folder

   3. Q3.You create an object with…

      A.`new Car()`B.`Car()`C.`create Car`

   Check answersPick one answer per question.
2. 02

   Various Constructors and Custom Getter/Setter

   A constructor is the recipe for making a new object. The main one is written in the class header. You can add extra recipes with `constructor(...)`. A property can also run a small piece of code every time you read it (getter) or write to it (setter).

   Real life · Sign-up forms: one long form asks for everything (main), a short form uses defaults for the rest (extra). A setter that trims spaces is like a form that tidies your typing automatically.
   kotlin

   Copy

   class User(firstName: String, val lastName: String) {
   
       0
       var firstName: String = firstName
           set(value) {
               field = value.trim()   1
           }
   
       2
       val fullName: String
           get() = "$firstName $lastName"
   
       3
       constructor(only: String) : this(only, "—")
   }
   
   val u = User("  Rakib  ", "Hasan")
   println(u.fullName)   4

   Practice · 3 quick questions

   1. Q1.A primary constructor sits…

      A.In a separate fileB.Beside the class nameC.Inside main()

   2. Q2.A custom setter lets you…

      A.Change how a value is storedB.Delete the propertyC.Run without a class

   3. Q3.Inside a setter, `field` means…

      A.Any variableB.The backing storage of this propertyC.Null

   Check answersPick one answer per question.
3. 03

   Inheritance and Method Overriding

   Inheritance means one class can build on another. The child class gets everything the parent has, and can add or change parts. In Kotlin you must mark a class or function `open` before it can be extended or changed. `override` means 'I am replacing the parent's version'.

   Real life · Every worker in a company has a name and can write a report. A Manager is still a worker, but their report also includes the team's progress.
   kotlin

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   open class Employee(val name: String) {           0
       open fun dailyReport() = "$name: worked 8 hours" 1
   }
   
   class Manager(name: String) : Employee(name) {
       override fun dailyReport() =                     2
           "${super.dailyReport()} + reviewed team"    3
   }
   
   println(Manager("Rima").dailyReport())

   Practice · 3 quick questions

   1. Q1.To allow a class to be inherited, mark it…

      A.finalB.openC.static

   2. Q2.To replace a parent's method, use…

      A.overrideB.extendC.super

   3. Q3.Which keyword refers to the parent?

      A.thisB.superC.self

   Check answersPick one answer per question.
4. 04

   Runtime Polymorphism

   Polymorphism means 'many shapes'. The SAME line of code can behave differently depending on the actual object it runs on. Kotlin picks the right version at run time.

   Real life · One 'Play' button on a universal remote: on the TV it plays a show, on the speaker it plays music. Same button, different behavior.
   kotlin

   Copy

   open class Shape {
       open fun area(): Double = 0.0
   }
   class Circle(val r: Double) : Shape() {
       override fun area() = Math.PI * r * r
   }
   class Square(val s: Double) : Shape() {
       override fun area() = s * s
   }
   
   0
   val shapes: List<Shape> = listOf(Circle(2.0), Square(3.0))
   shapes.forEach { println(it.area()) }
   1

   Practice · 3 quick questions

   1. Q1.Runtime polymorphism means…

      A.The correct method is picked at run time based on the object's real typeB.The code runs twiceC.There is no inheritance

   2. Q2.It requires…

      A.Only one classB.Inheritance + overridingC.Firebase

   3. Q3.Which is a benefit?

      A.One variable, many behaviorsB.Faster typingC.Less memory always

   Check answersPick one answer per question.

Real-life analogies

Code examples

open class Vehicle(val brand: String, val wheels: Int) {
    open fun describe() = "$brand vehicle with $wheels wheels"
}

class Car(brand: String) : Vehicle(brand, 4) {
    override fun describe() = "${super.describe()} — a car"
}

class Bike(brand: String) : Vehicle(brand, 2) {
    override fun describe() = "${super.describe()} — a bike"
}

val fleet: List<Vehicle> = listOf(Car("Toyota"), Bike("Yamaha"))
fleet.forEach { println(it.describe()) } 0

class User(firstName: String, lastName: String) {
    var firstName: String = firstName
        set(value) { field = value.trim() }

    val fullName: String
        get() = "$firstName $lastName"

    val lastName: String = lastName
}

Hands-on checklist

• Model Vehicle → Car, Bike with an override
• Add a secondary constructor
• Add a computed property using a custom getter
• Loop through a mixed list and observe polymorphism

Milestone

Common pitfalls

• Forgetting `open` on the parent class — child won't compile.
• Using `this.name = value` in a custom setter instead of `field = value` — infinite recursion.