KOTLIN Tutorial

Object-Oriented Programming (OOP) in Kotlin

Object-Oriented Programming (OOP) is a paradigm that organizes code into "objects" that combine data and methods. Kotlin fully supports OOP principles like classes, inheritance, and polymorphism.

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1. Classes and Objects

In Kotlin, a class is a blueprint for creating objects. You can define properties and methods inside a class, and then instantiate objects from that class.

// Define a class
class Car(val make: String, val model: String) {
    fun drive() {
        println("Driving the $make $model")
    }
}

// Create an object from the class
val myCar = Car("Toyota", "Camry")
myCar.drive()  // Output: Driving the Toyota Camry
  

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2. Constructors

Constructors are used to initialize the object’s properties when it's created. In Kotlin, you can define a primary constructor directly in the class header.

// Class with a primary constructor
class Person(val name: String, val age: Int) {
    fun greet() {
        println("Hello, my name is $name and I am $age years old.")
    }
}

val person1 = Person("Alice", 25)
person1.greet()  // Output: Hello, my name is Alice and I am 25 years old.
  

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3. Inheritance

Inheritance allows one class to inherit properties and methods from another class. In Kotlin, inheritance is implemented using the open keyword.

// Base class
open class Animal(val name: String) {
    fun speak() {
        println("$name makes a sound.")
    }
}

// Derived class inherits from Animal
class Dog(name: String) : Animal(name) {
    fun bark() {
        println("$name is barking!")
    }
}

val dog = Dog("Buddy")
dog.speak()  // Output: Buddy makes a sound.
dog.bark()   // Output: Buddy is barking!
  

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4. Polymorphism

Polymorphism allows methods to behave differently based on the object calling them. In Kotlin, polymorphism is achieved through method overriding.

// Base class with open function
open class Shape {
    open fun draw() {
        println("Drawing a shape")
    }
}

// Derived class overrides the draw function
class Circle : Shape() {
    override fun draw() {
        println("Drawing a circle")
    }
}

// Derived class overrides the draw function
class Square : Shape() {
    override fun draw() {
        println("Drawing a square")
    }
}

val circle = Circle()
val square = Square()

circle.draw()  // Output: Drawing a circle
square.draw()  // Output: Drawing a square
  

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5. Abstraction

Abstraction allows you to hide complex implementation details and expose only essential features. In Kotlin, abstraction is implemented using abstract classes and abstract methods.

// Abstract class
abstract class Vehicle {
    abstract fun start()
}

// Derived class provides implementation of the abstract method
class Car : Vehicle() {
    override fun start() {
        println("Car is starting.")
    }
}

val myCar = Car()
myCar.start()  // Output: Car is starting.
  

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6. Interface

An interface defines a contract that implementing classes must fulfill. In Kotlin, interfaces can contain method declarations without a body.

// Interface
interface Drivable {
    fun drive()
}

// Class implements the interface
class Car : Drivable {
    override fun drive() {
        println("The car is driving.")
    }
}

val car = Car()
car.drive()  // Output: The car is driving.
  

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🧪 Try It Yourself

Here are some challenges for you to practice:

• Create a Person class with properties for name, age, and occupation, and include a method to introduce themself.
• Define a Shape class with subclasses like Circle and Rectangle, and implement the draw method.
• Design an interface called Flyable and implement it in classes like Bird and Airplane.