KOTLIN Tutorial

Classes and Objects in Kotlin

In Kotlin, a class is a blueprint for creating objects. Each object created from a class can have its own properties (variables) and methods (functions). Let's dive into how to define and use classes and objects in Kotlin!

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1. Defining a Class

To define a class, use the class keyword followed by the class name. You can define properties and methods inside the class.

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

This Car class has two properties: make and model, and a method called drive that prints a message when invoked.

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2. Creating an Object

An object is created from a class using the new keyword in other languages, but in Kotlin, we simply instantiate a class directly.

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

Here, myCar is an object of the class Car. It holds the make and model of the car, and we can call its drive method.

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3. Properties and Initializer

You can also initialize properties directly in the class constructor. In Kotlin, properties are defined inside the class constructor and automatically initialized with values.

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

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

In this example, the Person class has a constructor with two parameters: name and age. The object person1 is created with these values, and we can call the greet method to display the information.

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4. Constructor with Default Values

You can also define default values for the class properties in the constructor. This allows you to create objects without passing all the values explicitly.

// Class with default values
class Car(val make: String = "Toyota", val model: String = "Camry") {
    fun drive() {
        println("Driving the $make $model")
    }
}

// Creating an object with default values
val defaultCar = Car()
defaultCar.drive()  // Output: Driving the Toyota Camry
  

Here, if no values are passed, the object defaultCar will use the default values ("Toyota" and "Camry").

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5. Primary Constructor vs. Secondary Constructor

Kotlin allows you to define a primary constructor and secondary constructors. The primary constructor is declared directly in the class header, while secondary constructors are declared inside the class body.

// Class with secondary constructor
class Person {
    var name: String
    var age: Int
    
    constructor(name: String, age: Int) {
        this.name = name
        this.age = age
    }
    
    fun introduce() {
        println("Hello, my name is $name and I am $age years old.")
    }
}

// Creating an object using the secondary constructor
val person2 = Person("Bob", 30)
person2.introduce()  // Output: Hello, my name is Bob and I am 30 years old.
  

The Person class uses a secondary constructor to initialize the name and age properties. The object person2 is created with values passed to the secondary constructor.

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6. Data Classes

Kotlin has a special class type called data classes that automatically provides methods like toString(), hashCode(), and equals() for you. This is useful when you only need to store data in an object.

// Defining a data class
data class Car(val make: String, val model: String)

// Creating an object
val car1 = Car("Honda", "Civic")
println(car1)  // Output: Car(make=Honda, model=Civic)
  

With the data class, Kotlin automatically generates methods for comparing objects and converting them to a string format.

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🧪 Try It Yourself

Here are some challenges for you to practice:

• Define a class called Book with properties for title, author, and price. Create an object and display the book details.
• Define a BankAccount class with methods to deposit, withdraw, and check balance. Create an object and test the methods.
• Use a data class to define a Product with properties like name, category, and price, then create and print an object of that class.