Classes, Objects, Inheritance, Interfaces, and Data Classes

Defining Classes and Objects

class User(val id: Int, var name: String)

val user = User(1, "Meera")
println(user.name)

Kotlin makes class construction compact. Students should understand primary constructors, properties, initialization, and object instances clearly before moving further.

Data Classes

data class Order(
    val id: Int,
    val amount: Double,
    val status: String
)

Data classes automatically provide useful methods such as toString, equals, hashCode, and copy. They are ideal for DTOs, domain models, API payloads, and UI state models.

Inheritance and Open Classes

open class PaymentProcessor {
    open fun process() {
        println("Base processing")
    }
}

class CardProcessor : PaymentProcessor() {
    override fun process() {
        println("Card payment processing")
    }
}

Classes are final by default in Kotlin, which helps avoid accidental inheritance and encourages deliberate design.

Interfaces and Abstraction

interface Notifier {
    fun send(message: String)
}

Interfaces are important for architecture because they let you separate behavior contracts from concrete implementation details. This makes testing and substitution easier.

Object Keyword and Singletons

object Config {
    const val BASE_URL = "https://example.com"
}

Kotlin’s object declaration is often used for stateless helpers, constants, registries, and singleton-like structures.

Real Project Modeling

In a hospital management system, data classes may represent patients and appointments, interfaces may abstract notification or billing services, and specific processor classes may handle different insurance flows. This is how object-oriented design becomes practical rather than theoretical.