Classes, Objects, Inheritance, Interfaces, and Data Classes
Learn Kotlin’s object-oriented features and how they support clean domain modeling, abstraction, and maintainable program design.
Inside this chapter
- Defining Classes and Objects
- Data Classes
- Inheritance and Open Classes
- Interfaces and Abstraction
- Object Keyword and Singletons
- Real Project Modeling
Series navigation
Study the chapters in order for the clearest path from Kotlin setup and syntax to coroutines, backend work, clean design, multiplatform thinking, and advanced engineering practice. Use the navigation at the bottom to move smoothly through the full tutorial series.
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.