Classes, Data Classes and Objects

Classes and constructors

Kotlin folds the constructor and property declarations into the class header:

class Person(val name: String, var age: Int) {
    fun birthday() { age++ }
}

val p = Person("Ada", 36)   // no 'new' keyword
p.birthday()

val name in the header declares a read-only property and assigns it from the constructor argument – one line replaces a field, a constructor parameter and a getter.

Data classes

Classes that just hold data are declared with data class. The compiler generates equals(), hashCode(), toString(), copy() and destructuring for you:

data class User(val name: String, val age: Int)

val u = User("Ada", 36)
println(u)                 // User(name=Ada, age=36)
val older = u.copy(age = 37)   // copy with one field changed
val (n, a) = u             // destructuring

Singletons with object

The object keyword declares a singleton in one step – no static, no manual instance management:

object AppConfig {
    val version = "1.0"
    fun printInfo() = println("Version $version")
}

AppConfig.printInfo()

Interfaces and inheritance

Classes are final by default; mark a class open to allow subclassing. Interfaces work as in Java but can hold default method bodies:

interface Shape { fun area(): Double }
open class Base
class Circle(val r: Double) : Base(), Shape {
    override fun area() = Math.PI * r * r
}

Key points

• Primary constructors and properties are declared in the class header – no boilerplate.
• data class generates equals/hashCode/toString/copy and destructuring.
• object declares a singleton directly.
• Classes are final by default; use open to allow inheritance.