Classes in Kotlin

A class in Kotlin groups state and behavior like Java classes, but properties replace many getter/setter pairs and constructors are concise. Instances are created without new—just call the constructor like a function.

Properties and constructors

class User(val name: String, var score: Int = 0) {
    fun greet() = "Hi, $name"
}

val ada = User("Ada")
println(ada.greet())

Primary constructor parameters in the header can declare val or var properties automatically. Use an init { } block for validation logic.

Compared to Java and C#

Unlike C# auto-properties declared separately, Kotlin often declares them in the constructor header. Everything is reference-typed on the JVM like Java—no new keyword.

Important interview questions and answers

1. Q: Class vs object in Kotlin?
   A: Class is the blueprint; object is a heap instance—same JVM model as Java, with lighter syntax.
2. Q: val vs var on class properties?
   A: val exposes a read-only property; var allows reassignment after construction.

Self-check

1. How do you create an instance without new?
2. Where can you put validation in a class?

Tip: Classes are final by default—use open only when inheritance is intentional.