Kotlin: Classes & Data Classes

What you will learn

Kotlin's concise class syntax, how constructor parameters become properties, and what you get for free with data class.

Classes with constructor properties

Kotlin combines constructor, property declaration, and initializer into one expression:

class Person(val name: String, var age: Int)

val alice = Person("Alice", 25)
println(alice.name)   // "Alice" (getter)
alice.age = 26        // setter works because var

val in the constructor creates a read-only property (getter only). var creates a mutable property (getter + setter).

Data classes

Add data before class and Kotlin automatically generates equals(), hashCode(), toString(), copy(), and componentN():

data class User(val id: Int, val email: String)

val u1 = User(1, "[email protected]")
val u2 = User(1, "[email protected]")
println(u1 == u2)          // true (structural equality, not reference)
println(u1.toString())     // User(id=1, [email protected])
val u3 = u1.copy(email = "[email protected]")  // copy with modification

This replaces dozens of lines of boilerplate that you'd write in Java.

Sealed classes (restricted hierarchies)

sealed class Result {
    data class Success(val data: String) : Result()
    data class Error(val message: String) : Result()
}

Sealed classes work with when expressions — the compiler checks that every subclass is handled.

Common mistakes

• Forgetting that data class properties must be declared in the primary constructor — properties in the body are not included in equals/toString
• Using == vs === — == calls equals() (structural), === checks reference identity
• Thinking data class makes deep copies — copy() does a shallow copy; nested objects are still shared

Quick check below!