Classes & Constructors

Kotlin is a modern, concise language that makes classes remarkably compact — a primary constructor can declare and initialise properties in a single line, no boilerplate required.

Learn Classes & Constructors in our free Kotlin course — a beginner-friendly interactive lesson with worked examples, a practice exercise and a quick reference.

Part of the free Kotlin course at LearnCodingFast — hands-on lessons with examples you run in your browser, plus practice exercises and a quick quiz.

By the end of this lesson you'll define classes with primary and secondary constructors, add methods, and run initialisation logic in init blocks.

What You'll Learn in This Lesson

1️⃣ Classes and Primary Constructors

A Kotlin class declares its primary constructor right after the class name. If you prefix a parameter with val or var , it becomes a property automatically. You create an instance by calling the class — there is no new keyword.

In Java this class would take a dozen lines of fields, a constructor, and getters. Kotlin condenses it to one expressive line plus the methods you actually care about.

2️⃣ init Blocks and Secondary Constructors

Setup logic that can't fit in the primary constructor goes in an init block, which runs during construction. require validates inputs and throws if they're invalid. A secondary constructor offers an alternative way to build the object and must delegate to the primary one with : this(...) .

Notice the order: the primary constructor's parameters are set, then the init block runs, then any secondary-constructor body. The require guard makes invalid objects impossible to create.

Your turn. Replace the TODO , then run and compare.

Mini-Challenge: A Counter Class

Define a small class with mutable state and two methods, then drive it from main() .

Common Errors (and the fix)

• Using new — Kotlin has no new . Just call the class: Person("Ada", 36) .
• Forgetting val/var on a constructor param — Without it, the parameter isn't a property and obj.name won't compile. Add val or var .
• Reassigning a val property — You can't reassign a val property after construction. Use var if it changes.
• Secondary constructor not delegating — Every secondary constructor must call the primary with : this(...) , or another secondary that eventually does.

Pro Tips

• 💡 Default params over secondary constructors: often a primary constructor with defaults removes the need for extra constructors.
• 💡 Validate early: use require in init to reject bad input at construction time.
• 💡 Keep methods small: a class is easiest to use when each method does one clear thing.

📋 Quick Reference — Classes

Concept

Kotlin Syntax

Class + properties

class P(val n: String, var a: Int)

Create instance

val p = P("Ada", 36)

Method

fun describe() = "..."

init block

init { ... }

Validate

require(x > 0) { "..." }

Secondary ctor

constructor(s: Int) : this(s, s)

Frequently Asked Questions

🎉 Lesson Complete!

• ✅ Classes declare a primary constructor after the name
• ✅ val / var params become properties
• ✅ Create instances with no new keyword
• ✅ init blocks run setup; require validates
• ✅ Secondary constructors delegate with : this(...)
• ✅ Next lesson: Properties

Practice quiz

Continue this course

• Previous: Collection Operations (map, filter, fold)
• Next: Properties