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.

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

📋 Quick Reference — Classes

Practice quiz

Where does a Kotlin class declare its primary constructor?

  • Inside an init block
  • At the end of the class body
  • Right after the class name
  • In a companion object

Answer: Right after the class name. The primary constructor goes after the class name: class Person(...).

What does prefixing a primary constructor parameter with val or var do?

  • Promotes it to a property of the class
  • Makes it private
  • Makes it static
  • Marks it as nullable

Answer: Promotes it to a property of the class. val/var on a constructor parameter makes it a property automatically.

How do you create an instance of a class in Kotlin?

  • Use the new keyword: new Person()
  • Call Person.create()
  • Use alloc Person
  • Call the class like a function: Person("Ada", 36)

Answer: Call the class like a function: Person("Ada", 36). Kotlin has no new keyword; you call the class directly.

What is the init block used for?

  • Declaring the primary constructor
  • Initialization logic that runs during construction
  • Defining methods
  • Importing packages

Answer: Initialization logic that runs during construction. The primary constructor can't hold code, so setup logic goes in init.

What does require(width > 0) { "..." } do?

  • Throws if the condition is false, validating input
  • Logs a warning
  • Returns false silently
  • Defines a default value

Answer: Throws if the condition is false, validating input. require validates inputs and throws when the condition fails.

What must every secondary constructor do?

  • Return a value
  • Be private
  • Delegate to the primary constructor with : this(...)
  • Have no parameters

Answer: Delegate to the primary constructor with : this(...). Secondary constructors must ultimately call the primary one.

Can you reassign a val property after construction?

  • Yes, always
  • No, val properties are read-only once set
  • Only inside init
  • Only with var keyword

Answer: No, val properties are read-only once set. Use var if the property needs to change after construction.

In class Person(val name: String, var age: Int), which property can birthday() mutate with age += 1?

  • name, because it is a val
  • Both
  • Neither
  • age, because it is a var

Answer: age, because it is a var. Only var properties can be reassigned; name is a val.

If a constructor parameter has no val or var, can you access it as obj.property?

  • Yes, always
  • No, it is only available inside the constructor and init blocks
  • Only if it is an Int
  • Only from a companion object

Answer: No, it is only available inside the constructor and init blocks. Without val/var the parameter is not a property.

What often removes the need for a secondary constructor?

  • Using new
  • Adding more init blocks
  • A primary constructor with default parameter values
  • Making the class abstract

Answer: A primary constructor with default parameter values. Default parameters frequently replace extra constructors idiomatically.